[compiler] Remove local CompilerError accumulators, emit directly to env.recordError()

Removes unnecessary indirection in 17 compiler passes that previously
accumulated errors in a local `CompilerError` instance before flushing
them to `env.recordErrors()` at the end of each pass. Errors are now
emitted directly via `env.recordError()` as they're discovered.

For passes with recursive error-detection patterns (ValidateNoRefAccessInRender,
ValidateNoSetStateInRender), the internal accumulator is kept but flushed
via individual `recordError()` calls. For InferMutationAliasingRanges,
a `shouldRecordErrors` flag preserves the conditional suppression logic.
For TransformFire, the throw-based error propagation is replaced with
direct recording plus an early-exit check in Pipeline.ts.

.claude/instructions.md

@@ -0,0 +1,46 @@
+# React
+
+**Scope**: All code EXCEPT `/compiler/` (compiler has its own instructions).
+
+## Project Structure
+
+| Directory | Purpose |
+|-----------|---------|
+| `/packages/` | Publishable packages (react, react-dom, scheduler, etc.) |
+| `/scripts/` | Build, test, and development scripts |
+| `/fixtures/` | Test applications for manual testing |
+| `/compiler/` | React Compiler (separate sub-project) |
+
+## Key Packages
+
+| Package | Purpose |
+|---------|---------|
+| `react` | Core React library |
+| `react-dom` | DOM renderer |
+| `react-reconciler` | Core reconciliation algorithm |
+| `scheduler` | Cooperative scheduling |
+| `react-server-dom-*` | Server Components |
+| `react-devtools-*` | Developer Tools |
+| `react-refresh` | Fast Refresh runtime |
+
+## Requirements
+
+- **Node**: Must be installed. Stop and prompt user if missing.
+- **Package Manager**: Use `yarn` only.
+
+## Verification
+
+**IMPORTANT**: Use `/verify` to validate all changes before committing.
+
+## Commands
+
+| Command  | Purpose              |
+|----------|----------------------|
+| `/fix`   | Lint and format code |
+| `/test`  | Run tests            |
+| `/flow`  | Type check with Flow |
+| `/flags` | Check feature flags  |
+
+## Building
+
+Builds are handled by CI. Do not run locally unless instructed.

.claude/settings.json

@@ -0,0 +1,44 @@
+{
+  "hooks": {
+    "SessionStart": [
+      {
+        "hooks": [
+          {
+            "type": "command",
+            "command": "if [[ \"$PWD\" != */compiler* ]]; then cat .claude/instructions.md 2>/dev/null || true; fi"
+          }
+        ]
+      }
+    ]
+  },
+  "permissions": {
+    "allow": [
+      "Skill(extract-errors)",
+      "Skill(feature-flags)",
+      "Skill(fix)",
+      "Skill(flags)",
+      "Skill(flow)",
+      "Skill(test)",
+      "Skill(verify)",
+      "Bash(yarn test:*)",
+      "Bash(yarn test-www:*)",
+      "Bash(yarn test-classic:*)",
+      "Bash(yarn test-stable:*)",
+      "Bash(yarn linc:*)",
+      "Bash(yarn lint:*)",
+      "Bash(yarn flow:*)",
+      "Bash(yarn prettier:*)",
+      "Bash(yarn build:*)",
+      "Bash(yarn extract-errors:*)",
+      "Bash(yarn flags:*)"
+    ],
+    "deny": [
+      "Bash(yarn download-build:*)",
+      "Bash(yarn download-build-for-head:*)",
+      "Bash(npm:*)",
+      "Bash(pnpm:*)",
+      "Bash(bun:*)",
+      "Bash(npx:*)"
+    ]
+  }
+}

.claude/skills/extract-errors/SKILL.md

@@ -0,0 +1,12 @@
+---
+name: extract-errors
+description: Use when adding new error messages to React, or seeing "unknown error code" warnings.
+---
+
+# Extract Error Codes
+
+## Instructions
+
+1. Run `yarn extract-errors`
+2. Report if any new errors need codes assigned
+3. Check if error codes are up to date

.claude/skills/feature-flags/SKILL.md

@@ -0,0 +1,79 @@
+---
+name: feature-flags
+description: Use when feature flag tests fail, flags need updating, understanding @gate pragmas, debugging channel-specific test failures, or adding new flags to React.
+---
+
+# React Feature Flags
+
+## Flag Files
+
+| File | Purpose |
+|------|---------|
+| `packages/shared/ReactFeatureFlags.js` | Default flags (canary), `__EXPERIMENTAL__` overrides |
+| `packages/shared/forks/ReactFeatureFlags.www.js` | www channel, `__VARIANT__` overrides |
+| `packages/shared/forks/ReactFeatureFlags.native-fb.js` | React Native, `__VARIANT__` overrides |
+| `packages/shared/forks/ReactFeatureFlags.test-renderer.js` | Test renderer |
+
+## Gating Tests
+
+### `@gate` pragma (test-level)
+
+Use when the feature is completely unavailable without the flag:
+
+```javascript
+// @gate enableViewTransition
+it('supports view transitions', () => {
+  // This test only runs when enableViewTransition is true
+  // and is SKIPPED (not failed) when false
+});
+```
+
+### `gate()` inline (assertion-level)
+
+Use when the feature exists but behavior differs based on flag:
+
+```javascript
+it('renders component', async () => {
+  await act(() => root.render(<App />));
+
+  if (gate(flags => flags.enableNewBehavior)) {
+    expect(container.textContent).toBe('new output');
+  } else {
+    expect(container.textContent).toBe('legacy output');
+  }
+});
+```
+
+## Adding a New Flag
+
+1. Add to `ReactFeatureFlags.js` with default value
+2. Add to each fork file (`*.www.js`, `*.native-fb.js`, etc.)
+3. If it should vary in www/RN, set to `__VARIANT__` in the fork file
+4. Gate tests with `@gate flagName` or inline `gate()`
+
+## Checking Flag States
+
+Use `/flags` to view states across channels. See the `flags` skill for full command options.
+
+## `__VARIANT__` Flags (GKs)
+
+Flags set to `__VARIANT__` simulate gatekeepers - tested twice (true and false):
+
+```bash
+/test www <pattern>              # __VARIANT__ = true
+/test www variant false <pattern> # __VARIANT__ = false
+```
+
+## Debugging Channel-Specific Failures
+
+1. Run `/flags --diff <channel1> <channel2>` to compare values
+2. Check `@gate` conditions - test may be gated to specific channels
+3. Run `/test <channel> <pattern>` to isolate the failure
+4. Verify flag exists in all fork files if newly added
+
+## Common Mistakes
+
+- **Forgetting both variants** - Always test `www` AND `www variant false` for `__VARIANT__` flags
+- **Using @gate for behavior differences** - Use inline `gate()` if both paths should run
+- **Missing fork files** - New flags must be added to ALL fork files, not just the main one
+- **Wrong gate syntax** - It's `gate(flags => flags.name)`, not `gate('name')`

.claude/skills/fix/SKILL.md

@@ -0,0 +1,17 @@
+---
+name: fix
+description: Use when you have lint errors, formatting issues, or before committing code to ensure it passes CI.
+---
+
+# Fix Lint and Formatting
+
+## Instructions
+
+1. Run `yarn prettier` to fix formatting
+2. Run `yarn linc` to check for remaining lint issues
+3. Report any remaining manual fixes needed
+
+## Common Mistakes
+
+- **Running prettier on wrong files** - `yarn prettier` only formats changed files
+- **Ignoring linc errors** - These will fail CI, fix them before committing

.claude/skills/flags/SKILL.md

@@ -0,0 +1,39 @@
+---
+name: flags
+description: Use when you need to check feature flag states, compare channels, or debug why a feature behaves differently across release channels.
+---
+
+# Feature Flags
+
+Arguments:
+- $ARGUMENTS: Optional flags
+
+## Options
+
+| Option | Purpose |
+|--------|---------|
+| (none) | Show all flags across all channels |
+| `--diff <ch1> <ch2>` | Compare flags between channels |
+| `--cleanup` | Show flags grouped by cleanup status |
+| `--csv` | Output in CSV format |
+
+## Channels
+
+- `www`, `www-modern` - Meta internal
+- `canary`, `next`, `experimental` - OSS channels
+- `rn`, `rn-fb`, `rn-next` - React Native
+
+## Legend
+
+✅ enabled, ❌ disabled, 🧪 `__VARIANT__`, 📊 profiling-only
+
+## Instructions
+
+1. Run `yarn flags $ARGUMENTS`
+2. Explain the output to the user
+3. For --diff, highlight meaningful differences
+
+## Common Mistakes
+
+- **Forgetting `__VARIANT__` flags** - These are tested both ways in www; check both variants
+- **Comparing wrong channels** - Use `--diff` to see exact differences

.claude/skills/flow/SKILL.md

@@ -0,0 +1,30 @@
+---
+name: flow
+description: Use when you need to run Flow type checking, or when seeing Flow type errors in React code.
+---
+
+# Flow Type Checking
+
+Arguments:
+- $ARGUMENTS: Renderer to check (default: dom-node)
+
+## Renderers
+
+| Renderer | When to Use |
+|----------|-------------|
+| `dom-node` | Default, recommended for most changes |
+| `dom-browser` | Browser-specific DOM code |
+| `native` | React Native |
+| `fabric` | React Native Fabric |
+
+## Instructions
+
+1. Run `yarn flow $ARGUMENTS` (use `dom-node` if no argument)
+2. Report type errors with file locations
+3. For comprehensive checking (slow), use `yarn flow-ci`
+
+## Common Mistakes
+
+- **Running without a renderer** - Always specify or use default `dom-node`
+- **Ignoring suppressions** - Check if `$FlowFixMe` comments are masking real issues
+- **Missing type imports** - Ensure types are imported from the correct package

.claude/skills/test/SKILL.md

@@ -0,0 +1,46 @@
+---
+name: test
+description: Use when you need to run tests for React core. Supports source, www, stable, and experimental channels.
+---
+
+Run tests for the React codebase.
+
+Arguments:
+- $ARGUMENTS: Channel, flags, and test pattern
+
+Usage Examples:
+- `/test ReactFiberHooks` - Run with source channel (default)
+- `/test experimental ReactFiberHooks` - Run with experimental channel
+- `/test www ReactFiberHooks` - Run with www-modern channel
+- `/test www variant false ReactFiberHooks` - Test __VARIANT__=false
+- `/test stable ReactFiberHooks` - Run with stable channel
+- `/test classic ReactFiberHooks` - Run with www-classic channel
+- `/test watch ReactFiberHooks` - Run in watch mode (TDD)
+
+Release Channels:
+- `(default)` - Source/canary channel, uses ReactFeatureFlags.js defaults
+- `experimental` - Source/experimental channel with __EXPERIMENTAL__ flags = true
+- `www` - www-modern channel with __VARIANT__ flags = true
+- `www variant false` - www channel with __VARIANT__ flags = false
+- `stable` - What ships to npm
+- `classic` - Legacy www-classic (rarely needed)
+
+Instructions:
+1. Parse channel from arguments (default: source)
+2. Map to yarn command:
+   - (default) → `yarn test --silent --no-watchman <pattern>`
+   - experimental → `yarn test -r=experimental --silent --no-watchman <pattern>`
+   - stable → `yarn test-stable --silent --no-watchman <pattern>`
+   - classic → `yarn test-classic --silent --no-watchman <pattern>`
+   - www → `yarn test-www --silent --no-watchman <pattern>`
+   - www variant false → `yarn test-www --variant=false --silent --no-watchman <pattern>`
+3. Report test results and any failures
+
+Hard Rules:
+1. **Use --silent to see failures** - This limits the test output to only failures.
+2. **Use --no-watchman** - This is a common failure in sandboxing.
+
+Common Mistakes:
+- **Running without a pattern** - Runs ALL tests, very slow. Always specify a pattern.
+- **Forgetting both www variants** - Test `www` AND `www variant false` for `__VARIANT__` flags.
+- **Test skipped unexpectedly** - Check for `@gate` pragma; see `feature-flags` skill.

.claude/skills/verify/SKILL.md

@@ -0,0 +1,24 @@
+---
+name: verify
+description: Use when you want to validate changes before committing, or when you need to check all React contribution requirements.
+---
+
+# Verification
+
+Run all verification steps.
+
+Arguments:
+- $ARGUMENTS: Test pattern for the test step
+
+## Instructions
+
+Run these first in sequence:
+1. Run `yarn prettier` - format code (stop if fails)
+2. Run `yarn linc` - lint changed files (stop if fails)
+
+Then run these with subagents in parallel:
+1. Use `/flow` to type check (stop if fails)
+2. Use `/test` to test changes in source (stop if fails)
+3. Use `/test www` to test changes in www (stop if fails)
+
+If all pass, show success summary. On failure, stop immediately and report the issue with suggested fixes.

.codesandbox/ci.json

@@ -1,7 +1,7 @@
 {
   "packages": ["packages/react", "packages/react-dom", "packages/react-server-dom-webpack", "packages/scheduler"],
   "buildCommand": "download-build-in-codesandbox-ci",
-  "node": "18",
+  "node": "20",
   "publishDirectory": {
     "react": "build/oss-experimental/react",
     "react-dom": "build/oss-experimental/react-dom",

.eslintignore

@@ -28,3 +28,6 @@ packages/react-devtools-shared/src/hooks/__tests__/__source__/__untransformed__/
 packages/react-devtools-shell/dist
 packages/react-devtools-timeline/dist
 packages/react-devtools-timeline/static
+
+# Imported third-party Flow types
+flow-typed/

.eslintrc.js

@@ -331,6 +331,7 @@ module.exports = {
         'packages/react-server-dom-turbopack/**/*.js',
         'packages/react-server-dom-parcel/**/*.js',
         'packages/react-server-dom-fb/**/*.js',
+        'packages/react-server-dom-unbundled/**/*.js',
         'packages/react-test-renderer/**/*.js',
         'packages/react-debug-tools/**/*.js',
         'packages/react-devtools-extensions/**/*.js',
@@ -468,6 +469,7 @@ module.exports = {
       files: ['packages/react-server-dom-webpack/**/*.js'],
       globals: {
         __webpack_chunk_load__: 'readonly',
+        __webpack_get_script_filename__: 'readonly',
         __webpack_require__: 'readonly',
       },
     },
@@ -505,7 +507,6 @@ module.exports = {
         __IS_FIREFOX__: 'readonly',
         __IS_EDGE__: 'readonly',
         __IS_NATIVE__: 'readonly',
-        __IS_INTERNAL_MCP_BUILD__: 'readonly',
         __IS_INTERNAL_VERSION__: 'readonly',
         chrome: 'readonly',
       },
@@ -516,6 +517,14 @@ module.exports = {
         __IS_INTERNAL_VERSION__: 'readonly',
       },
     },
+    {
+      files: ['packages/react-devtools-*/**/*.js'],
+      excludedFiles: '**/__tests__/**/*.js',
+      plugins: ['eslint-plugin-react-hooks-published'],
+      rules: {
+        'react-hooks-published/rules-of-hooks': ERROR,
+      },
+    },
     {
       files: ['packages/eslint-plugin-react-hooks/src/**/*'],
       extends: ['plugin:@typescript-eslint/recommended'],
@@ -546,13 +555,10 @@ module.exports = {
   },
 
   globals: {
-    $Call: 'readonly',
-    $ElementType: 'readonly',
     $Flow$ModuleRef: 'readonly',
     $FlowFixMe: 'readonly',
     $Keys: 'readonly',
     $NonMaybeType: 'readonly',
-    $PropertyType: 'readonly',
     $ReadOnly: 'readonly',
     $ReadOnlyArray: 'readonly',
     $ArrayBufferView: 'readonly',
@@ -567,6 +573,7 @@ module.exports = {
     BigInt: 'readonly',
     BigInt64Array: 'readonly',
     BigUint64Array: 'readonly',
+    CacheType: 'readonly',
     Class: 'readonly',
     ClientRect: 'readonly',
     CopyInspectedElementPath: 'readonly',
@@ -578,16 +585,20 @@ module.exports = {
     $AsyncIterator: 'readonly',
     Iterator: 'readonly',
     AsyncIterator: 'readonly',
+    IntervalID: 'readonly',
     IteratorResult: 'readonly',
     JSONValue: 'readonly',
     JSResourceReference: 'readonly',
+    mixin$Animatable: 'readonly',
     MouseEventHandler: 'readonly',
     NavigateEvent: 'readonly',
+    Partial: 'readonly',
+    PerformanceMeasureOptions: 'readonly',
     PropagationPhases: 'readonly',
     PropertyDescriptor: 'readonly',
-    React$AbstractComponent: 'readonly',
+    PropertyDescriptorMap: 'readonly',
+    Proxy$traps: 'readonly',
     React$Component: 'readonly',
-    React$ComponentType: 'readonly',
     React$Config: 'readonly',
     React$Context: 'readonly',
     React$Element: 'readonly',
@@ -619,12 +630,12 @@ module.exports = {
     PropertyIndexedKeyframes: 'readonly',
     KeyframeAnimationOptions: 'readonly',
     GetAnimationsOptions: 'readonly',
-    Animatable: 'readonly',
     ScrollTimeline: 'readonly',
     EventListenerOptionsOrUseCapture: 'readonly',
     FocusOptions: 'readonly',
     OptionalEffectTiming: 'readonly',
 
+    __REACT_ROOT_PATH_TEST__: 'readonly',
     spyOnDev: 'readonly',
     spyOnDevAndProd: 'readonly',
     spyOnProd: 'readonly',

.github/ISSUE_TEMPLATE/compiler_bug_report.yml

@@ -11,7 +11,7 @@ body:
     options:
       - label: React Compiler core (the JS output is incorrect, or your app works incorrectly after optimization)
       - label: babel-plugin-react-compiler (build issue installing or using the Babel plugin)
-      - label: eslint-plugin-react-compiler (build issue installing or using the eslint plugin)
+      - label: eslint-plugin-react-hooks (build issue installing or using the eslint plugin)
       - label: react-compiler-healthcheck (build issue installing or using the healthcheck script)
 - type: input
   attributes:

.github/workflows/compiler_playground.yml

@@ -57,8 +57,6 @@ jobs:
           key: playwright-browsers-v6-${{ runner.arch }}-${{ runner.os }}-${{ steps.playwright_version.outputs.playwright_version }}
       - run: npx playwright install --with-deps chromium
         if: steps.cache_playwright_browsers.outputs.cache-hit != 'true'
-      - run: npx playwright install-deps
-        if: steps.cache_playwright_browsers.outputs.cache-hit == 'true'
       - run: CI=true yarn test
       - run: ls -R test-results
         if: '!cancelled()'

.github/workflows/compiler_prereleases.yml

@@ -19,6 +19,9 @@ on:
       tag_version:
         required: false
         type: string
+      dry_run:
+        required: false
+        type: boolean
     secrets:
       NPM_TOKEN:
         required: true
@@ -55,7 +58,13 @@ jobs:
           key: compiler-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('compiler/yarn.lock') }}
       - run: yarn install --frozen-lockfile
         if: steps.node_modules.outputs.cache-hit != 'true'
-      - name: Publish packages to npm
+      - if: inputs.dry_run == true
+        name: Publish packages to npm (dry run)
+        run: |
+          cp ./scripts/release/ci-npmrc ~/.npmrc
+          scripts/release/publish.js --frfr --debug --ci --versionName=${{ inputs.version_name }} --tag=${{ inputs.dist_tag }} ${{ inputs.tag_version && format('--tagVersion={0}', inputs.tag_version) || '' }}
+      - if: inputs.dry_run != true
+        name: Publish packages to npm
         run: |
           cp ./scripts/release/ci-npmrc ~/.npmrc
           scripts/release/publish.js --frfr --ci --versionName=${{ inputs.version_name }} --tag=${{ inputs.dist_tag }} ${{ inputs.tag_version && format('--tagVersion={0}', inputs.tag_version) || '' }}

.github/workflows/compiler_prereleases_manual.yml

@@ -17,6 +17,9 @@ on:
       tag_version:
         required: false
         type: string
+      dry_run:
+        required: false
+        type: boolean
 
 permissions: {}
 
@@ -33,5 +36,6 @@ jobs:
       dist_tag: ${{ inputs.dist_tag }}
       version_name: ${{ inputs.version_name }}
       tag_version: ${{ inputs.tag_version }}
+      dry_run: ${{ inputs.dry_run }}
     secrets:
       NPM_TOKEN: ${{ secrets.NPM_TOKEN }}

.github/workflows/compiler_prereleases_nightly.yml

@@ -19,5 +19,6 @@ jobs:
       release_channel: experimental
       dist_tag: experimental
       version_name: '0.0.0'
+      dry_run: false
     secrets:
       NPM_TOKEN: ${{ secrets.NPM_TOKEN }}

.github/workflows/devtools_discord_notify.yml

@@ -0,0 +1,49 @@
+name: (DevTools) Discord Notify
+
+on:
+  pull_request_target:
+    types: [opened, ready_for_review]
+    paths:
+      - packages/react-devtools**
+      - .github/workflows/devtools_**.yml
+
+permissions: {}
+
+jobs:
+  check_access:
+    if: ${{ github.event.pull_request.draft == false }}
+    runs-on: ubuntu-latest
+    outputs:
+      is_member_or_collaborator: ${{ steps.check_is_member_or_collaborator.outputs.is_member_or_collaborator }}
+    steps:
+      - run: echo ${{ github.event.pull_request.author_association }}
+      - name: Check is member or collaborator
+        id: check_is_member_or_collaborator
+        if: ${{ github.event.pull_request.author_association == 'MEMBER' || github.event.pull_request.author_association == 'COLLABORATOR' }}
+        run: echo "is_member_or_collaborator=true" >> "$GITHUB_OUTPUT"
+
+  check_maintainer:
+    if: ${{ needs.check_access.outputs.is_member_or_collaborator == 'true' || needs.check_access.outputs.is_member_or_collaborator == true }}
+    needs: [check_access]
+    uses: facebook/react/.github/workflows/shared_check_maintainer.yml@main
+    permissions:
+      # Used by check_maintainer
+      contents: read
+    with:
+      actor: ${{ github.event.pull_request.user.login }}
+
+  notify:
+    if: ${{ needs.check_maintainer.outputs.is_core_team == 'true' }}
+    needs: check_maintainer
+    runs-on: ubuntu-latest
+    steps:
+      - name: Discord Webhook Action
+        uses: tsickert/discord-webhook@86dc739f3f165f16dadc5666051c367efa1692f4
+        with:
+          webhook-url: ${{ secrets.DEVTOOLS_DISCORD_WEBHOOK_URL }}
+          embed-author-name: ${{ github.event.pull_request.user.login }}
+          embed-author-url: ${{ github.event.pull_request.user.html_url }}
+          embed-author-icon-url: ${{ github.event.pull_request.user.avatar_url }}
+          embed-title: '#${{ github.event.number }} (+${{github.event.pull_request.additions}} -${{github.event.pull_request.deletions}}): ${{ github.event.pull_request.title }}'
+          embed-description: ${{ github.event.pull_request.body }}
+          embed-url: ${{ github.event.pull_request.html_url }}

.github/workflows/devtools_regression_tests.yml

@@ -92,7 +92,7 @@ jobs:
         uses: actions/upload-artifact@v4
         with:
           name: react-devtools
-          path: build/devtools.tgz
+          path: build/devtools
           if-no-files-found: error
       # Simplifies getting the extension for local testing
       - name: Archive chrome extension
@@ -201,5 +201,5 @@ jobs:
       - uses: actions/upload-artifact@v4
         with:
           name: screenshots
-          path: ./tmp/screenshots
+          path: ./tmp/playwright-artifacts
           if-no-files-found: warn

.github/workflows/runtime_build_and_test.yml

@@ -3,6 +3,10 @@ name: (Runtime) Build and Test
 on:
   push:
     branches: [main]
+    tags:
+      # To get CI for backport releases.
+      # This will duplicate CI for releases from main which is acceptable
+      - "v*"
   pull_request:
     paths-ignore:
       - compiler/**
@@ -41,7 +45,7 @@ jobs:
         with:
           path: |
             **/node_modules
-          key: runtime-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock') }}
+          key: runtime-node_modules-v7-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock') }}
           lookup-only: true
       - uses: actions/setup-node@v4
         if: steps.node_modules.outputs.cache-hit != 'true'
@@ -55,10 +59,8 @@ jobs:
         with:
           path: |
             **/node_modules
-          key: runtime-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock') }}
-          restore-keys: |
-            runtime-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-
-            runtime-node_modules-v6-
+          key: runtime-node_modules-v7-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock') }}
+          # Don't use restore-keys here. Otherwise the cache grows indefinitely.
       - run: yarn install --frozen-lockfile
         if: steps.node_modules.outputs.cache-hit != 'true'
       - name: Save cache
@@ -67,7 +69,7 @@ jobs:
         with:
           path: |
             **/node_modules
-          key: runtime-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock') }}
+          key: runtime-node_modules-v7-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock') }}
 
   runtime_compiler_node_modules_cache:
     name: Cache Runtime, Compiler node_modules
@@ -82,7 +84,7 @@ jobs:
         with:
           path: |
             **/node_modules
-          key: runtime-and-compiler-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock', 'compiler/yarn.lock') }}
+          key: runtime-and-compiler-node_modules-v7-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock', 'compiler/yarn.lock') }}
           lookup-only: true
       - uses: actions/setup-node@v4
         if: steps.node_modules.outputs.cache-hit != 'true'
@@ -98,10 +100,8 @@ jobs:
         with:
           path: |
             **/node_modules
-          key: runtime-and-compiler-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock', 'compiler/yarn.lock') }}
-          restore-keys: |
-            runtime-and-compiler-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-
-            runtime-and-compiler-node_modules-v6-
+          key: runtime-and-compiler-node_modules-v7-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock', 'compiler/yarn.lock') }}
+          # Don't use restore-keys here. Otherwise the cache grows indefinitely.
       - run: yarn install --frozen-lockfile
         if: steps.node_modules.outputs.cache-hit != 'true'
       - run: yarn --cwd compiler install --frozen-lockfile
@@ -112,7 +112,7 @@ jobs:
         with:
           path: |
             **/node_modules
-          key: runtime-and-compiler-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock', 'compiler/yarn.lock') }}
+          key: runtime-and-compiler-node_modules-v7-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock', 'compiler/yarn.lock') }}
 
   # ----- FLOW -----
   discover_flow_inline_configs:
@@ -154,10 +154,8 @@ jobs:
         with:
           path: |
             **/node_modules
-          key: runtime-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock') }}
-          restore-keys: |
-            runtime-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-
-            runtime-node_modules-v6-
+          key: runtime-node_modules-v7-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock') }}
+          # Don't use restore-keys here. Otherwise the cache grows indefinitely.
       - name: Ensure clean build directory
         run: rm -rf build
       - run: yarn install --frozen-lockfile
@@ -184,17 +182,15 @@ jobs:
         with:
           path: |
             **/node_modules
-          key: runtime-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock') }}
-          restore-keys: |
-            runtime-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-
-            runtime-node_modules-v6-
+          key: runtime-node_modules-v7-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock') }}
+          # Don't use restore-keys here. Otherwise the cache grows indefinitely.
       - name: Ensure clean build directory
         run: rm -rf build
       - run: yarn install --frozen-lockfile
         if: steps.node_modules.outputs.cache-hit != 'true'
       - run: |
           yarn generate-inline-fizz-runtime
-          git diff --quiet || (echo "There was a change to the Fizz runtime. Run `yarn generate-inline-fizz-runtime` and check in the result." && false)
+          git diff --exit-code || (echo "There was a change to the Fizz runtime. Run \`yarn generate-inline-fizz-runtime\` and check in the result." && false)
 
   # ----- FEATURE FLAGS -----
   flags:
@@ -216,7 +212,7 @@ jobs:
         with:
           path: |
             **/node_modules
-          key: runtime-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock') }}
+          key: runtime-node_modules-v7-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock') }}
       - name: Ensure clean build directory
         run: rm -rf build
       - run: yarn install --frozen-lockfile
@@ -274,16 +270,15 @@ jobs:
         with:
           path: |
             **/node_modules
-          key: runtime-and-compiler-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock', 'compiler/yarn.lock') }}
-          restore-keys: |
-            runtime-and-compiler-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-
-            runtime-and-compiler-node_modules-v6-
+          key: runtime-and-compiler-node_modules-v7-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock', 'compiler/yarn.lock') }}
+          # Don't use restore-keys here. Otherwise the cache grows indefinitely.
       - name: Ensure clean build directory
         run: rm -rf build
       - run: yarn install --frozen-lockfile
         if: steps.node_modules.outputs.cache-hit != 'true'
       - run: yarn --cwd compiler install --frozen-lockfile
         if: steps.node_modules.outputs.cache-hit != 'true'
+      - run: node --version
       - run: yarn test ${{ matrix.params }} --ci --shard=${{ matrix.shard }}
 
   # Hardcoded to improve parallelism
@@ -306,7 +301,7 @@ jobs:
         with:
           path: |
             **/node_modules
-          key: runtime-and-compiler-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock', 'compiler/yarn.lock') }}
+          key: runtime-and-compiler-node_modules-v7-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock', 'compiler/yarn.lock') }}
       - name: Install runtime dependencies
         run: yarn install --frozen-lockfile
         if: steps.node_modules.outputs.cache-hit != 'true'
@@ -316,7 +311,7 @@ jobs:
         if: steps.node_modules.outputs.cache-hit != 'true'
       - run: ./scripts/react-compiler/build-compiler.sh && ./scripts/react-compiler/link-compiler.sh
       - run: yarn workspace eslint-plugin-react-hooks test
-        
+
   # ----- BUILD -----
   build_and_lint:
     name: yarn build and lint
@@ -349,10 +344,8 @@ jobs:
         with:
           path: |
             **/node_modules
-          key: runtime-and-compiler-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock', 'compiler/yarn.lock') }}
-          restore-keys: |
-            runtime-and-compiler-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-
-            runtime-and-compiler-node_modules-v6-
+          key: runtime-and-compiler-node_modules-v7-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock', 'compiler/yarn.lock') }}
+          # Don't use restore-keys here. Otherwise the cache grows indefinitely.
       - name: Ensure clean build directory
         run: rm -rf build
       - run: yarn install --frozen-lockfile
@@ -390,9 +383,6 @@ jobs:
           -r=experimental --env=development,
           -r=experimental --env=production,
 
-          # Dev Tools
-          --project=devtools -r=experimental,
-
           # TODO: Update test config to support www build tests
           # - "-r=www-classic --env=development --variant=false"
           # - "-r=www-classic --env=production --variant=false"
@@ -440,10 +430,8 @@ jobs:
         with:
           path: |
             **/node_modules
-          key: runtime-and-compiler-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock', 'compiler/yarn.lock') }}
-          restore-keys: |
-            runtime-and-compiler-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-
-            runtime-and-compiler-node_modules-v6-
+          key: runtime-and-compiler-node_modules-v7-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock', 'compiler/yarn.lock') }}
+          # Don't use restore-keys here. Otherwise the cache grows indefinitely.
       - name: Ensure clean build directory
         run: rm -rf build
       - run: yarn install --frozen-lockfile
@@ -458,8 +446,54 @@ jobs:
           merge-multiple: true
       - name: Display structure of build
         run: ls -R build
+      - run: node --version
       - run: yarn test --build ${{ matrix.test_params }} --shard=${{ matrix.shard }} --ci
 
+  test_build_devtools:
+    name: yarn test-build (devtools)
+    needs: [build_and_lint, runtime_node_modules_cache]
+    strategy:
+      fail-fast: false
+      matrix:
+        shard:
+          - 1/5
+          - 2/5
+          - 3/5
+          - 4/5
+          - 5/5
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          ref: ${{ github.event.inputs.commit_sha != '' && github.event.inputs.commit_sha || github.event.pull_request.head.sha || github.sha }}
+      - uses: actions/setup-node@v4
+        with:
+          node-version-file: '.nvmrc'
+          cache: yarn
+          cache-dependency-path: yarn.lock
+      - name: Restore cached node_modules
+        uses: actions/cache/restore@v4
+        id: node_modules
+        with:
+          path: |
+            **/node_modules
+          key: runtime-node_modules-v7-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock') }}
+          # Don't use restore-keys here. Otherwise the cache grows indefinitely.
+      - name: Ensure clean build directory
+        run: rm -rf build
+      - run: yarn install --frozen-lockfile
+        if: steps.node_modules.outputs.cache-hit != 'true'
+      - name: Restore archived build
+        uses: actions/download-artifact@v4
+        with:
+          pattern: _build_*
+          path: build
+          merge-multiple: true
+      - name: Display structure of build
+        run: ls -R build
+      - run: node --version
+      - run: yarn test --build --project=devtools -r=experimental --shard=${{ matrix.shard }} --ci
+
   process_artifacts_combined:
     name: Process artifacts combined
     needs: [build_and_lint, runtime_node_modules_cache]
@@ -483,10 +517,8 @@ jobs:
         with:
           path: |
             **/node_modules
-          key: runtime-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock') }}
-          restore-keys: |
-            runtime-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-
-            runtime-node_modules-v6-
+          key: runtime-node_modules-v7-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock') }}
+          # Don't use restore-keys here. Otherwise the cache grows indefinitely.
       - name: Ensure clean build directory
         run: rm -rf build
       - run: yarn install --frozen-lockfile
@@ -548,10 +580,8 @@ jobs:
         with:
           path: |
             **/node_modules
-          key: runtime-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock') }}
-          restore-keys: |
-            runtime-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-
-            runtime-node_modules-v6-
+          key: runtime-node_modules-v7-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock') }}
+          # Don't use restore-keys here. Otherwise the cache grows indefinitely.
       - name: Ensure clean build directory
         run: rm -rf build
       - run: yarn install --frozen-lockfile
@@ -567,7 +597,7 @@ jobs:
       - name: Search build artifacts for unminified errors
         run: |
           yarn extract-errors
-          git diff --quiet || (echo "Found unminified errors. Either update the error codes map or disable error minification for the affected build, if appropriate." && false)
+          git diff --exit-code || (echo "Found unminified errors. Either update the error codes map or disable error minification for the affected build, if appropriate." && false)
 
   check_release_dependencies:
     name: Check release dependencies
@@ -588,10 +618,8 @@ jobs:
         with:
           path: |
             **/node_modules
-          key: runtime-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock') }}
-          restore-keys: |
-            runtime-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-
-            runtime-node_modules-v6-
+          key: runtime-node_modules-v7-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock') }}
+          # Don't use restore-keys here. Otherwise the cache grows indefinitely.
       - name: Ensure clean build directory
         run: rm -rf build
       - run: yarn install --frozen-lockfile
@@ -740,10 +768,8 @@ jobs:
         with:
           path: |
             **/node_modules
-          key: runtime-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock') }}
-          restore-keys: |
-            runtime-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-
-            runtime-node_modules-v6-
+          key: runtime-node_modules-v7-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock') }}
+          # Don't use restore-keys here. Otherwise the cache grows indefinitely.
       - name: Ensure clean build directory
         run: rm -rf build
       - run: yarn install --frozen-lockfile
@@ -766,6 +792,11 @@ jobs:
           name: react-devtools-${{ matrix.browser }}-extension
           path: build/devtools/${{ matrix.browser }}-extension.zip
           if-no-files-found: error
+      - name: Archive ${{ matrix.browser }} metadata
+        uses: actions/upload-artifact@v4
+        with:
+          name: react-devtools-${{ matrix.browser }}-metadata
+          path: build/devtools/webpack-stats.*.json
 
   merge_devtools_artifacts:
     name: Merge DevTools artifacts
@@ -776,7 +807,7 @@ jobs:
         uses: actions/upload-artifact/merge@v4
         with:
           name: react-devtools
-          pattern: react-devtools-*-extension
+          pattern: react-devtools-*
 
   run_devtools_e2e_tests:
     name: Run DevTools e2e tests
@@ -797,10 +828,8 @@ jobs:
         with:
           path: |
             **/node_modules
-          key: runtime-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock') }}
-          restore-keys: |
-            runtime-node_modules-v6-${{ runner.arch }}-${{ runner.os }}-
-            runtime-node_modules-v6-
+          key: runtime-node_modules-v7-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock') }}
+          # Don't use restore-keys here. Otherwise the cache grows indefinitely.
       - name: Ensure clean build directory
         run: rm -rf build
       - run: yarn install --frozen-lockfile
@@ -811,12 +840,27 @@ jobs:
           pattern: _build_*
           path: build
           merge-multiple: true
-      - run: |
-          npx playwright install
-          sudo npx playwright install-deps
+      - name: Check Playwright version
+        id: playwright_version
+        run: echo "playwright_version=$(npm ls @playwright/test | grep @playwright | sed 's/.*@//' | head -1)" >> "$GITHUB_OUTPUT"
+      - name: Cache Playwright Browsers for version ${{ steps.playwright_version.outputs.playwright_version }}
+        id: cache_playwright_browsers
+        uses: actions/cache@v4
+        with:
+          path: ~/.cache/ms-playwright
+          key: playwright-browsers-v6-${{ runner.arch }}-${{ runner.os }}-${{ steps.playwright_version.outputs.playwright_version }}
+      - name: Playwright install deps
+        if: steps.cache_playwright_browsers.outputs.cache-hit != 'true'
+        run: npx playwright install --with-deps chromium
       - run: ./scripts/ci/run_devtools_e2e_tests.js
         env:
           RELEASE_CHANNEL: experimental
+      - name: Archive Playwright report
+        uses: actions/upload-artifact@v4
+        with:
+          name: devtools-playwright-artifacts
+          path: tmp/playwright-artifacts
+          if-no-files-found: warn
 
   # ----- SIZEBOT -----
   sizebot:

.github/workflows/runtime_commit_artifacts.yml

@@ -162,10 +162,13 @@ jobs:
           mv build/facebook-react-native/react-is/cjs/ $BASE_FOLDER/RKJSModules/vendor/react/react-is/
           mv build/facebook-react-native/react-test-renderer/cjs/ $BASE_FOLDER/RKJSModules/vendor/react/react-test-renderer/
 
-          # Delete OSS renderer. OSS renderer is synced through internal script.
+          # Delete the OSS renderers, these are sync'd to RN separately.
           RENDERER_FOLDER=$BASE_FOLDER/react-native-github/Libraries/Renderer/implementations/
           rm $RENDERER_FOLDER/ReactFabric-{dev,prod,profiling}.js
-          rm $RENDERER_FOLDER/ReactNativeRenderer-{dev,prod,profiling}.js
+          
+          # Delete the legacy renderer shim, this is not sync'd and will get deleted in the future.
+          SHIM_FOLDER=$BASE_FOLDER/react-native-github/Libraries/Renderer/shims/
+          rm $SHIM_FOLDER/ReactNative.js
 
           # Copy eslint-plugin-react-hooks
           # NOTE: This is different from www, here we include the full package

.github/workflows/runtime_discord_notify.yml

@@ -4,8 +4,10 @@ on:
   pull_request_target:
     types: [opened, ready_for_review]
     paths-ignore:
+      - packages/react-devtools**
       - compiler/**
       - .github/workflows/compiler_**.yml
+      - .github/workflows/devtools**.yml
 
 permissions: {}
 

.github/workflows/runtime_eslint_plugin_e2e.yml

@@ -29,6 +29,7 @@ jobs:
           - "7"
           - "8"
           - "9"
+          - "10"
     steps:
       - uses: actions/checkout@v4
         with:

.github/workflows/runtime_prereleases.yml

@@ -82,7 +82,6 @@ jobs:
         run: |
           scripts/release/publish.js \
             --ci \
-            --skipTests \
             --tags=${{ inputs.dist_tag }} \
             --onlyPackages=${{ inputs.only_packages }} ${{ (inputs.dry && '') || '\'}}
             ${{ inputs.dry && '--dry' || '' }}
@@ -91,11 +90,10 @@ jobs:
         run: |
           scripts/release/publish.js \
             --ci \
-            --skipTests \
             --tags=${{ inputs.dist_tag }} \
             --skipPackages=${{ inputs.skip_packages }} ${{ (inputs.dry && '') || '\'}}
             ${{ inputs.dry && '--dry' || '' }}
-      - if: '${{ !(inputs.skip_packages && inputs.only_packages) }}'
+      - if: '${{ !inputs.skip_packages && !inputs.only_packages }}'
         name: 'Publish all packages'
         run: |
           scripts/release/publish.js \

.github/workflows/shared_close_direct_sync_branch_prs.yml

@@ -18,6 +18,7 @@ jobs:
     permissions:
       # Used to create a review and close PRs
       pull-requests: write
+      contents: write
     steps:
       - name: Close PR
         uses: actions/github-script@v7

.gitignore

@@ -23,6 +23,9 @@ chrome-user-data
 .vscode
 *.swp
 *.swo
+/tmp
+/.worktrees
+.claude/*.local.*
 
 packages/react-devtools-core/dist
 packages/react-devtools-extensions/chrome/build

CHANGELOG.md

@@ -1,3 +1,88 @@
+## 19.2.1 (Dec 3, 2025)
+
+### React Server Components
+
+- Bring React Server Component fixes to Server Actions (@sebmarkbage [#35277](https://github.com/facebook/react/pull/35277))
+
+## 19.2.0 (October 1st, 2025)
+
+Below is a list of all new features, APIs, and bug fixes.
+
+Read the [React 19.2 release post](https://react.dev/blog/2025/10/01/react-19-2) for more information.
+
+### New React Features
+
+- [`<Activity>`](https://react.dev/reference/react/Activity): A new API to hide and restore the UI and internal state of its children.
+- [`useEffectEvent`](https://react.dev/reference/react/useEffectEvent) is a React Hook that lets you extract non-reactive logic into an [Effect Event](https://react.dev/learn/separating-events-from-effects#declaring-an-effect-event).
+- [`cacheSignal`](https://react.dev/reference/react/cacheSignal) (for RSCs) lets your know when the `cache()` lifetime is over.
+- [React Performance tracks](https://react.dev/reference/dev-tools/react-performance-tracks) appear on the Performance panel’s timeline in your browser developer tools
+
+### New React DOM Features
+
+- Added resume APIs for partial pre-rendering with Web Streams:
+  - [`resume`](https://react.dev/reference/react-dom/server/resume): to resume a prerender to a stream.
+  - [`resumeAndPrerender`](https://react.dev/reference/react-dom/static/resumeAndPrerender): to resume a prerender to HTML.
+- Added resume APIs for partial pre-rendering with Node Streams:
+  - [`resumeToPipeableStream`](https://react.dev/reference/react-dom/server/resumeToPipeableStream): to resume a prerender to a stream.
+  - [`resumeAndPrerenderToNodeStream`](https://react.dev/reference/react-dom/static/resumeAndPrerenderToNodeStream): to resume a prerender to HTML.
+- Updated [`prerender`](https://react.dev/reference/react-dom/static/prerender) APIs to return a `postponed` state that can be passed to the `resume` APIs.
+
+### Notable changes
+
+- React DOM now batches suspense boundary reveals, matching the behavior of client side rendering. This change is especially noticeable when animating the reveal of Suspense boundaries e.g. with the upcoming `<ViewTransition>` Component. React will batch as much reveals as possible before the first paint while trying to hit popular first-contentful paint metrics.
+- Add Node Web Streams (`prerender`, `renderToReadableStream`) to server-side-rendering APIs for Node.js
+- Use underscore instead of `:` IDs generated by useId
+
+### All Changes
+
+#### React
+
+- `<Activity />` was developed over many years, starting before `ClassComponent.setState` (@acdlite @sebmarkbage and many others)
+- Stringify context as "SomeContext" instead of "SomeContext.Provider" (@kassens [#33507](https://github.com/facebook/react/pull/33507))
+- Include stack of cause of React instrumentation errors with `%o` placeholder (@eps1lon [#34198](https://github.com/facebook/react/pull/34198))
+- Fix infinite `useDeferredValue` loop in popstate event (@acdlite [#32821](https://github.com/facebook/react/pull/32821))
+- Fix a bug when an initial value was passed to `useDeferredValue` (@acdlite [#34376](https://github.com/facebook/react/pull/34376))
+- Fix a crash when submitting forms with Client Actions (@sebmarkbage [#33055](https://github.com/facebook/react/pull/33055))
+- Hide/unhide the content of dehydrated suspense boundaries if they resuspend (@sebmarkbage [#32900](https://github.com/facebook/react/pull/32900))
+- Avoid stack overflow on wide trees during Hot Reload (@sophiebits [#34145](https://github.com/facebook/react/pull/34145))
+- Improve Owner and Component stacks in various places (@sebmarkbage, @eps1lon: [#33629](https://github.com/facebook/react/pull/33629), [#33724](https://github.com/facebook/react/pull/33724), [#32735](https://github.com/facebook/react/pull/32735), [#33723](https://github.com/facebook/react/pull/33723))
+- Add `cacheSignal` (@sebmarkbage [#33557](https://github.com/facebook/react/pull/33557))
+
+#### React DOM
+
+- Block on Suspensey Fonts during reveal of server-side-rendered content (@sebmarkbage [#33342](https://github.com/facebook/react/pull/33342))
+- Use underscore instead of `:` for IDs generated by `useId` (@sebmarkbage, @eps1lon: [#32001](https://github.com/facebook/react/pull/32001), [https://github.com/facebook/react/pull/33342](https://github.com/facebook/react/pull/33342)[#33099](https://github.com/facebook/react/pull/33099), [#33422](https://github.com/facebook/react/pull/33422))
+- Stop warning when ARIA 1.3 attributes are used (@Abdul-Omira [#34264](https://github.com/facebook/react/pull/34264))
+- Allow `nonce` to be used on hoistable styles (@Andarist [#32461](https://github.com/facebook/react/pull/32461))
+- Warn for using a React owned node as a Container if it also has text content (@sebmarkbage [#32774](https://github.com/facebook/react/pull/32774))
+- s/HTML/text for for error messages if text hydration mismatches (@rickhanlonii [#32763](https://github.com/facebook/react/pull/32763))
+- Fix a bug with `React.use` inside `React.lazy`\-ed Component (@hi-ogawa [#33941](https://github.com/facebook/react/pull/33941))
+- Enable the `progressiveChunkSize` option for server-side-rendering APIs (@sebmarkbage [#33027](https://github.com/facebook/react/pull/33027))
+- Fix a bug with deeply nested Suspense inside Suspense fallback when server-side-rendering (@gnoff [#33467](https://github.com/facebook/react/pull/33467))
+- Avoid hanging when suspending after aborting while rendering (@gnoff [#34192](https://github.com/facebook/react/pull/34192))
+- Add Node Web Streams to server-side-rendering APIs for Node.js (@sebmarkbage [#33475](https://github.com/facebook/react/pull/33475))
+
+#### React Server Components
+
+- Preload `<img>` and `<link>` using hints before they're rendered (@sebmarkbage [#34604](https://github.com/facebook/react/pull/34604))
+- Log error if production elements are rendered during development (@eps1lon [#34189](https://github.com/facebook/react/pull/34189))
+- Fix a bug when returning a Temporary reference (e.g. a Client Reference) from Server Functions (@sebmarkbage [#34084](https://github.com/facebook/react/pull/34084), @denk0403 [#33761](https://github.com/facebook/react/pull/33761))
+- Pass line/column to `filterStackFrame` (@eps1lon [#33707](https://github.com/facebook/react/pull/33707))
+- Support Async Modules in Turbopack Server References (@lubieowoce [#34531](https://github.com/facebook/react/pull/34531))
+- Add support for .mjs file extension in Webpack (@jennyscript [#33028](https://github.com/facebook/react/pull/33028))
+- Fix a wrong missing key warning (@unstubbable [#34350](https://github.com/facebook/react/pull/34350))
+- Make console log resolve in predictable order (@sebmarkbage [#33665](https://github.com/facebook/react/pull/33665))
+
+#### React Reconciler
+
+- [createContainer](https://github.com/facebook/react/blob/v19.2.0/packages/react-reconciler/src/ReactFiberReconciler.js#L255-L261) and [createHydrationContainer](https://github.com/facebook/react/blob/v19.2.0/packages/react-reconciler/src/ReactFiberReconciler.js#L305-L312) had their parameter order adjusted after `on*` handlers to account for upcoming experimental APIs
+
+## 19.1.2 (Dec 3, 2025)
+
+### React Server Components
+
+- Bring React Server Component fixes to Server Actions (@sebmarkbage [#35277](https://github.com/facebook/react/pull/35277))
+
 ## 19.1.1 (July 28, 2025)
 
 ### React
@@ -50,6 +135,12 @@ An Owner Stack is a string representing the components that are directly respons
 * Exposed `registerServerReference` in client builds to handle server references in different environments. [#32534](https://github.com/facebook/react/pull/32534)
 * Added react-server-dom-parcel package which integrates Server Components with the [Parcel bundler](https://parceljs.org/) [#31725](https://github.com/facebook/react/pull/31725), [#32132](https://github.com/facebook/react/pull/32132), [#31799](https://github.com/facebook/react/pull/31799), [#32294](https://github.com/facebook/react/pull/32294), [#31741](https://github.com/facebook/react/pull/31741)
 
+## 19.0.1 (Dec 3, 2025)
+
+### React Server Components
+
+- Bring React Server Component fixes to Server Actions (@sebmarkbage [#35277](https://github.com/facebook/react/pull/35277))
+
 ## 19.0.0 (December 5, 2024)
 
 Below is a list of all new features, APIs, deprecations, and breaking changes. Read [React 19 release post](https://react.dev/blog/2024/04/25/react-19) and [React 19 upgrade guide](https://react.dev/blog/2024/04/25/react-19-upgrade-guide) for more information.

CLAUDE.md

@@ -0,0 +1,8 @@
+# React
+
+React is a JavaScript library for building user interfaces.
+
+## Monorepo Overview
+
+- **React**: All files outside `/compiler/`
+- **React Compiler**: `/compiler/` directory (has its own instructions)

MAINTAINERS

@@ -1,5 +1,6 @@
 acdlite
 eps1lon
+EugeneChoi4
 gaearon
 gnoff
 unstubbable

ReactVersions.js

@@ -7,18 +7,18 @@
 //
 // The @latest channel uses the version as-is, e.g.:
 //
-//   19.1.0
+//   19.3.0
 //
 // The @canary channel appends additional information, with the scheme
 // <version>-<label>-<commit_sha>, e.g.:
 //
-//   19.1.0-canary-a1c2d3e4
+//   19.3.0-canary-a1c2d3e4
 //
 // The @experimental channel doesn't include a version, only a date and a sha, e.g.:
 //
 //   0.0.0-experimental-241c4467e-20200129
 
-const ReactVersion = '19.2.0';
+const ReactVersion = '19.3.0';
 
 // The label used by the @canary channel. Represents the upcoming release's
 // stability. Most of the time, this will be "canary", but we may temporarily
@@ -33,8 +33,8 @@ const canaryChannelLabel = 'canary';
 const rcNumber = 0;
 
 const stablePackages = {
-  'eslint-plugin-react-hooks': '6.1.0',
-  'jest-react': '0.17.0',
+  'eslint-plugin-react-hooks': '7.1.0',
+  'jest-react': '0.18.0',
   react: ReactVersion,
   'react-art': ReactVersion,
   'react-dom': ReactVersion,
@@ -42,12 +42,12 @@ const stablePackages = {
   'react-server-dom-turbopack': ReactVersion,
   'react-server-dom-parcel': ReactVersion,
   'react-is': ReactVersion,
-  'react-reconciler': '0.33.0',
-  'react-refresh': '0.18.0',
+  'react-reconciler': '0.34.0',
+  'react-refresh': '0.19.0',
   'react-test-renderer': ReactVersion,
-  'use-subscription': '1.12.0',
-  'use-sync-external-store': '1.6.0',
-  scheduler: '0.27.0',
+  'use-subscription': '1.13.0',
+  'use-sync-external-store': '1.7.0',
+  scheduler: '0.28.0',
 };
 
 // These packages do not exist in the @canary or @latest channel, only

babel.config-ts.js

@@ -8,6 +8,7 @@ module.exports = {
     '@babel/plugin-syntax-jsx',
     '@babel/plugin-transform-flow-strip-types',
     ['@babel/plugin-transform-class-properties', {loose: true}],
+    ['@babel/plugin-transform-private-methods', {loose: true}],
     '@babel/plugin-transform-classes',
   ],
   presets: [

compiler/.claude/agents/investigate-error.md

@@ -0,0 +1,113 @@
+---
+name: investigate-error
+description: Investigates React compiler errors to determine the root cause and identify potential mitigation(s). Use this agent when the user asks to 'investigate a bug', 'debug why this fixture errors', 'understand why the compiler is failing', 'find the root cause of a compiler issue', or when they provide a snippet of code and ask to debug. Use automatically when encountering a failing test case, in order to understand the root cause.
+model: opus
+color: pink
+---
+
+You are an expert React Compiler debugging specialist with deep knowledge of compiler internals, intermediate representations, and optimization passes. Your mission is to systematically investigate compiler bugs to identify root causes and provide actionable information for fixes.
+
+## Your Investigation Process
+
+### Step 1: Create Test Fixture
+Create a new fixture file at `packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/<fixture-name>.js` containing the problematic code. Use a descriptive name that reflects the issue (e.g., `bug-optional-chain-in-effect.js`).
+
+### Step 2: Run Debug Compilation
+Execute `yarn snap -d -p <fixture-name>` to compile the fixture with full debug output. This shows the state of the program after each compilation pass. You can also use `yarn snap compile -d <path-to-fixture>`.
+
+### Step 3: Analyze Compilation Results
+
+### Step 3a: If the fixture compiles successfully
+- Compare the output against the user's expected behavior
+- Review each compilation pass output from the `-d` flag
+- Identify the first pass where the output diverges from expected behavior
+- Proceed to binary search simplification
+
+### Step 3b: If the fixture errors
+Execute `yarn snap minimize --update <path-to-fixture>` to remove non-critical aspects of the failing test case. This **updates the fixture in place**.
+
+Re-read the fixture file to see the latest, minimal reproduction of the error.
+
+### Step 4: Iteratively adjust the fixture until it stops erroring
+After the previous step the fixture will have all extraneous aspects removed. Try to make further edits to determine the specific feature that is causing the error.
+
+Ideas:
+* Replace immediately-invoked function expressions with labeled blocks
+* Remove statements
+* Simplify calls (remove arguments, replace the call with its lone argument)
+* Simplify control flow statements by picking a single branch. Try using a labeled block with just the selected block
+* Replace optional member/call expressions with non-optional versions
+* Remove items in array/object expressions
+* Remove properties from member expressions
+
+Try to make the minimal possible edit to get the fixture stop erroring.
+
+### Step 5: Compare Debug Outputs
+With both minimal versions (failing and non-failing):
+- Run `yarn snap -d -p <fixture-name>` on both
+- Compare the debug output pass-by-pass
+- Identify the exact pass where behavior diverges
+- Note specific differences in HIR, effects, or generated code
+
+### Step 6: Investigate Compiler Logic
+- Read the documentation for the problematic pass in `packages/babel-plugin-react-compiler/docs/passes/`
+- Examine the pass implementation in `packages/babel-plugin-react-compiler/src/`
+- Key directories to investigate:
+  - `src/HIR/` - IR definitions and utilities
+  - `src/Inference/` - Effect inference (aliasing, mutation)
+  - `src/Validation/` - Validation passes
+  - `src/Optimization/` - Optimization passes
+  - `src/ReactiveScopes/` - Reactive scope analysis
+- Identify specific code locations that may be handling the pattern incorrectly
+
+## Output Format
+
+Provide a structured investigation report:
+
+```
+## Investigation Summary
+
+### Bug Description
+[Brief description of the issue]
+
+### Minimal Failing Fixture
+```javascript
+// packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/<name>.js
+[minimal code that reproduces the error]
+```
+
+### Minimal Non-Failing Fixture
+```javascript
+// The simplest change that makes it work
+[code that compiles correctly]
+```
+
+### Problematic Compiler Pass
+[Name of the pass where the issue occurs]
+
+### Root Cause Analysis
+[Explanation of what the compiler is doing wrong]
+
+### Suspect Code Locations
+- `packages/babel-plugin-react-compiler/src/<path>:<line>:<column>` - [description of what may be incorrect]
+- [additional locations if applicable]
+
+### Suggested Fix Direction
+[Brief suggestion of how the bug might be fixed]
+```
+
+## Key Debugging Tips
+
+1. The debug output (`-d` flag) shows the program state after each pass - use this to pinpoint where things go wrong
+2. Look for `@aliasingEffects=` on FunctionExpressions to understand data flow
+3. Check for `Impure`, `Render`, `Capture` effects on instructions
+4. The pass ordering in `Pipeline.ts` shows when effects are populated vs validated
+5. Todo errors indicate unsupported but known patterns; Invariant errors indicate unexpected states
+
+## Important Reminders
+
+- Always create the fixture file before running tests
+- Use descriptive fixture names that indicate the bug being investigated
+- Keep both failing and non-failing minimal versions for your report
+- Provide specific file:line:column references when identifying suspect code
+- Read the relevant pass documentation before making conclusions about the cause

compiler/.claude/settings.json

@@ -0,0 +1,18 @@
+{
+  "permissions": {
+    "allow": [
+      "Bash(yarn snap:*)",
+      "Bash(yarn snap:build)",
+      "Bash(node scripts/enable-feature-flag.js:*)"
+    ],
+    "deny": [
+      "Skill(extract-errors)",
+      "Skill(feature-flags)",
+      "Skill(fix)",
+      "Skill(flags)",
+      "Skill(flow)",
+      "Skill(test)",
+      "Skill(verify)"
+    ]
+  }
+}

compiler/.gitignore

@@ -8,7 +8,9 @@ dist
 .vscode
 !packages/playground/.vscode
 testfilter.txt
+.claude/settings.local.json
 
 # forgive
 *.vsix
 .vscode-test
+

compiler/CLAUDE.md

@@ -0,0 +1,248 @@
+# React Compiler Knowledge Base
+
+This document contains knowledge about the React Compiler gathered during development sessions. It serves as a reference for understanding the codebase architecture and key concepts.
+
+## Project Structure
+
+When modifying the compiler, you MUST read the documentation about that pass in `compiler/packages/babel-plugin-react-compiler/docs/passes/` to learn more about the role of that pass within the compiler.
+
+- `packages/babel-plugin-react-compiler/` - Main compiler package
+  - `src/HIR/` - High-level Intermediate Representation types and utilities
+  - `src/Inference/` - Effect inference passes (aliasing, mutation, etc.)
+  - `src/Validation/` - Validation passes that check for errors
+  - `src/Entrypoint/Pipeline.ts` - Main compilation pipeline with pass ordering
+  - `src/__tests__/fixtures/compiler/` - Test fixtures
+    - `error.todo-*.js` - Unsupported feature, correctly throws Todo error (graceful bailout)
+    - `error.bug-*.js` - Known bug, throws wrong error type or incorrect behavior
+    - `*.expect.md` - Expected output for each fixture
+
+## Running Tests
+
+```bash
+# Run all tests
+yarn snap
+
+# Run tests matching a pattern
+# Example: yarn snap -p 'error.*'
+yarn snap -p <pattern>
+
+# Run a single fixture in debug mode. Use the path relative to the __tests__/fixtures/compiler directory
+# For each step of compilation, outputs the step name and state of the compiled program
+# Example: yarn snap -p simple.js -d
+yarn snap -p <file-basename> -d
+
+# Update fixture outputs (also works with -p)
+yarn snap -u
+```
+
+## Compiling Arbitrary Files
+
+Use `yarn snap compile` to compile any file (not just fixtures) with the React Compiler:
+
+```bash
+# Compile a file and see the output
+yarn snap compile <path>
+
+# Compile with debug logging to see the state after each compiler pass
+# This is an alternative to `yarn snap -d -p <pattern>` when you don't have a fixture file yet
+yarn snap compile --debug <path>
+```
+
+## Minimizing Test Cases
+
+Use `yarn snap minimize` to automatically reduce a failing test case to its minimal reproduction:
+
+```bash
+# Minimize a file that causes a compiler error
+yarn snap minimize <path>
+
+# Minimize and update the file in-place with the minimized version
+yarn snap minimize --update <path>
+```
+
+## Version Control
+
+This repository uses Sapling (`sl`) for version control. Sapling is similar to Mercurial: there is not staging area, but new/deleted files must be explicitlyu added/removed.
+
+```bash
+# Check status
+sl status
+
+# Add new files, remove deleted files
+sl addremove
+
+# Commit all changes
+sl commit -m "Your commit message"
+
+# Commit with multi-line message using heredoc
+sl commit -m "$(cat <<'EOF'
+Summary line
+
+Detailed description here
+EOF
+)"
+```
+
+## Key Concepts
+
+### HIR (High-level Intermediate Representation)
+
+The compiler converts source code to HIR for analysis. Key types in `src/HIR/HIR.ts`:
+
+- **HIRFunction** - A function being compiled
+  - `body.blocks` - Map of BasicBlocks
+  - `context` - Captured variables from outer scope
+  - `params` - Function parameters
+  - `returns` - The function's return place
+  - `aliasingEffects` - Effects that describe the function's behavior when called
+
+- **Instruction** - A single operation
+  - `lvalue` - The place being assigned to
+  - `value` - The instruction kind (CallExpression, FunctionExpression, LoadLocal, etc.)
+  - `effects` - Array of AliasingEffects for this instruction
+
+- **Terminal** - Block terminators (return, branch, etc.)
+  - `effects` - Array of AliasingEffects
+
+- **Place** - A reference to a value
+  - `identifier.id` - Unique IdentifierId
+
+- **Phi nodes** - Join points for values from different control flow paths
+  - Located at `block.phis`
+  - `phi.place` - The result place
+  - `phi.operands` - Map of predecessor block to source place
+
+### AliasingEffects System
+
+Effects describe data flow and operations. Defined in `src/Inference/AliasingEffects.ts`:
+
+**Data Flow Effects:**
+- `Impure` - Marks a place as containing an impure value (e.g., Date.now() result, ref.current)
+- `Capture a -> b` - Value from `a` is captured into `b` (mutable capture)
+- `Alias a -> b` - `b` aliases `a`
+- `ImmutableCapture a -> b` - Immutable capture (like Capture but read-only)
+- `Assign a -> b` - Direct assignment
+- `MaybeAlias a -> b` - Possible aliasing
+- `CreateFrom a -> b` - Created from source
+
+**Mutation Effects:**
+- `Mutate value` - Value is mutated
+- `MutateTransitive value` - Value and transitive captures are mutated
+- `MutateConditionally value` - May mutate
+- `MutateTransitiveConditionally value` - May mutate transitively
+
+**Other Effects:**
+- `Render place` - Place is used in render context (JSX props, component return)
+- `Freeze place` - Place is frozen (made immutable)
+- `Create place` - New value created
+- `CreateFunction` - Function expression created, includes `captures` array
+- `Apply` - Function application with receiver, function, args, and result
+
+### Hook Aliasing Signatures
+
+Located in `src/HIR/Globals.ts`, hooks can define custom aliasing signatures to control how data flows through them.
+
+**Structure:**
+```typescript
+aliasing: {
+  receiver: '@receiver',    // The hook function itself
+  params: ['@param0'],      // Named positional parameters
+  rest: '@rest',            // Rest parameters (or null)
+  returns: '@returns',      // Return value
+  temporaries: [],          // Temporary values during execution
+  effects: [                // Array of effects to apply when hook is called
+    {kind: 'Freeze', value: '@param0', reason: ValueReason.HookCaptured},
+    {kind: 'Assign', from: '@param0', into: '@returns'},
+  ],
+}
+```
+
+**Common patterns:**
+
+1. **RenderHookAliasing** (useState, useContext, useMemo, useCallback):
+   - Freezes arguments (`Freeze @rest`)
+   - Marks arguments as render-time (`Render @rest`)
+   - Creates frozen return value
+   - Aliases arguments to return
+
+2. **EffectHookAliasing** (useEffect, useLayoutEffect, useInsertionEffect):
+   - Freezes function and deps
+   - Creates internal effect object
+   - Captures function and deps into effect
+   - Returns undefined
+
+3. **Event handler hooks** (useEffectEvent):
+   - Freezes callback (`Freeze @fn`)
+   - Aliases input to return (`Assign @fn -> @returns`)
+   - NO Render effect (callback not called during render)
+
+**Example: useEffectEvent**
+```typescript
+const UseEffectEventHook = addHook(
+  DEFAULT_SHAPES,
+  {
+    positionalParams: [Effect.Freeze],  // Takes one positional param
+    restParam: null,
+    returnType: {kind: 'Function', ...},
+    calleeEffect: Effect.Read,
+    hookKind: 'useEffectEvent',
+    returnValueKind: ValueKind.Frozen,
+    aliasing: {
+      receiver: '@receiver',
+      params: ['@fn'],              // Name for the callback parameter
+      rest: null,
+      returns: '@returns',
+      temporaries: [],
+      effects: [
+        {kind: 'Freeze', value: '@fn', reason: ValueReason.HookCaptured},
+        {kind: 'Assign', from: '@fn', into: '@returns'},
+        // Note: NO Render effect - callback is not called during render
+      ],
+    },
+  },
+  BuiltInUseEffectEventId,
+);
+
+// Add as both names for compatibility
+['useEffectEvent', UseEffectEventHook],
+['experimental_useEffectEvent', UseEffectEventHook],
+```
+
+**Key insight:** If a hook is missing an `aliasing` config, it falls back to `DefaultNonmutatingHook` which includes a `Render` effect on all arguments. This can cause false positives for hooks like `useEffectEvent` whose callbacks are not called during render.
+
+## Feature Flags
+
+Feature flags are configured in `src/HIR/Environment.ts`, for example `enableJsxOutlining`. Test fixtures can override the active feature flags used for that fixture via a comment pragma on the first line of the fixture input, for example:
+
+```javascript
+// enableJsxOutlining @enableNameAnonymousFunctions:false
+
+...code...
+```
+
+Would enable the `enableJsxOutlining` feature and disable the `enableNameAnonymousFunctions` feature.
+
+## Debugging Tips
+
+1. Run `yarn snap -p <fixture>` to see full HIR output with effects
+2. Look for `@aliasingEffects=` on FunctionExpressions
+3. Look for `Impure`, `Render`, `Capture` effects on instructions
+4. Check the pass ordering in Pipeline.ts to understand when effects are populated vs validated
+
+## Error Handling for Unsupported Features
+
+When the compiler encounters an unsupported but known pattern, use `CompilerError.throwTodo()` instead of `CompilerError.invariant()`. Todo errors cause graceful bailouts in production; Invariant errors are hard failures indicating unexpected/invalid states.
+
+```typescript
+// Unsupported but expected pattern - graceful bailout
+CompilerError.throwTodo({
+  reason: `Support [description of unsupported feature]`,
+  loc: terminal.loc,
+});
+
+// Invariant is for truly unexpected/invalid states - hard failure
+CompilerError.invariant(false, {
+  reason: `Unexpected [thing]`,
+  loc: terminal.loc,
+});
+```

compiler/apps/playground/.gitignore

@@ -12,6 +12,7 @@
 # next.js
 /.next/
 /out/
+/next-env.d.ts
 
 # production
 /build

compiler/apps/playground/__tests__/e2e/__snapshots__/page.spec.ts/compilationMode-all-output.txt

@@ -1,5 +1,4 @@
-import { c as _c } from "react/compiler-runtime"; // 
-@compilationMode:"all"
+import { c as _c } from "react/compiler-runtime"; // @compilationMode:"all"
 function nonReactFn() {
   const $ = _c(1);
   let t0;

compiler/apps/playground/__tests__/e2e/__snapshots__/page.spec.ts/default-config.txt

@@ -0,0 +1,5 @@
+import type { PluginOptions } from 
+'babel-plugin-react-compiler/dist';
+({
+  //compilationMode: "all"
+} satisfies PluginOptions);

compiler/apps/playground/__tests__/e2e/page.spec.ts

@@ -5,8 +5,9 @@
  * LICENSE file in the root directory of this source tree.
  */
 
-import {expect, test} from '@playwright/test';
+import {expect, test, type Page} from '@playwright/test';
 import {encodeStore, type Store} from '../../lib/stores';
+import {defaultConfig} from '../../lib/defaultStore';
 import {format} from 'prettier';
 
 function isMonacoLoaded(): boolean {
@@ -20,6 +21,16 @@ function formatPrint(data: Array<string>): Promise<string> {
   return format(data.join(''), {parser: 'babel'});
 }
 
+async function expandConfigs(page: Page): Promise<void> {
+  const expandButton = page.locator('[title="Expand config editor"]');
+  await expandButton.click();
+  await page.waitForSelector('.monaco-editor-config', {state: 'visible'});
+}
+
+const TEST_SOURCE = `export default function TestComponent({ x }) {
+  return <Button>{x}</Button>;
+}`;
+
 const TEST_CASE_INPUTS = [
   {
     name: 'module-scope-use-memo',
@@ -121,10 +132,9 @@ test('editor should open successfully', async ({page}) => {
 
 test('editor should compile from hash successfully', async ({page}) => {
   const store: Store = {
-    source: `export default function TestComponent({ x }) {
-      return <Button>{x}</Button>;
-    }
-    `,
+    source: TEST_SOURCE,
+    config: defaultConfig,
+    showInternals: false,
   };
   const hash = encodeStore(store);
   await page.goto(`/#${hash}`, {waitUntil: 'networkidle'});
@@ -136,7 +146,7 @@ test('editor should compile from hash successfully', async ({page}) => {
     path: 'test-results/01-compiles-from-hash.png',
   });
   const text =
-    (await page.locator('.monaco-editor').nth(1).allInnerTexts()) ?? [];
+    (await page.locator('.monaco-editor-output').allInnerTexts()) ?? [];
   const output = await formatPrint(text);
 
   expect(output).not.toEqual('');
@@ -145,10 +155,9 @@ test('editor should compile from hash successfully', async ({page}) => {
 
 test('reset button works', async ({page}) => {
   const store: Store = {
-    source: `export default function TestComponent({ x }) {
-      return <Button>{x}</Button>;
-    }
-    `,
+    source: TEST_SOURCE,
+    config: defaultConfig,
+    showInternals: false,
   };
   const hash = encodeStore(store);
   await page.goto(`/#${hash}`, {waitUntil: 'networkidle'});
@@ -157,33 +166,170 @@ test('reset button works', async ({page}) => {
   // Reset button works
   page.on('dialog', dialog => dialog.accept());
   await page.getByRole('button', {name: 'Reset'}).click();
+  await expandConfigs(page);
+
   await page.screenshot({
     fullPage: true,
     path: 'test-results/02-reset-button-works.png',
   });
   const text =
-    (await page.locator('.monaco-editor').nth(1).allInnerTexts()) ?? [];
+    (await page.locator('.monaco-editor-output').allInnerTexts()) ?? [];
   const output = await formatPrint(text);
 
+  const configText =
+    (await page.locator('.monaco-editor-config').allInnerTexts()) ?? [];
+  const configOutput = configText.join('');
+
   expect(output).not.toEqual('');
   expect(output).toMatchSnapshot('02-default-output.txt');
+  expect(configOutput).not.toEqual('');
+  expect(configOutput).toMatchSnapshot('default-config.txt');
+});
+
+test('defaults load when only source is in Store', async ({page}) => {
+  // Test for backwards compatibility
+  const partial = {
+    source: TEST_SOURCE,
+  };
+  const hash = encodeStore(partial as Store);
+  await page.goto(`/#${hash}`, {waitUntil: 'networkidle'});
+  await page.waitForFunction(isMonacoLoaded);
+  await expandConfigs(page);
+
+  await page.screenshot({
+    fullPage: true,
+    path: 'test-results/03-missing-defaults.png',
+  });
+
+  // Config editor has default config
+  const configText =
+    (await page.locator('.monaco-editor-config').allInnerTexts()) ?? [];
+  const configOutput = configText.join('');
+
+  expect(configOutput).not.toEqual('');
+  expect(configOutput).toMatchSnapshot('default-config.txt');
+
+  const checkbox = page.locator('label.show-internals');
+  await expect(checkbox).not.toBeChecked();
+  const ssaTab = page.locator('text=SSA');
+  await expect(ssaTab).not.toBeVisible();
+});
+
+test('show internals button toggles correctly', async ({page}) => {
+  await page.goto(`/`, {waitUntil: 'networkidle'});
+  await page.waitForFunction(isMonacoLoaded);
+
+  // show internals should be off
+  const checkbox = page.locator('label.show-internals');
+  await checkbox.click();
+
+  await page.screenshot({
+    fullPage: true,
+    path: 'test-results/04-show-internals-on.png',
+  });
+
+  await expect(checkbox).toBeChecked();
+
+  const ssaTab = page.locator('text=SSA');
+  await expect(ssaTab).toBeVisible();
+});
+
+test('error is displayed when config has syntax error', async ({page}) => {
+  const store: Store = {
+    source: TEST_SOURCE,
+    config: `compilationMode: `,
+    showInternals: false,
+  };
+  const hash = encodeStore(store);
+  await page.goto(`/#${hash}`, {waitUntil: 'networkidle'});
+  await page.waitForFunction(isMonacoLoaded);
+  await expandConfigs(page);
+  await page.screenshot({
+    fullPage: true,
+    path: 'test-results/05-config-syntax-error.png',
+  });
+
+  const text =
+    (await page.locator('.monaco-editor-output').allInnerTexts()) ?? [];
+  const output = text.join('');
+
+  // Remove hidden chars
+  expect(output.replace(/\s+/g, ' ')).toContain('Invalid override format');
+});
+
+test('error is displayed when config has validation error', async ({page}) => {
+  const store: Store = {
+    source: TEST_SOURCE,
+    config: `import type { PluginOptions } from 'babel-plugin-react-compiler/dist';
+
+({
+  compilationMode: "123"
+} satisfies PluginOptions);`,
+    showInternals: false,
+  };
+  const hash = encodeStore(store);
+  await page.goto(`/#${hash}`, {waitUntil: 'networkidle'});
+  await page.waitForFunction(isMonacoLoaded);
+  await expandConfigs(page);
+  await page.screenshot({
+    fullPage: true,
+    path: 'test-results/06-config-validation-error.png',
+  });
+
+  const text =
+    (await page.locator('.monaco-editor-output').allInnerTexts()) ?? [];
+  const output = text.join('');
+
+  expect(output.replace(/\s+/g, ' ')).toContain('Unexpected compilationMode');
+});
+
+test('error is displayed when source has syntax error', async ({page}) => {
+  const syntaxErrorSource = `function TestComponent(props) {
+  const oops = props.
+  return (
+    <>{oops}</>
+  );
+}`;
+  const store: Store = {
+    source: syntaxErrorSource,
+    config: defaultConfig,
+    showInternals: false,
+  };
+  const hash = encodeStore(store);
+  await page.goto(`/#${hash}`);
+  await page.waitForFunction(isMonacoLoaded);
+  await expandConfigs(page);
+  await page.screenshot({
+    fullPage: true,
+    path: 'test-results/08-source-syntax-error.png',
+  });
+
+  const text =
+    (await page.locator('.monaco-editor-output').allInnerTexts()) ?? [];
+  const output = text.join('');
+
+  expect(output.replace(/\s+/g, ' ')).toContain(
+    'Expected identifier to be defined before being used',
+  );
 });
 
 TEST_CASE_INPUTS.forEach((t, idx) =>
   test(`playground compiles: ${t.name}`, async ({page}) => {
     const store: Store = {
       source: t.input,
+      config: defaultConfig,
+      showInternals: false,
     };
     const hash = encodeStore(store);
     await page.goto(`/#${hash}`, {waitUntil: 'networkidle'});
     await page.waitForFunction(isMonacoLoaded);
     await page.screenshot({
       fullPage: true,
-      path: `test-results/03-0${idx}-${t.name}.png`,
+      path: `test-results/08-0${idx}-${t.name}.png`,
     });
 
     const text =
-      (await page.locator('.monaco-editor').nth(1).allInnerTexts()) ?? [];
+      (await page.locator('.monaco-editor-output').allInnerTexts()) ?? [];
     let output: string;
     if (t.noFormat) {
       output = text.join('');

compiler/apps/playground/app/index.tsx

@@ -1,56 +0,0 @@
-/**
- * Copyright (c) Meta Platforms, Inc. and affiliates.
- *
- * This source code is licensed under the MIT license found in the
- * LICENSE file in the root directory of this source tree.
- */
-
-import type {NextPage} from 'next';
-import Head from 'next/head';
-import {SnackbarProvider} from 'notistack';
-import {Editor, Header, StoreProvider} from '../components';
-import MessageSnackbar from '../components/Message';
-
-const Home: NextPage = () => {
-  return (
-    <div className="flex flex-col w-screen h-screen font-light">
-      <Head>
-        <title>
-          {process.env.NODE_ENV === 'development'
-            ? '[DEV] React Compiler Playground'
-            : 'React Compiler Playground'}
-        </title>
-        <meta
-          name="viewport"
-          content="width=device-width, initial-scale=1, maximum-scale=1, user-scalable=0"></meta>
-        <link rel="icon" href="/favicon.ico" />
-        <link rel="manifest" href="/site.webmanifest" />
-        <link
-          rel="preload"
-          href="/fonts/Source-Code-Pro-Regular.woff2"
-          as="font"
-          type="font/woff2"
-          crossOrigin="anonymous"
-        />
-        <link
-          rel="preload"
-          href="/fonts/Optimistic_Display_W_Lt.woff2"
-          as="font"
-          type="font/woff2"
-          crossOrigin="anonymous"
-        />
-      </Head>
-      <StoreProvider>
-        <SnackbarProvider
-          preventDuplicate
-          maxSnack={10}
-          Components={{message: MessageSnackbar}}>
-          <Header />
-          <Editor />
-        </SnackbarProvider>
-      </StoreProvider>
-    </div>
-  );
-};
-
-export default Home;

compiler/apps/playground/components/AccordionWindow.tsx

@@ -0,0 +1,126 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import {Resizable} from 're-resizable';
+import React, {
+  useId,
+  unstable_ViewTransition as ViewTransition,
+  unstable_addTransitionType as addTransitionType,
+  startTransition,
+} from 'react';
+import {EXPAND_ACCORDION_TRANSITION} from '../lib/transitionTypes';
+
+type TabsRecord = Map<string, React.ReactNode>;
+
+export default function AccordionWindow(props: {
+  defaultTab: string | null;
+  tabs: TabsRecord;
+  tabsOpen: Set<string>;
+  setTabsOpen: (newTab: Set<string>) => void;
+  changedPasses: Set<string>;
+}): React.ReactElement {
+  return (
+    <div className="flex-1 min-w-[550px] sm:min-w-0">
+      <div className="flex flex-row h-full">
+        {Array.from(props.tabs.keys()).map(name => {
+          return (
+            <AccordionWindowItem
+              name={name}
+              key={name}
+              tabs={props.tabs}
+              tabsOpen={props.tabsOpen}
+              setTabsOpen={props.setTabsOpen}
+              hasChanged={props.changedPasses.has(name)}
+            />
+          );
+        })}
+      </div>
+    </div>
+  );
+}
+
+function AccordionWindowItem({
+  name,
+  tabs,
+  tabsOpen,
+  setTabsOpen,
+  hasChanged,
+}: {
+  name: string;
+  tabs: TabsRecord;
+  tabsOpen: Set<string>;
+  setTabsOpen: (newTab: Set<string>) => void;
+  hasChanged: boolean;
+  isFailure: boolean;
+}): React.ReactElement {
+  const id = useId();
+  const isShow = tabsOpen.has(name);
+
+  const transitionName = `accordion-window-item-${id}`;
+
+  const toggleTabs = (): void => {
+    startTransition(() => {
+      addTransitionType(EXPAND_ACCORDION_TRANSITION);
+      const nextState = new Set(tabsOpen);
+      if (nextState.has(name)) {
+        nextState.delete(name);
+      } else {
+        nextState.add(name);
+      }
+      setTabsOpen(nextState);
+    });
+  };
+
+  // Replace spaces with non-breaking spaces
+  const displayName = name.replace(/ /g, '\u00A0');
+
+  return (
+    <div key={name} className="flex flex-row">
+      {isShow ? (
+        <ViewTransition
+          name={transitionName}
+          update={{
+            [EXPAND_ACCORDION_TRANSITION]: 'expand-accordion',
+            default: 'none',
+          }}>
+          <Resizable className="border-r" minWidth={550} enable={{right: true}}>
+            <h2
+              title="Minimize tab"
+              aria-label="Minimize tab"
+              onClick={toggleTabs}
+              className={`p-4 duration-150 ease-in border-b cursor-pointer border-grey-200 ${
+                hasChanged ? 'font-bold' : 'font-light'
+              } text-secondary hover:text-link`}>
+              - {displayName}
+            </h2>
+            {tabs.get(name) ?? <div>No output for {name}</div>}
+          </Resizable>
+        </ViewTransition>
+      ) : (
+        <ViewTransition
+          name={transitionName}
+          update={{
+            [EXPAND_ACCORDION_TRANSITION]: 'expand-accordion',
+            default: 'none',
+          }}>
+          <div className="relative items-center h-full px-1 py-6 align-middle border-r border-grey-200">
+            <button
+              title={`Expand compiler tab: ${name}`}
+              aria-label={`Expand compiler tab: ${name}`}
+              style={{transform: 'rotate(90deg) translate(-50%)'}}
+              onClick={toggleTabs}
+              className={`flex-grow-0 w-5 transition-colors duration-150 ease-in ${
+                hasChanged ? 'font-bold' : 'font-light'
+              } text-secondary hover:text-link`}>
+              {displayName}
+            </button>
+          </div>
+        </ViewTransition>
+      )}
+    </div>
+  );
+}

compiler/apps/playground/components/Editor/ConfigEditor.tsx

@@ -0,0 +1,221 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import MonacoEditor, {loader, type Monaco} from '@monaco-editor/react';
+import type {editor} from 'monaco-editor';
+import * as monaco from 'monaco-editor';
+import React, {
+  useState,
+  useRef,
+  unstable_ViewTransition as ViewTransition,
+  unstable_addTransitionType as addTransitionType,
+  startTransition,
+} from 'react';
+import {Resizable} from 're-resizable';
+import {useStore, useStoreDispatch} from '../StoreContext';
+import {monacoConfigOptions} from './monacoOptions';
+import {IconChevron} from '../Icons/IconChevron';
+import {CONFIG_PANEL_TRANSITION} from '../../lib/transitionTypes';
+
+// @ts-expect-error - webpack asset/source loader handles .d.ts files as strings
+import compilerTypeDefs from 'babel-plugin-react-compiler/dist/index.d.ts';
+
+loader.config({monaco});
+
+export default function ConfigEditor({
+  formattedAppliedConfig,
+}: {
+  formattedAppliedConfig: string;
+}): React.ReactElement {
+  const [isExpanded, setIsExpanded] = useState(false);
+
+  // TODO: Add back <Activity> after upgrading next.js
+  return (
+    <>
+      <div
+        style={{
+          display: isExpanded ? 'block' : 'none',
+        }}>
+        {/* <Activity mode={isExpanded ? 'visible' : 'hidden'}> */}
+        <ExpandedEditor
+          onToggle={() => {
+            startTransition(() => {
+              addTransitionType(CONFIG_PANEL_TRANSITION);
+              setIsExpanded(false);
+            });
+          }}
+          formattedAppliedConfig={formattedAppliedConfig}
+        />
+      </div>
+      <div
+        style={{
+          display: !isExpanded ? 'block' : 'none',
+        }}>
+        {/* </Activity>
+        <Activity mode={isExpanded ? 'hidden' : 'visible'}></Activity> */}
+        <CollapsedEditor
+          onToggle={() => {
+            startTransition(() => {
+              addTransitionType(CONFIG_PANEL_TRANSITION);
+              setIsExpanded(true);
+            });
+          }}
+        />
+      </div>
+      {/* </Activity> */}
+    </>
+  );
+}
+
+function ExpandedEditor({
+  onToggle,
+  formattedAppliedConfig,
+}: {
+  onToggle: (expanded: boolean) => void;
+  formattedAppliedConfig: string;
+}): React.ReactElement {
+  const store = useStore();
+  const dispatchStore = useStoreDispatch();
+  const debounceTimerRef = useRef<NodeJS.Timeout | null>(null);
+
+  const handleChange: (value: string | undefined) => void = (
+    value: string | undefined,
+  ) => {
+    if (value === undefined) return;
+
+    if (debounceTimerRef.current) {
+      clearTimeout(debounceTimerRef.current);
+    }
+
+    debounceTimerRef.current = setTimeout(() => {
+      dispatchStore({
+        type: 'updateConfig',
+        payload: {
+          config: value,
+        },
+      });
+    }, 500); // 500ms debounce delay
+  };
+
+  const handleMount: (
+    _: editor.IStandaloneCodeEditor,
+    monaco: Monaco,
+  ) => void = (_, monaco) => {
+    // Add the babel-plugin-react-compiler type definitions to Monaco
+    monaco.languages.typescript.typescriptDefaults.addExtraLib(
+      //@ts-expect-error - compilerTypeDefs is a string
+      compilerTypeDefs,
+      'file:///node_modules/babel-plugin-react-compiler/dist/index.d.ts',
+    );
+    monaco.languages.typescript.typescriptDefaults.setCompilerOptions({
+      target: monaco.languages.typescript.ScriptTarget.Latest,
+      allowNonTsExtensions: true,
+      moduleResolution: monaco.languages.typescript.ModuleResolutionKind.NodeJs,
+      module: monaco.languages.typescript.ModuleKind.ESNext,
+      noEmit: true,
+      strict: false,
+      esModuleInterop: true,
+      allowSyntheticDefaultImports: true,
+      jsx: monaco.languages.typescript.JsxEmit.React,
+    });
+  };
+
+  return (
+    <ViewTransition
+      update={{[CONFIG_PANEL_TRANSITION]: 'slide-in', default: 'none'}}>
+      {/* enter={{[CONFIG_PANEL_TRANSITION]: 'slide-in', default: 'none'}}
+      exit={{[CONFIG_PANEL_TRANSITION]: 'slide-out', default: 'none'}}> */}
+      <Resizable
+        minWidth={300}
+        maxWidth={600}
+        defaultSize={{width: 350}}
+        enable={{right: true, bottom: false}}>
+        <div className="bg-blue-10 relative h-full flex flex-col !h-[calc(100vh_-_3.5rem)] border border-gray-300">
+          <div
+            className="absolute w-8 h-16 bg-blue-10 rounded-r-full flex items-center justify-center z-[2] cursor-pointer border border-l-0 border-gray-300"
+            title="Minimize config editor"
+            onClick={onToggle}
+            style={{
+              top: '50%',
+              marginTop: '-32px',
+              right: '-32px',
+              borderTopLeftRadius: 0,
+              borderBottomLeftRadius: 0,
+            }}>
+            <IconChevron displayDirection="left" className="text-blue-50" />
+          </div>
+
+          <div className="flex-1 flex flex-col m-2 mb-2">
+            <div className="pb-2">
+              <h2 className="inline-block text-blue-50 py-1.5 px-1.5 xs:px-3 sm:px-4 text-sm">
+                Config Overrides
+              </h2>
+            </div>
+            <div className="flex-1 border border-gray-300">
+              <MonacoEditor
+                path={'config.ts'}
+                language={'typescript'}
+                value={store.config}
+                onMount={handleMount}
+                onChange={handleChange}
+                loading={''}
+                className="monaco-editor-config"
+                options={monacoConfigOptions}
+              />
+            </div>
+          </div>
+          <div className="flex-1 flex flex-col m-2">
+            <div className="pb-2">
+              <h2 className="inline-block text-blue-50 py-1.5 px-1.5 xs:px-3 sm:px-4 text-sm">
+                Applied Configs
+              </h2>
+            </div>
+            <div className="flex-1 border border-gray-300">
+              <MonacoEditor
+                path={'applied-config.js'}
+                language={'javascript'}
+                value={formattedAppliedConfig}
+                loading={''}
+                className="monaco-editor-applied-config"
+                options={{
+                  ...monacoConfigOptions,
+                  readOnly: true,
+                }}
+              />
+            </div>
+          </div>
+        </div>
+      </Resizable>
+    </ViewTransition>
+  );
+}
+
+function CollapsedEditor({
+  onToggle,
+}: {
+  onToggle: () => void;
+}): React.ReactElement {
+  return (
+    <div
+      className="w-4 !h-[calc(100vh_-_3.5rem)]"
+      style={{position: 'relative'}}>
+      <div
+        className="absolute w-10 h-16 bg-blue-10 hover:translate-x-2 transition-transform rounded-r-full flex items-center justify-center z-[2] cursor-pointer border border-gray-300"
+        title="Expand config editor"
+        onClick={onToggle}
+        style={{
+          top: '50%',
+          marginTop: '-32px',
+          left: '-8px',
+          borderTopLeftRadius: 0,
+          borderBottomLeftRadius: 0,
+        }}>
+        <IconChevron displayDirection="right" className="text-blue-50" />
+      </div>
+    </div>
+  );
+}

compiler/apps/playground/components/Editor/EditorImpl.tsx

@@ -5,313 +5,30 @@
  * LICENSE file in the root directory of this source tree.
  */
 
-import {parse as babelParse, ParseResult} from '@babel/parser';
-import * as HermesParser from 'hermes-parser';
-import * as t from '@babel/types';
-import BabelPluginReactCompiler, {
-  CompilerError,
+import {
   CompilerErrorDetail,
   CompilerDiagnostic,
-  Effect,
-  ErrorSeverity,
-  parseConfigPragmaForTests,
-  ValueKind,
-  type Hook,
-  PluginOptions,
-  CompilerPipelineValue,
-  parsePluginOptions,
-  printReactiveFunctionWithOutlined,
-  printFunctionWithOutlined,
 } from 'babel-plugin-react-compiler';
-import clsx from 'clsx';
-import invariant from 'invariant';
-import {useSnackbar} from 'notistack';
-import {useDeferredValue, useMemo} from 'react';
-import {useMountEffect} from '../../hooks';
-import {defaultStore} from '../../lib/defaultStore';
-import {
-  createMessage,
-  initStoreFromUrlOrLocalStorage,
-  MessageLevel,
-  MessageSource,
-  type Store,
-} from '../../lib/stores';
-import {useStore, useStoreDispatch} from '../StoreContext';
+import {useDeferredValue, useMemo, useState} from 'react';
+import {useStore} from '../StoreContext';
+import ConfigEditor from './ConfigEditor';
 import Input from './Input';
-import {
-  CompilerOutput,
-  CompilerTransformOutput,
-  default as Output,
-  PrintedCompilerPipelineValue,
-} from './Output';
-import {transformFromAstSync} from '@babel/core';
-import {LoggerEvent} from 'babel-plugin-react-compiler/dist/Entrypoint';
-
-function parseInput(
-  input: string,
-  language: 'flow' | 'typescript',
-): ParseResult<t.File> {
-  // Extract the first line to quickly check for custom test directives
-  if (language === 'flow') {
-    return HermesParser.parse(input, {
-      babel: true,
-      flow: 'all',
-      sourceType: 'module',
-      enableExperimentalComponentSyntax: true,
-    });
-  } else {
-    return babelParse(input, {
-      plugins: ['typescript', 'jsx'],
-      sourceType: 'module',
-    }) as ParseResult<t.File>;
-  }
-}
-
-function invokeCompiler(
-  source: string,
-  language: 'flow' | 'typescript',
-  options: PluginOptions,
-): CompilerTransformOutput {
-  const ast = parseInput(source, language);
-  let result = transformFromAstSync(ast, source, {
-    filename: '_playgroundFile.js',
-    highlightCode: false,
-    retainLines: true,
-    plugins: [[BabelPluginReactCompiler, options]],
-    ast: true,
-    sourceType: 'module',
-    configFile: false,
-    sourceMaps: true,
-    babelrc: false,
-  });
-  if (result?.ast == null || result?.code == null || result?.map == null) {
-    throw new Error('Expected successful compilation');
-  }
-  return {
-    code: result.code,
-    sourceMaps: result.map,
-    language,
-  };
-}
-
-const COMMON_HOOKS: Array<[string, Hook]> = [
-  [
-    'useFragment',
-    {
-      valueKind: ValueKind.Frozen,
-      effectKind: Effect.Freeze,
-      noAlias: true,
-      transitiveMixedData: true,
-    },
-  ],
-  [
-    'usePaginationFragment',
-    {
-      valueKind: ValueKind.Frozen,
-      effectKind: Effect.Freeze,
-      noAlias: true,
-      transitiveMixedData: true,
-    },
-  ],
-  [
-    'useRefetchableFragment',
-    {
-      valueKind: ValueKind.Frozen,
-      effectKind: Effect.Freeze,
-      noAlias: true,
-      transitiveMixedData: true,
-    },
-  ],
-  [
-    'useLazyLoadQuery',
-    {
-      valueKind: ValueKind.Frozen,
-      effectKind: Effect.Freeze,
-      noAlias: true,
-      transitiveMixedData: true,
-    },
-  ],
-  [
-    'usePreloadedQuery',
-    {
-      valueKind: ValueKind.Frozen,
-      effectKind: Effect.Freeze,
-      noAlias: true,
-      transitiveMixedData: true,
-    },
-  ],
-];
-
-function compile(
-  source: string,
-  mode: 'compiler' | 'linter',
-): [CompilerOutput, 'flow' | 'typescript'] {
-  const results = new Map<string, Array<PrintedCompilerPipelineValue>>();
-  const error = new CompilerError();
-  const otherErrors: Array<CompilerErrorDetail | CompilerDiagnostic> = [];
-  const upsert: (result: PrintedCompilerPipelineValue) => void = result => {
-    const entry = results.get(result.name);
-    if (Array.isArray(entry)) {
-      entry.push(result);
-    } else {
-      results.set(result.name, [result]);
-    }
-  };
-  let language: 'flow' | 'typescript';
-  if (source.match(/\@flow/)) {
-    language = 'flow';
-  } else {
-    language = 'typescript';
-  }
-  let transformOutput;
-  try {
-    // Extract the first line to quickly check for custom test directives
-    const pragma = source.substring(0, source.indexOf('\n'));
-    const logIR = (result: CompilerPipelineValue): void => {
-      switch (result.kind) {
-        case 'ast': {
-          break;
-        }
-        case 'hir': {
-          upsert({
-            kind: 'hir',
-            fnName: result.value.id,
-            name: result.name,
-            value: printFunctionWithOutlined(result.value),
-          });
-          break;
-        }
-        case 'reactive': {
-          upsert({
-            kind: 'reactive',
-            fnName: result.value.id,
-            name: result.name,
-            value: printReactiveFunctionWithOutlined(result.value),
-          });
-          break;
-        }
-        case 'debug': {
-          upsert({
-            kind: 'debug',
-            fnName: null,
-            name: result.name,
-            value: result.value,
-          });
-          break;
-        }
-        default: {
-          const _: never = result;
-          throw new Error(`Unhandled result ${result}`);
-        }
-      }
-    };
-    const parsedOptions = parseConfigPragmaForTests(pragma, {
-      compilationMode: 'infer',
-      environment:
-        mode === 'linter'
-          ? {
-              // enabled in compiler
-              validateRefAccessDuringRender: false,
-              // enabled in linter
-              validateNoSetStateInRender: true,
-              validateNoSetStateInEffects: true,
-              validateNoJSXInTryStatements: true,
-              validateNoImpureFunctionsInRender: true,
-              validateStaticComponents: true,
-              validateNoFreezingKnownMutableFunctions: true,
-              validateNoVoidUseMemo: true,
-            }
-          : {
-              /* use defaults for compiler mode */
-            },
-    });
-    const opts: PluginOptions = parsePluginOptions({
-      ...parsedOptions,
-      environment: {
-        ...parsedOptions.environment,
-        customHooks: new Map([...COMMON_HOOKS]),
-      },
-      logger: {
-        debugLogIRs: logIR,
-        logEvent: (_filename: string | null, event: LoggerEvent) => {
-          if (event.kind === 'CompileError') {
-            otherErrors.push(event.detail);
-          }
-        },
-      },
-    });
-    transformOutput = invokeCompiler(source, language, opts);
-  } catch (err) {
-    /**
-     * error might be an invariant violation or other runtime error
-     * (i.e. object shape that is not CompilerError)
-     */
-    if (err instanceof CompilerError && err.details.length > 0) {
-      error.merge(err);
-    } else {
-      /**
-       * Handle unexpected failures by logging (to get a stack trace)
-       * and reporting
-       */
-      console.error(err);
-      error.details.push(
-        new CompilerErrorDetail({
-          severity: ErrorSeverity.Invariant,
-          reason: `Unexpected failure when transforming input! ${err}`,
-          loc: null,
-          suggestions: null,
-        }),
-      );
-    }
-  }
-  // Only include logger errors if there weren't other errors
-  if (!error.hasErrors() && otherErrors.length !== 0) {
-    otherErrors.forEach(e => error.details.push(e));
-  }
-  if (error.hasErrors()) {
-    return [{kind: 'err', results, error}, language];
-  }
-  return [
-    {kind: 'ok', results, transformOutput, errors: error.details},
-    language,
-  ];
-}
+import {CompilerOutput, default as Output} from './Output';
+import {compile} from '../../lib/compilation';
+import prettyFormat from 'pretty-format';
 
 export default function Editor(): JSX.Element {
   const store = useStore();
   const deferredStore = useDeferredValue(store);
-  const dispatchStore = useStoreDispatch();
-  const {enqueueSnackbar} = useSnackbar();
-  const [compilerOutput, language] = useMemo(
-    () => compile(deferredStore.source, 'compiler'),
-    [deferredStore.source],
+  const [compilerOutput, language, appliedOptions] = useMemo(
+    () => compile(deferredStore.source, 'compiler', deferredStore.config),
+    [deferredStore.source, deferredStore.config],
   );
   const [linterOutput] = useMemo(
-    () => compile(deferredStore.source, 'linter'),
-    [deferredStore.source],
+    () => compile(deferredStore.source, 'linter', deferredStore.config),
+    [deferredStore.source, deferredStore.config],
   );
-
-  useMountEffect(() => {
-    let mountStore: Store;
-    try {
-      mountStore = initStoreFromUrlOrLocalStorage();
-    } catch (e) {
-      invariant(e instanceof Error, 'Only Error may be caught.');
-      enqueueSnackbar(e.message, {
-        variant: 'warning',
-        ...createMessage(
-          'Bad URL - fell back to the default Playground.',
-          MessageLevel.Info,
-          MessageSource.Playground,
-        ),
-      });
-      mountStore = defaultStore;
-    }
-    dispatchStore({
-      type: 'setStore',
-      payload: {store: mountStore},
-    });
-  });
+  const [formattedAppliedConfig, setFormattedAppliedConfig] = useState('');
 
   let mergedOutput: CompilerOutput;
   let errors: Array<CompilerErrorDetail | CompilerDiagnostic>;
@@ -325,13 +42,25 @@ export default function Editor(): JSX.Element {
     mergedOutput = compilerOutput;
     errors = compilerOutput.error.details;
   }
+
+  if (appliedOptions) {
+    const formatted = prettyFormat(appliedOptions, {
+      printFunctionName: false,
+      printBasicPrototype: false,
+    });
+    if (formatted !== formattedAppliedConfig) {
+      setFormattedAppliedConfig(formatted);
+    }
+  }
+
   return (
     <>
-      <div className="relative flex basis top-14">
-        <div className={clsx('relative sm:basis-1/4')}>
-          <Input language={language} errors={errors} />
+      <div className="relative flex top-14">
+        <div className="flex-shrink-0">
+          <ConfigEditor formattedAppliedConfig={formattedAppliedConfig} />
         </div>
-        <div className={clsx('flex sm:flex flex-wrap')}>
+        <div className="flex flex-1 min-w-0">
+          <Input language={language} errors={errors} />
           <Output store={deferredStore} compilerOutput={mergedOutput} />
         </div>
       </div>

compiler/apps/playground/components/Editor/Input.tsx

@@ -6,22 +6,31 @@
  */
 
 import MonacoEditor, {loader, type Monaco} from '@monaco-editor/react';
-import {CompilerErrorDetail} from 'babel-plugin-react-compiler';
+import {
+  CompilerErrorDetail,
+  CompilerDiagnostic,
+} from 'babel-plugin-react-compiler';
 import invariant from 'invariant';
 import type {editor} from 'monaco-editor';
 import * as monaco from 'monaco-editor';
-import {Resizable} from 're-resizable';
-import {useEffect, useState} from 'react';
+import {
+  useEffect,
+  useState,
+  unstable_ViewTransition as ViewTransition,
+} from 'react';
 import {renderReactCompilerMarkers} from '../../lib/reactCompilerMonacoDiagnostics';
 import {useStore, useStoreDispatch} from '../StoreContext';
+import TabbedWindow from '../TabbedWindow';
 import {monacoOptions} from './monacoOptions';
+import {CONFIG_PANEL_TRANSITION} from '../../lib/transitionTypes';
+
 // @ts-expect-error TODO: Make TS recognize .d.ts files, in addition to loading them with webpack.
 import React$Types from '../../node_modules/@types/react/index.d.ts';
 
 loader.config({monaco});
 
 type Props = {
-  errors: Array<CompilerErrorDetail>;
+  errors: Array<CompilerErrorDetail | CompilerDiagnostic>;
   language: 'flow' | 'typescript';
 };
 
@@ -42,11 +51,6 @@ export default function Input({errors, language}: Props): JSX.Element {
       details: errors,
       source: store.source,
     });
-    /**
-     * N.B. that `tabSize` is a model property, not an editor property.
-     * So, the tab size has to be set per model.
-     */
-    model.updateOptions({tabSize: 2});
   }, [monaco, errors, store.source]);
 
   useEffect(() => {
@@ -79,11 +83,11 @@ export default function Input({errors, language}: Props): JSX.Element {
     });
   }, [monaco, language]);
 
-  const handleChange: (value: string | undefined) => void = value => {
+  const handleChange: (value: string | undefined) => void = async value => {
     if (!value) return;
 
     dispatchStore({
-      type: 'updateFile',
+      type: 'updateSource',
       payload: {
         source: value,
       },
@@ -135,30 +139,42 @@ export default function Input({errors, language}: Props): JSX.Element {
     });
   };
 
+  const editorContent = (
+    <MonacoEditor
+      path={'index.js'}
+      /**
+       * .js and .jsx files are specified to be TS so that Monaco can actually
+       * check their syntax using its TS language service. They are still JS files
+       * due to their extensions, so TS language features don't work.
+       */
+      language={'javascript'}
+      value={store.source}
+      onMount={handleMount}
+      onChange={handleChange}
+      className="monaco-editor-input"
+      options={monacoOptions}
+      loading={''}
+    />
+  );
+
+  const tabs = new Map([['Input', editorContent]]);
+  const [activeTab, setActiveTab] = useState('Input');
+
   return (
-    <div className="relative flex flex-col flex-none border-r border-gray-200">
-      <Resizable
-        minWidth={650}
-        enable={{right: true}}
-        /**
-         * Restrict MonacoEditor's height, since the config autoLayout:true
-         * will grow the editor to fit within parent element
-         */
-        className="!h-[calc(100vh_-_3.5rem)]">
-        <MonacoEditor
-          path={'index.js'}
-          /**
-           * .js and .jsx files are specified to be TS so that Monaco can actually
-           * check their syntax using its TS language service. They are still JS files
-           * due to their extensions, so TS language features don't work.
-           */
-          language={'javascript'}
-          value={store.source}
-          onMount={handleMount}
-          onChange={handleChange}
-          options={monacoOptions}
-        />
-      </Resizable>
-    </div>
+    <ViewTransition
+      update={{
+        [CONFIG_PANEL_TRANSITION]: 'container',
+        default: 'none',
+      }}>
+      <div className="flex-1 min-w-[550px] sm:min-w-0">
+        <div className="flex flex-col h-full !h-[calc(100vh_-_3.5rem)] border-r border-gray-200">
+          <TabbedWindow
+            tabs={tabs}
+            activeTab={activeTab}
+            onTabChange={setActiveTab}
+          />
+        </div>
+      </div>
+    </ViewTransition>
   );
 }

compiler/apps/playground/components/Editor/Output.tsx

@@ -19,15 +19,38 @@ import {
 import parserBabel from 'prettier/plugins/babel';
 import * as prettierPluginEstree from 'prettier/plugins/estree';
 import * as prettier from 'prettier/standalone';
-import {memo, ReactNode, useEffect, useState} from 'react';
 import {type Store} from '../../lib/stores';
+import {
+  memo,
+  ReactNode,
+  use,
+  useState,
+  Suspense,
+  unstable_ViewTransition as ViewTransition,
+  unstable_addTransitionType as addTransitionType,
+  startTransition,
+} from 'react';
+import AccordionWindow from '../AccordionWindow';
 import TabbedWindow from '../TabbedWindow';
 import {monacoOptions} from './monacoOptions';
 import {BabelFileResult} from '@babel/core';
+import {
+  CONFIG_PANEL_TRANSITION,
+  TOGGLE_INTERNALS_TRANSITION,
+  EXPAND_ACCORDION_TRANSITION,
+} from '../../lib/transitionTypes';
+import {LRUCache} from 'lru-cache';
+
 const MemoizedOutput = memo(Output);
 
 export default MemoizedOutput;
 
+export const BASIC_OUTPUT_TAB_NAMES = ['Output', 'SourceMap'];
+
+const tabifyCache = new LRUCache<Store, Promise<Map<string, ReactNode>>>({
+  max: 5,
+});
+
 export type PrintedCompilerPipelineValue =
   | {
       kind: 'hir';
@@ -64,12 +87,16 @@ type Props = {
 async function tabify(
   source: string,
   compilerOutput: CompilerOutput,
+  showInternals: boolean,
 ): Promise<Map<string, ReactNode>> {
   const tabs = new Map<string, React.ReactNode>();
   const reorderedTabs = new Map<string, React.ReactNode>();
   const concattedResults = new Map<string, string>();
   // Concat all top level function declaration results into a single tab for each pass
   for (const [passName, results] of compilerOutput.results) {
+    if (!showInternals && !BASIC_OUTPUT_TAB_NAMES.includes(passName)) {
+      continue;
+    }
     for (const result of results) {
       switch (result.kind) {
         case 'hir': {
@@ -192,6 +219,25 @@ ${code}
   return reorderedTabs;
 }
 
+function tabifyCached(
+  store: Store,
+  compilerOutput: CompilerOutput,
+): Promise<Map<string, ReactNode>> {
+  const cached = tabifyCache.get(store);
+  if (cached) return cached;
+  const result = tabify(store.source, compilerOutput, store.showInternals);
+  tabifyCache.set(store, result);
+  return result;
+}
+
+function Fallback(): JSX.Element {
+  return (
+    <div className="w-full h-monaco_small sm:h-monaco flex items-center justify-center">
+      Loading...
+    </div>
+  );
+}
+
 function utf16ToUTF8(s: string): string {
   return unescape(encodeURIComponent(s));
 }
@@ -205,12 +251,18 @@ function getSourceMapUrl(code: string, map: string): string | null {
 }
 
 function Output({store, compilerOutput}: Props): JSX.Element {
+  return (
+    <Suspense fallback={<Fallback />}>
+      <OutputContent store={store} compilerOutput={compilerOutput} />
+    </Suspense>
+  );
+}
+
+function OutputContent({store, compilerOutput}: Props): JSX.Element {
   const [tabsOpen, setTabsOpen] = useState<Set<string>>(
     () => new Set(['Output']),
   );
-  const [tabs, setTabs] = useState<Map<string, React.ReactNode>>(
-    () => new Map(),
-  );
+  const [activeTab, setActiveTab] = useState<string>('Output');
 
   /*
    * Update the active tab back to the output or errors tab when the compilation state
@@ -219,17 +271,19 @@ function Output({store, compilerOutput}: Props): JSX.Element {
   const [previousOutputKind, setPreviousOutputKind] = useState(
     compilerOutput.kind,
   );
+  const isFailure = compilerOutput.kind !== 'ok';
+
   if (compilerOutput.kind !== previousOutputKind) {
     setPreviousOutputKind(compilerOutput.kind);
-    setTabsOpen(new Set(['Output']));
+    if (isFailure) {
+      startTransition(() => {
+        addTransitionType(EXPAND_ACCORDION_TRANSITION);
+        setTabsOpen(prev => new Set(prev).add('Output'));
+        setActiveTab('Output');
+      });
+    }
   }
 
-  useEffect(() => {
-    tabify(store.source, compilerOutput).then(tabs => {
-      setTabs(tabs);
-    });
-  }, [store.source, compilerOutput]);
-
   const changedPasses: Set<string> = new Set(['Output', 'HIR']); // Initial and final passes should always be bold
   let lastResult: string = '';
   for (const [passName, results] of compilerOutput.results) {
@@ -244,17 +298,40 @@ function Output({store, compilerOutput}: Props): JSX.Element {
       lastResult = currResult;
     }
   }
+  const tabs = use(tabifyCached(store, compilerOutput));
+
+  if (!store.showInternals) {
+    return (
+      <ViewTransition
+        update={{
+          [CONFIG_PANEL_TRANSITION]: 'container',
+          [TOGGLE_INTERNALS_TRANSITION]: '',
+          default: 'none',
+        }}>
+        <TabbedWindow
+          tabs={tabs}
+          activeTab={activeTab}
+          onTabChange={setActiveTab}
+        />
+      </ViewTransition>
+    );
+  }
 
   return (
-    <>
-      <TabbedWindow
-        defaultTab="HIR"
+    <ViewTransition
+      update={{
+        [CONFIG_PANEL_TRANSITION]: 'accordion-container',
+        [TOGGLE_INTERNALS_TRANSITION]: '',
+        default: 'none',
+      }}>
+      <AccordionWindow
+        defaultTab={store.showInternals ? 'HIR' : 'Output'}
         setTabsOpen={setTabsOpen}
         tabsOpen={tabsOpen}
         tabs={tabs}
         changedPasses={changedPasses}
       />
-    </>
+    </ViewTransition>
   );
 }
 
@@ -306,20 +383,29 @@ function TextTabContent({
         <DiffEditor
           original={diff}
           modified={output}
+          loading={''}
           options={{
             ...monacoOptions,
+            scrollbar: {
+              vertical: 'hidden',
+            },
+            dimension: {
+              width: 0,
+              height: 0,
+            },
             readOnly: true,
             lineNumbers: 'off',
             glyphMargin: false,
             // Undocumented see https://github.com/Microsoft/vscode/issues/30795#issuecomment-410998882
-            lineDecorationsWidth: 0,
-            lineNumbersMinChars: 0,
+            overviewRulerLanes: 0,
           }}
         />
       ) : (
         <MonacoEditor
           language={language ?? 'javascript'}
           value={output}
+          loading={''}
+          className="monaco-editor-output"
           options={{
             ...monacoOptions,
             readOnly: true,

compiler/apps/playground/components/Editor/monacoOptions.ts

@@ -29,4 +29,17 @@ export const monacoOptions: Partial<EditorProps['options']> = {
   automaticLayout: true,
   wordWrap: 'on',
   wrappingIndent: 'same',
+
+  tabSize: 2,
+};
+
+export const monacoConfigOptions: Partial<EditorProps['options']> = {
+  ...monacoOptions,
+  lineNumbers: 'off',
+  renderLineHighlight: 'none',
+  overviewRulerBorder: false,
+  overviewRulerLanes: 0,
+  fontSize: 12,
+  scrollBeyondLastLine: false,
+  glyphMargin: false,
 };

compiler/apps/playground/components/Header.tsx

@@ -10,14 +10,20 @@ import {CheckIcon} from '@heroicons/react/solid';
 import clsx from 'clsx';
 import Link from 'next/link';
 import {useSnackbar} from 'notistack';
-import {useState} from 'react';
+import {
+  useState,
+  startTransition,
+  unstable_addTransitionType as addTransitionType,
+} from 'react';
 import {defaultStore} from '../lib/defaultStore';
 import {IconGitHub} from './Icons/IconGitHub';
 import Logo from './Logo';
-import {useStoreDispatch} from './StoreContext';
+import {useStore, useStoreDispatch} from './StoreContext';
+import {TOGGLE_INTERNALS_TRANSITION} from '../lib/transitionTypes';
 
 export default function Header(): JSX.Element {
   const [showCheck, setShowCheck] = useState(false);
+  const store = useStore();
   const dispatchStore = useStoreDispatch();
   const {enqueueSnackbar, closeSnackbar} = useSnackbar();
 
@@ -56,6 +62,32 @@ export default function Header(): JSX.Element {
         <p className="hidden select-none sm:block">React Compiler Playground</p>
       </div>
       <div className="flex items-center text-[15px] gap-4">
+        <div className="flex items-center gap-2">
+          <label className="show-internals relative inline-block w-[34px] h-5">
+            <input
+              type="checkbox"
+              checked={store.showInternals}
+              onChange={() =>
+                startTransition(() => {
+                  addTransitionType(TOGGLE_INTERNALS_TRANSITION);
+                  dispatchStore({type: 'toggleInternals'});
+                })
+              }
+              className="absolute opacity-0 cursor-pointer h-full w-full m-0"
+            />
+            <span
+              className={clsx(
+                'absolute inset-0 rounded-full cursor-pointer transition-all duration-250',
+                "before:content-[''] before:absolute before:w-4 before:h-4 before:left-0.5 before:bottom-0.5",
+                'before:bg-white before:rounded-full before:transition-transform before:duration-250',
+                'focus-within:shadow-[0_0_1px_#2196F3]',
+                store.showInternals
+                  ? 'bg-link before:translate-x-3.5'
+                  : 'bg-gray-300',
+              )}></span>
+          </label>
+          <span className="text-secondary">Show Internals</span>
+        </div>
         <button
           title="Reset Playground"
           aria-label="Reset Playground"

compiler/apps/playground/components/Icons/IconChevron.tsx

@@ -0,0 +1,41 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import {memo} from 'react';
+
+export const IconChevron = memo<
+  JSX.IntrinsicElements['svg'] & {
+    /**
+     * The direction the arrow should point.
+     */
+    displayDirection: 'right' | 'left';
+  }
+>(function IconChevron({className, displayDirection, ...props}) {
+  const rotationClass =
+    displayDirection === 'left' ? 'rotate-90' : '-rotate-90';
+  const classes = className ? `${rotationClass} ${className}` : rotationClass;
+
+  return (
+    <svg
+      className={classes}
+      xmlns="http://www.w3.org/2000/svg"
+      width="20"
+      height="20"
+      viewBox="0 0 20 20"
+      {...props}>
+      <g fill="none" fillRule="evenodd" transform="translate(-446 -398)">
+        <path
+          fill="currentColor"
+          fillRule="nonzero"
+          d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z"
+          transform="translate(356.5 164.5)"
+        />
+        <polygon points="446 418 466 418 466 398 446 398" />
+      </g>
+    </svg>
+  );
+});

compiler/apps/playground/components/StoreContext.tsx

@@ -6,10 +6,14 @@
  */
 
 import type {Dispatch, ReactNode} from 'react';
-import {useEffect, useReducer} from 'react';
+import {useState, useEffect, useReducer} from 'react';
 import createContext from '../lib/createContext';
-import {emptyStore} from '../lib/defaultStore';
-import {saveStore, type Store} from '../lib/stores';
+import {emptyStore, defaultStore} from '../lib/defaultStore';
+import {
+  saveStore,
+  initStoreFromUrlOrLocalStorage,
+  type Store,
+} from '../lib/stores';
 
 const StoreContext = createContext<Store>();
 
@@ -30,6 +34,20 @@ export const useStoreDispatch = StoreDispatchContext.useContext;
  */
 export function StoreProvider({children}: {children: ReactNode}): JSX.Element {
   const [store, dispatch] = useReducer(storeReducer, emptyStore);
+  const [isPageReady, setIsPageReady] = useState<boolean>(false);
+
+  useEffect(() => {
+    let mountStore: Store;
+    try {
+      mountStore = initStoreFromUrlOrLocalStorage();
+    } catch (e) {
+      console.error('Failed to initialize store from URL or local storage', e);
+      mountStore = defaultStore;
+    }
+    dispatch({type: 'setStore', payload: {store: mountStore}});
+    setIsPageReady(true);
+  }, []);
+
   useEffect(() => {
     if (store !== emptyStore) {
       saveStore(store);
@@ -39,7 +57,7 @@ export function StoreProvider({children}: {children: ReactNode}): JSX.Element {
   return (
     <StoreContext.Provider value={store}>
       <StoreDispatchContext.Provider value={dispatch}>
-        {children}
+        {isPageReady ? children : null}
       </StoreDispatchContext.Provider>
     </StoreContext.Provider>
   );
@@ -53,10 +71,19 @@ type ReducerAction =
       };
     }
   | {
-      type: 'updateFile';
+      type: 'updateSource';
       payload: {
         source: string;
       };
+    }
+  | {
+      type: 'updateConfig';
+      payload: {
+        config: string;
+      };
+    }
+  | {
+      type: 'toggleInternals';
     };
 
 function storeReducer(store: Store, action: ReducerAction): Store {
@@ -65,13 +92,28 @@ function storeReducer(store: Store, action: ReducerAction): Store {
       const newStore = action.payload.store;
       return newStore;
     }
-    case 'updateFile': {
-      const {source} = action.payload;
+    case 'updateSource': {
+      const source = action.payload.source;
       const newStore = {
         ...store,
         source,
       };
       return newStore;
     }
+    case 'updateConfig': {
+      const config = action.payload.config;
+      const newStore = {
+        ...store,
+        config,
+      };
+      return newStore;
+    }
+    case 'toggleInternals': {
+      const newStore = {
+        ...store,
+        showInternals: !store.showInternals,
+      };
+      return newStore;
+    }
   }
 }

compiler/apps/playground/components/TabbedWindow.tsx

@@ -4,103 +4,78 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  */
+import React, {
+  startTransition,
+  useId,
+  unstable_ViewTransition as ViewTransition,
+  unstable_addTransitionType as addTransitionType,
+} from 'react';
+import clsx from 'clsx';
+import {TOGGLE_TAB_TRANSITION} from '../lib/transitionTypes';
 
-import {Resizable} from 're-resizable';
-import React, {useCallback} from 'react';
-
-type TabsRecord = Map<string, React.ReactNode>;
-
-export default function TabbedWindow(props: {
-  defaultTab: string | null;
-  tabs: TabsRecord;
-  tabsOpen: Set<string>;
-  setTabsOpen: (newTab: Set<string>) => void;
-  changedPasses: Set<string>;
-}): React.ReactElement {
-  if (props.tabs.size === 0) {
-    return (
-      <div
-        className="flex items-center justify-center"
-        style={{width: 'calc(100vw - 650px)'}}>
-        No compiler output detected, see errors below
-      </div>
-    );
-  }
-  return (
-    <div className="flex flex-row">
-      {Array.from(props.tabs.keys()).map(name => {
-        return (
-          <TabbedWindowItem
-            name={name}
-            key={name}
-            tabs={props.tabs}
-            tabsOpen={props.tabsOpen}
-            setTabsOpen={props.setTabsOpen}
-            hasChanged={props.changedPasses.has(name)}
-          />
-        );
-      })}
-    </div>
-  );
-}
-
-function TabbedWindowItem({
-  name,
+export default function TabbedWindow({
   tabs,
-  tabsOpen,
-  setTabsOpen,
-  hasChanged,
+  activeTab,
+  onTabChange,
 }: {
-  name: string;
-  tabs: TabsRecord;
-  tabsOpen: Set<string>;
-  setTabsOpen: (newTab: Set<string>) => void;
-  hasChanged: boolean;
+  tabs: Map<string, React.ReactNode>;
+  activeTab: string;
+  onTabChange: (tab: string) => void;
 }): React.ReactElement {
-  const isShow = tabsOpen.has(name);
+  const id = useId();
+  const transitionName = `tab-highlight-${id}`;
 
-  const toggleTabs = useCallback(() => {
-    const nextState = new Set(tabsOpen);
-    if (nextState.has(name)) {
-      nextState.delete(name);
-    } else {
-      nextState.add(name);
-    }
-    setTabsOpen(nextState);
-  }, [tabsOpen, name, setTabsOpen]);
-
-  // Replace spaces with non-breaking spaces
-  const displayName = name.replace(/ /g, '\u00A0');
+  const handleTabChange = (tab: string): void => {
+    startTransition(() => {
+      addTransitionType(TOGGLE_TAB_TRANSITION);
+      onTabChange(tab);
+    });
+  };
 
   return (
-    <div key={name} className="flex flex-row">
-      {isShow ? (
-        <Resizable className="border-r" minWidth={550} enable={{right: true}}>
-          <h2
-            title="Minimize tab"
-            aria-label="Minimize tab"
-            onClick={toggleTabs}
-            className={`p-4 duration-150 ease-in border-b cursor-pointer border-grey-200 ${
-              hasChanged ? 'font-bold' : 'font-light'
-            } text-secondary hover:text-link`}>
-            - {displayName}
-          </h2>
-          {tabs.get(name) ?? <div>No output for {name}</div>}
-        </Resizable>
-      ) : (
-        <div className="relative items-center h-full px-1 py-6 align-middle border-r border-grey-200">
-          <button
-            title={`Expand compiler tab: ${name}`}
-            aria-label={`Expand compiler tab: ${name}`}
-            style={{transform: 'rotate(90deg) translate(-50%)'}}
-            onClick={toggleTabs}
-            className={`flex-grow-0 w-5 transition-colors duration-150 ease-in ${
-              hasChanged ? 'font-bold' : 'font-light'
-            } text-secondary hover:text-link`}>
-            {displayName}
-          </button>
+    <div className="flex-1 min-w-[550px] sm:min-w-0">
+      <div className="flex flex-col h-full max-w-full">
+        <div className="flex p-2 flex-shrink-0">
+          {Array.from(tabs.keys()).map(tab => {
+            const isActive = activeTab === tab;
+            return (
+              <button
+                key={tab}
+                onClick={() => handleTabChange(tab)}
+                className={clsx(
+                  'transition-transform py-1.5 px-1.5 xs:px-3 sm:px-4 rounded-full text-sm relative',
+                  isActive ? 'text-link' : 'hover:bg-primary/5',
+                )}>
+                {isActive && (
+                  <ViewTransition
+                    name={transitionName}
+                    enter={{default: 'none'}}
+                    exit={{default: 'none'}}
+                    share={{
+                      [TOGGLE_TAB_TRANSITION]: 'tab-highlight',
+                      default: 'none',
+                    }}
+                    update={{default: 'none'}}>
+                    <div className="absolute inset-0 bg-highlight rounded-full" />
+                  </ViewTransition>
+                )}
+                <ViewTransition
+                  enter={{default: 'none'}}
+                  exit={{default: 'none'}}
+                  update={{
+                    [TOGGLE_TAB_TRANSITION]: 'tab-text',
+                    default: 'none',
+                  }}>
+                  <span className="relative z-1">{tab}</span>
+                </ViewTransition>
+              </button>
+            );
+          })}
         </div>
-      )}
+        <div className="flex-1 overflow-hidden w-full h-full">
+          {tabs.get(activeTab)}
+        </div>
+      </div>
     </div>
   );
 }

compiler/apps/playground/lib/compilation.ts

@@ -0,0 +1,308 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import {parse as babelParse, ParseResult} from '@babel/parser';
+import * as HermesParser from 'hermes-parser';
+import * as t from '@babel/types';
+import BabelPluginReactCompiler, {
+  CompilerError,
+  CompilerErrorDetail,
+  CompilerDiagnostic,
+  Effect,
+  ErrorCategory,
+  parseConfigPragmaForTests,
+  ValueKind,
+  type Hook,
+  PluginOptions,
+  CompilerPipelineValue,
+  parsePluginOptions,
+  printReactiveFunctionWithOutlined,
+  printFunctionWithOutlined,
+  type LoggerEvent,
+} from 'babel-plugin-react-compiler';
+import {transformFromAstSync} from '@babel/core';
+import type {
+  CompilerOutput,
+  CompilerTransformOutput,
+  PrintedCompilerPipelineValue,
+} from '../components/Editor/Output';
+
+function parseInput(
+  input: string,
+  language: 'flow' | 'typescript',
+): ParseResult<t.File> {
+  // Extract the first line to quickly check for custom test directives
+  if (language === 'flow') {
+    return HermesParser.parse(input, {
+      babel: true,
+      flow: 'all',
+      sourceType: 'module',
+      enableExperimentalComponentSyntax: true,
+    });
+  } else {
+    return babelParse(input, {
+      plugins: ['typescript', 'jsx'],
+      sourceType: 'module',
+    }) as ParseResult<t.File>;
+  }
+}
+
+function invokeCompiler(
+  source: string,
+  language: 'flow' | 'typescript',
+  options: PluginOptions,
+): CompilerTransformOutput {
+  const ast = parseInput(source, language);
+  let result = transformFromAstSync(ast, source, {
+    filename: '_playgroundFile.js',
+    highlightCode: false,
+    retainLines: true,
+    plugins: [[BabelPluginReactCompiler, options]],
+    ast: true,
+    sourceType: 'module',
+    configFile: false,
+    sourceMaps: true,
+    babelrc: false,
+  });
+  if (result?.ast == null || result?.code == null || result?.map == null) {
+    throw new Error('Expected successful compilation');
+  }
+  return {
+    code: result.code,
+    sourceMaps: result.map,
+    language,
+  };
+}
+
+const COMMON_HOOKS: Array<[string, Hook]> = [
+  [
+    'useFragment',
+    {
+      valueKind: ValueKind.Frozen,
+      effectKind: Effect.Freeze,
+      noAlias: true,
+      transitiveMixedData: true,
+    },
+  ],
+  [
+    'usePaginationFragment',
+    {
+      valueKind: ValueKind.Frozen,
+      effectKind: Effect.Freeze,
+      noAlias: true,
+      transitiveMixedData: true,
+    },
+  ],
+  [
+    'useRefetchableFragment',
+    {
+      valueKind: ValueKind.Frozen,
+      effectKind: Effect.Freeze,
+      noAlias: true,
+      transitiveMixedData: true,
+    },
+  ],
+  [
+    'useLazyLoadQuery',
+    {
+      valueKind: ValueKind.Frozen,
+      effectKind: Effect.Freeze,
+      noAlias: true,
+      transitiveMixedData: true,
+    },
+  ],
+  [
+    'usePreloadedQuery',
+    {
+      valueKind: ValueKind.Frozen,
+      effectKind: Effect.Freeze,
+      noAlias: true,
+      transitiveMixedData: true,
+    },
+  ],
+];
+
+function parseOptions(
+  source: string,
+  mode: 'compiler' | 'linter',
+  configOverrides: string,
+): PluginOptions {
+  // Extract the first line to quickly check for custom test directives
+  const pragma = source.substring(0, source.indexOf('\n'));
+
+  const parsedPragmaOptions = parseConfigPragmaForTests(pragma, {
+    compilationMode: 'infer',
+    environment:
+      mode === 'linter'
+        ? {
+            // enabled in compiler
+            validateRefAccessDuringRender: false,
+            // enabled in linter
+            validateNoSetStateInRender: true,
+            validateNoSetStateInEffects: true,
+            validateNoJSXInTryStatements: true,
+            validateNoImpureFunctionsInRender: true,
+            validateStaticComponents: true,
+            validateNoFreezingKnownMutableFunctions: true,
+            validateNoVoidUseMemo: true,
+          }
+        : {
+            /* use defaults for compiler mode */
+          },
+  });
+
+  // Parse config overrides from config editor
+  let configOverrideOptions: any = {};
+  const configMatch = configOverrides.match(/^\s*import.*?\n\n\((.*)\)/s);
+  if (configOverrides.trim()) {
+    if (configMatch && configMatch[1]) {
+      const configString = configMatch[1].replace(/satisfies.*$/, '').trim();
+      configOverrideOptions = new Function(`return (${configString})`)();
+    } else {
+      throw new Error('Invalid override format');
+    }
+  }
+
+  const opts: PluginOptions = parsePluginOptions({
+    ...parsedPragmaOptions,
+    ...configOverrideOptions,
+    environment: {
+      ...parsedPragmaOptions.environment,
+      ...configOverrideOptions.environment,
+      customHooks: new Map([...COMMON_HOOKS]),
+    },
+  });
+
+  return opts;
+}
+
+export function compile(
+  source: string,
+  mode: 'compiler' | 'linter',
+  configOverrides: string,
+): [CompilerOutput, 'flow' | 'typescript', PluginOptions | null] {
+  const results = new Map<string, Array<PrintedCompilerPipelineValue>>();
+  const error = new CompilerError();
+  const otherErrors: Array<CompilerErrorDetail | CompilerDiagnostic> = [];
+  const upsert: (result: PrintedCompilerPipelineValue) => void = result => {
+    const entry = results.get(result.name);
+    if (Array.isArray(entry)) {
+      entry.push(result);
+    } else {
+      results.set(result.name, [result]);
+    }
+  };
+  let language: 'flow' | 'typescript';
+  if (source.match(/\@flow/)) {
+    language = 'flow';
+  } else {
+    language = 'typescript';
+  }
+  let transformOutput;
+
+  let baseOpts: PluginOptions | null = null;
+  try {
+    baseOpts = parseOptions(source, mode, configOverrides);
+  } catch (err) {
+    error.details.push(
+      new CompilerErrorDetail({
+        category: ErrorCategory.Config,
+        reason: `Unexpected failure when transforming configs! \n${err}`,
+        loc: null,
+        suggestions: null,
+      }),
+    );
+  }
+  if (baseOpts) {
+    try {
+      const logIR = (result: CompilerPipelineValue): void => {
+        switch (result.kind) {
+          case 'ast': {
+            break;
+          }
+          case 'hir': {
+            upsert({
+              kind: 'hir',
+              fnName: result.value.id,
+              name: result.name,
+              value: printFunctionWithOutlined(result.value),
+            });
+            break;
+          }
+          case 'reactive': {
+            upsert({
+              kind: 'reactive',
+              fnName: result.value.id,
+              name: result.name,
+              value: printReactiveFunctionWithOutlined(result.value),
+            });
+            break;
+          }
+          case 'debug': {
+            upsert({
+              kind: 'debug',
+              fnName: null,
+              name: result.name,
+              value: result.value,
+            });
+            break;
+          }
+          default: {
+            const _: never = result;
+            throw new Error(`Unhandled result ${result}`);
+          }
+        }
+      };
+      // Add logger options to the parsed options
+      const opts = {
+        ...baseOpts,
+        logger: {
+          debugLogIRs: logIR,
+          logEvent: (_filename: string | null, event: LoggerEvent): void => {
+            if (event.kind === 'CompileError') {
+              otherErrors.push(event.detail);
+            }
+          },
+        },
+      };
+      transformOutput = invokeCompiler(source, language, opts);
+    } catch (err) {
+      /**
+       * error might be an invariant violation or other runtime error
+       * (i.e. object shape that is not CompilerError)
+       */
+      if (err instanceof CompilerError && err.details.length > 0) {
+        error.merge(err);
+      } else {
+        /**
+         * Handle unexpected failures by logging (to get a stack trace)
+         * and reporting
+         */
+        error.details.push(
+          new CompilerErrorDetail({
+            category: ErrorCategory.Invariant,
+            reason: `Unexpected failure when transforming input! \n${err}`,
+            loc: null,
+            suggestions: null,
+          }),
+        );
+      }
+    }
+  }
+  // Only include logger errors if there weren't other errors
+  if (!error.hasErrors() && otherErrors.length !== 0) {
+    otherErrors.forEach(e => error.details.push(e));
+  }
+  if (error.hasErrors() || !transformOutput) {
+    return [{kind: 'err', results, error}, language, baseOpts];
+  }
+  return [
+    {kind: 'ok', results, transformOutput, errors: error.details},
+    language,
+    baseOpts,
+  ];
+}

compiler/apps/playground/lib/defaultStore.ts

@@ -13,10 +13,21 @@ export default function MyApp() {
 }
 `;
 
+export const defaultConfig = `\
+import type { PluginOptions } from 'babel-plugin-react-compiler/dist';
+
+({
+  //compilationMode: "all"
+} satisfies PluginOptions);`;
+
 export const defaultStore: Store = {
   source: index,
+  config: defaultConfig,
+  showInternals: false,
 };
 
 export const emptyStore: Store = {
   source: '',
+  config: '',
+  showInternals: false,
 };

compiler/apps/playground/lib/stores/store.ts

@@ -10,18 +10,20 @@ import {
   compressToEncodedURIComponent,
   decompressFromEncodedURIComponent,
 } from 'lz-string';
-import {defaultStore} from '../defaultStore';
+import {defaultStore, defaultConfig} from '../defaultStore';
 
 /**
  * Global Store for Playground
  */
 export interface Store {
   source: string;
+  config: string;
+  showInternals: boolean;
 }
 export function encodeStore(store: Store): string {
   return compressToEncodedURIComponent(JSON.stringify(store));
 }
-export function decodeStore(hash: string): Store {
+export function decodeStore(hash: string): any {
   return JSON.parse(decompressFromEncodedURIComponent(hash));
 }
 
@@ -62,8 +64,14 @@ export function initStoreFromUrlOrLocalStorage(): Store {
    */
   if (!encodedSource) return defaultStore;
 
-  const raw = decodeStore(encodedSource);
+  const raw: any = decodeStore(encodedSource);
 
   invariant(isValidStore(raw), 'Invalid Store');
-  return raw;
+
+  // Make sure all properties are populated
+  return {
+    source: raw.source,
+    config: 'config' in raw && raw['config'] ? raw.config : defaultConfig,
+    showInternals: 'showInternals' in raw ? raw.showInternals : false,
+  };
 }

compiler/apps/playground/lib/transitionTypes.ts

@@ -0,0 +1,11 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+export const CONFIG_PANEL_TRANSITION = 'config-panel';
+export const TOGGLE_TAB_TRANSITION = 'toggle-tab';
+export const TOGGLE_INTERNALS_TRANSITION = 'toggle-internals';
+export const EXPAND_ACCORDION_TRANSITION = 'open-accordion';

compiler/apps/playground/next-env.d.ts

@@ -1,5 +0,0 @@
-/// <reference types="next" />
-/// <reference types="next/image-types/global" />
-
-// NOTE: This file should not be edited
-// see https://nextjs.org/docs/app/api-reference/config/typescript for more information.

compiler/apps/playground/next.config.js

@@ -11,6 +11,7 @@ const path = require('path');
 const nextConfig = {
   experimental: {
     reactCompiler: true,
+    viewTransition: true,
   },
   reactStrictMode: true,
   webpack: (config, options) => {

compiler/apps/playground/package.json

@@ -26,34 +26,39 @@
     "@babel/traverse": "^7.18.9",
     "@babel/types": "7.26.3",
     "@heroicons/react": "^1.0.6",
-    "@monaco-editor/react": "^4.4.6",
-    "@playwright/test": "^1.51.1",
+    "@monaco-editor/react": "^4.8.0-rc.2",
+    "@playwright/test": "^1.56.1",
     "@use-gesture/react": "^10.2.22",
     "hermes-eslint": "^0.25.0",
     "hermes-parser": "^0.25.0",
     "invariant": "^2.2.4",
+    "lru-cache": "^11.2.2",
     "lz-string": "^1.5.0",
     "monaco-editor": "^0.52.0",
-    "next": "^15.2.0-canary.64",
+    "next": "15.5.9",
     "notistack": "^3.0.0-alpha.7",
     "prettier": "^3.3.3",
     "pretty-format": "^29.3.1",
     "re-resizable": "^6.9.16",
-    "react": "^19.0.0",
-    "react-dom": "^19.0.0"
+    "react": "19.2.3",
+    "react-dom": "19.2.3"
   },
   "devDependencies": {
     "@types/node": "18.11.9",
-    "@types/react": "^19.0.0",
-    "@types/react-dom": "^19.0.0",
+    "@types/react": "19.2",
+    "@types/react-dom": "19.2",
     "autoprefixer": "^10.4.13",
     "clsx": "^1.2.1",
     "concurrently": "^7.4.0",
     "eslint": "^8.28.0",
-    "eslint-config-next": "^15.0.1",
+    "eslint-config-next": "15.5.2",
     "monaco-editor-webpack-plugin": "^7.1.0",
     "postcss": "^8.4.31",
     "tailwindcss": "^3.2.4",
     "wait-on": "^7.2.0"
+  },
+  "resolutions": {
+    "@types/react": "19.2",
+    "@types/react-dom": "19.2"
   }
 }

compiler/apps/playground/playwright.config.js

@@ -55,12 +55,16 @@ export default defineConfig({
     // contextOptions: {
     //   ignoreHTTPSErrors: true,
     // },
+    viewport: {width: 1920, height: 1080},
   },
 
   projects: [
     {
       name: 'chromium',
-      use: {...devices['Desktop Chrome']},
+      use: {
+        ...devices['Desktop Chrome'],
+        viewport: {width: 1920, height: 1080},
+      },
     },
     // {
     //   name: 'Desktop Firefox',

compiler/apps/playground/styles/globals.css

@@ -69,3 +69,75 @@
     scrollbar-width: none; /* Firefox */
   }
 }
+
+::view-transition-old(.slide-in) {
+  animation-name: slideOutLeft;
+}
+::view-transition-new(.slide-in) {
+  animation-name: slideInLeft;
+}
+::view-transition-group(.slide-in) {
+  z-index: 1;
+}
+::view-transition-old(.slide-out) {
+  animation-name: slideOutLeft;
+}
+::view-transition-new(.slide-out) {
+  animation-name: slideInLeft;
+}
+::view-transition-group(.slide-out) {
+  z-index: 1;
+}
+
+@keyframes slideOutLeft {
+  from {
+    transform: translateX(0);
+  }
+  to {
+    transform: translateX(-100%);
+  }
+}
+@keyframes slideInLeft {
+  from {
+    transform: translateX(-100%);
+  }
+  to {
+    transform: translateX(0);
+  }
+}
+
+::view-transition-old(.container),
+::view-transition-new(.container) {
+  height: 100%;
+}
+
+::view-transition-old(.accordion-container),
+::view-transition-new(.accordion-container) {
+  height: 100%;
+  object-fit: none;
+  object-position: left;
+}
+
+::view-transition-old(.tab-highlight),
+::view-transition-new(.tab-highlight) {
+  height: 100%;
+}
+::view-transition-group(.tab-text) {
+	z-index: 1;
+}
+
+::view-transition-old(.expand-accordion),
+::view-transition-new(.expand-accordion) {
+  width: auto;
+}
+::view-transition-group(.expand-accordion) {
+  overflow: clip;
+}
+
+/**
+ * For some reason, the original Monaco editor is still visible to the
+ * left of the DiffEditor. This is a workaround for better visual clarity.
+ */
+.monaco-diff-editor .editor.original{
+  visibility: hidden !important;
+}

compiler/apps/playground/tsconfig.json

@@ -6,6 +6,9 @@
       "dom.iterable",
       "esnext"
     ],
+    "types": [
+      "react/experimental"
+    ],
     "allowJs": true,
     "skipLibCheck": true,
     "strict": true,

compiler/docs/DEVELOPMENT_GUIDE.md

@@ -17,7 +17,32 @@ yarn snap:build
 yarn snap --watch
 ```
 
-`snap` is our custom test runner, which creates "golden" test files that have the expected output for each input fixture, as well as the results of executing a specific input (or sequence of inputs) in both the uncompiled and compiler versions of the input. 
+`snap` is our custom test runner, which creates "golden" test files that have the expected output for each input fixture, as well as the results of executing a specific input (or sequence of inputs) in both the uncompiled and compiler versions of the input.
+
+### Compiling Arbitrary Files
+
+You can compile any file (not just fixtures) using:
+
+```sh
+# Compile a file and see the output
+yarn snap compile <path>
+
+# Compile with debug output to see the state after each compiler pass
+# This is an alternative to `yarn snap -d -p <pattern>` when you don't have a fixture file yet
+yarn snap compile --debug <path>
+```
+
+### Minimizing Test Cases
+
+To reduce a failing test case to its minimal reproduction:
+
+```sh
+# Minimize a file that causes a compiler error
+yarn snap minimize <path>
+
+# Minimize and update the file in-place
+yarn snap minimize --update <path>
+```
 
 When contributing changes, we prefer to:
 * Add one or more fixtures that demonstrate the current compiled output for a particular combination of input and configuration. Send this as a first PR.

compiler/fault-tolerance-overview.md

@@ -0,0 +1,333 @@
+## React Compiler Fault Tolerance
+
+Update React Compiler (@compiler/ directory) to always run all passes and return either the transformed code (if no error) or a list of one or more compilation errors. 
+
+## Background
+
+Currently React Compiler runs through a series of passes in Pipeline.ts. If an error occurs in a pass the compiler will generally either throw the error in the pass where it occurs, or return a Result<_, CompilerError> which is then unwrapped in Pipeline.ts, throwing there. This means that a single error that triggers early can prevent later validation from running, meaning the user has to first fix one error in order to see another.
+
+## New Approach
+
+The compiler should always run all passes in the pipeline, up to and including CodegenReactiveFunction. During this process it should accumulate errors. If at the end of compilation there were no accumulated errors, return `Ok(generatedfunction)`. Else, return `Err(CompilerError)` with *all* the accumulated errors.
+
+Note that some errors may continue to cause an eager bailout:
+* If an error is not an instanceof CompilerError, throw it as it occurs
+* If an error is a CompilerError invariant, throw it as it occurs since this represents a truly exceptional, unexpected case
+
+## Detailed Design
+
+* The Environment needs a way to record errors as compilation proceeds. This should generally store the error (and log, if a logger is configured), but should immediately throw if the error is an invariant (see above).
+* BuildHIR should always produce an HIR without error. For syntax forms that are unsupported (currently throwing a Todo error), we should instead construct record the todo error on the environment, and construct a partial HIR. The exact form of the partial HIR can be situation specific:
+  * `var` is currently unsupported, but we could pretend it was `let`
+  * `finally` blocks are unsupported, we could just prune them, or move the code after the try/catch (put the finally logic in the consequent)
+  * This may mean updating the HIR to allow representing partial code
+  * `eval()` can just be an Unsupported InstructionValue variant
+* All of the passes need to be updated to stop returning Result or CompilerError, and instead record their errors on the environment. They should always be able to proceed even in the presence of errors. For example, in InferMutationAliasingEffects if we discover that the code mutates a frozen value, we can record this as an error and then just pretend the mutation didn't happen - ie construct a scope as if the mutating code was not a mutation after all.
+* Finally, the end of the pipeline should check for errors and either turn `Ok(GeneratedFunction)` or `Err(aggregatedErrors)`. The code calling into the pipeline then needs to handle this appropriately.
+
+## Detailed Plan
+
+### Phase 1: Environment Error Accumulation Infrastructure
+
+Add error accumulation to the `Environment` class so that any pass can record errors during compilation without halting.
+
+- [x] **1.1 Add error accumulator to Environment** (`src/HIR/Environment.ts`)
+  - Add a `#errors: CompilerError` field, initialized in the constructor
+  - Add a `recordError(error: CompilerDiagnostic | CompilerErrorDetail)` method that:
+    - If an Invariant-category detail, immediately throw it
+    - Otherwise, push the diagnostic/detail onto `#errors` (and log via `this.logger` if configured)
+  - Add a `recordErrors(error: CompilerError)` method that calls `recordError()` for each of the details on the given error.
+  - Add a `hasErrors(): boolean` getter
+  - Add a `aggregateErrors(): CompilerError` method that returns the accumulated error object
+  - Consider whether `recordError` should accept the same options as `CompilerError.push()` for convenience (reason, description, severity, loc, etc.)
+
+- [x] **1.2 Add a `tryRecord` helper on Environment** (`src/HIR/Environment.ts`)
+  - Add a `tryRecord(fn: () => void): void` method that wraps a callback in try/catch:
+    - If `fn` throws a `CompilerError` that is NOT an invariant, record it via `recordError`
+    - If `fn` throws a non-CompilerError or a CompilerError invariant, re-throw
+  - This helper is the migration path for passes that currently throw: wrap their call in `env.tryRecord(() => pass(hir))` so exceptions become recorded errors
+
+### Phase 2: Update Pipeline.ts to Accumulate Errors
+
+Change `runWithEnvironment` to run all passes and check for errors at the end instead of letting exceptions propagate.
+
+- [x] **2.1 Change `runWithEnvironment` return type** (`src/Entrypoint/Pipeline.ts`)
+  - Change return type from `CodegenFunction` to `Result<CodegenFunction, CompilerError>`
+  - At the end of the pipeline, check `env.hasErrors()`:
+    - If no errors: return `Ok(ast)`
+    - If errors: return `Err(env.aggregateErrors())`
+
+- [x] **2.2 Update `compileFn` to propagate the Result** (`src/Entrypoint/Pipeline.ts`)
+  - Change `compileFn` return type from `CodegenFunction` to `Result<CodegenFunction, CompilerError>`
+  - Propagate the Result from `runWithEnvironment`
+
+- [x] **2.3 Update `run` to propagate the Result** (`src/Entrypoint/Pipeline.ts`)
+  - Same change for the internal `run` function
+
+- [x] **2.4 Update callers in Program.ts** (`src/Entrypoint/Program.ts`)
+  - In `tryCompileFunction`, change from try/catch around `compileFn` to handling the `Result`:
+    - If `Ok(codegenFn)`: return the compiled function
+    - If `Err(compilerError)`: return `{kind: 'error', error: compilerError}`
+    - Keep the try/catch only for truly unexpected (non-CompilerError) exceptions and invariants
+  - The existing `handleError`/`logError`/`panicThreshold` logic in `processFn` should continue to work unchanged since it already handles `CompilerError` instances
+
+### Phase 3: Update BuildHIR (lower) to Always Produce HIR
+
+Currently `lower()` returns `Result<HIRFunction, CompilerError>`. It already accumulates errors internally via `builder.errors`, but returns `Err` when errors exist. Change it to always return `Ok(hir)` while recording errors on the environment.
+
+- [x] **3.1 Change `lower` to always return HIRFunction** (`src/HIR/BuildHIR.ts`)
+  - Change return type from `Result<HIRFunction, CompilerError>` to `HIRFunction`
+  - Instead of returning `Err(builder.errors)` at line 227-229, record errors on `env` via `env.recordErrors(builder.errors)` and return the (partial) HIR
+  - Update the pipeline to call `lower(func, env)` directly instead of `lower(func, env).unwrap()`
+  - Added try/catch around body lowering to catch thrown CompilerErrors (e.g., from `resolveBinding`) and record them
+
+- [x] **3.2 Handle `var` declarations as `let`** (`src/HIR/BuildHIR.ts`, line ~855)
+  - Record the Todo error, then treat `var` as `let` and continue lowering (instead of skipping the declaration)
+
+- [x] **3.3 Handle `try/finally` by pruning `finally`** (`src/HIR/BuildHIR.ts`, lines ~1281-1296)
+  - Already handled: `try` without `catch` pushes error and returns; `try` with `finally` pushes error and continues with `try/catch` portion only
+
+- [x] **3.4 Handle `eval()` via UnsupportedNode** (`src/HIR/BuildHIR.ts`, line ~3568)
+  - Already handled: records error via `builder.errors.push()` and continues
+
+- [x] **3.5 Handle `with` statement via UnsupportedNode** (`src/HIR/BuildHIR.ts`, line ~1382)
+  - Already handled: records error and emits `UnsupportedNode`
+
+- [x] **3.6 Handle inline `class` declarations** (`src/HIR/BuildHIR.ts`, line ~1402)
+  - Already handled: records error and emits `UnsupportedNode`
+
+- [x] **3.7 Handle remaining Todo errors in expression lowering** (`src/HIR/BuildHIR.ts`)
+  - Already handled: all ~60 error sites use `builder.errors.push()` to accumulate errors. The try/catch around body lowering provides a safety net for any that still throw.
+
+- [x] **3.8 Handle `throw` inside `try/catch`** (`src/HIR/BuildHIR.ts`, line ~284)
+  - Already handled: records error via `builder.errors.push()` and continues
+
+- [x] **3.9 Handle `for` loops with missing test or expression init** (`src/HIR/BuildHIR.ts`, lines ~559, ~632)
+  - For `for(;;)` (missing test): emit `true` as the test expression and add a branch terminal
+  - For empty init (`for (; ...)`): add a placeholder instruction to avoid invariant about empty blocks
+  - For expression init (`for (expr; ...)`): record error and lower the expression as best-effort
+  - Changed `'unsupported'` terminal to `'goto'` terminal for non-variable init to maintain valid CFG structure
+
+- [x] **3.10 Handle nested function lowering failures** (`src/HIR/BuildHIR.ts`, `lowerFunction` at line ~3504)
+  - `lowerFunction()` now always returns `LoweredFunction` since `lower()` always returns `HIRFunction`
+  - Errors from nested functions are recorded on the shared environment
+  - Removed the `null` return case and the corresponding `UnsupportedNode` fallback in callers
+
+- [x] **3.11 Handle unreachable functions in `build()`** (`src/HIR/HIRBuilder.ts`, `build()`)
+  - Changed `CompilerError.throwTodo()` for unreachable code with hoisted declarations to `this.errors.push()` to allow HIR construction to complete
+
+- [x] **3.12 Handle duplicate fbt tags** (`src/HIR/BuildHIR.ts`, line ~2279)
+  - Changed `CompilerError.throwDiagnostic()` to `builder.errors.pushDiagnostic()` to record instead of throw
+
+### Phase 4: Update Validation Passes
+
+All validation passes need to record errors on the environment instead of returning `Result` or throwing. They should still detect the same problems, but the pipeline should continue after each one.
+
+#### Pattern A passes (currently return `Result`, called with `.unwrap()`)
+
+These passes already accumulate errors internally and return `Result<void, CompilerError>`. The change is: instead of returning the Result, record errors on `env` and return void. Remove the `.unwrap()` call in Pipeline.ts.
+
+- [x] **4.1 `validateHooksUsage`** (`src/Validation/ValidateHooksUsage.ts`)
+  - Change signature from `(fn: HIRFunction): Result<void, CompilerError>` to `(fn: HIRFunction): void`
+  - Record errors on `fn.env` instead of returning `errors.asResult()`
+  - Update Pipeline.ts call site (line 211): remove `.unwrap()`
+
+- [x] **4.2 `validateNoCapitalizedCalls`** (`src/Validation/ValidateNoCapitalizedCalls.ts`)
+  - Change signature to return void
+  - Fix the hybrid pattern: the direct `CallExpression` path currently throws via `CompilerError.throwInvalidReact()` — change to record on env
+  - The `MethodCall` path already accumulates — change to record on env
+  - Update Pipeline.ts call site (line 214): remove `.unwrap()`
+
+- [x] **4.3 `validateUseMemo`** (`src/Validation/ValidateUseMemo.ts`)
+  - Change signature to return void
+  - Record hard errors on env instead of returning `errors.asResult()`
+  - The soft `voidMemoErrors` path already uses `env.logErrors()` — keep as-is or also record
+  - Update Pipeline.ts call site (line 170): remove `.unwrap()`
+
+- [x] **4.4 `dropManualMemoization`** (`src/Inference/DropManualMemoization.ts`)
+  - Change signature to return void
+  - Record errors on env instead of returning `errors.asResult()`
+  - Update Pipeline.ts call site (line 178): remove `.unwrap()`
+
+- [x] **4.5 `validateNoRefAccessInRender`** (`src/Validation/ValidateNoRefAccessInRender.ts`)
+  - Change signature to return void
+  - Record errors on env instead of returning Result
+  - Update Pipeline.ts call site (line 275): remove `.unwrap()`
+
+- [x] **4.6 `validateNoSetStateInRender`** (`src/Validation/ValidateNoSetStateInRender.ts`)
+  - Change signature to return void
+  - Record errors on env
+  - Update Pipeline.ts call site (line 279): remove `.unwrap()`
+
+- [x] **4.7 `validateNoImpureFunctionsInRender`** (`src/Validation/ValidateNoImpureFunctionsInRender.ts`)
+  - Change signature to return void
+  - Record errors on env
+  - Update Pipeline.ts call site (line 300): remove `.unwrap()`
+
+- [x] **4.8 `validateNoFreezingKnownMutableFunctions`** (`src/Validation/ValidateNoFreezingKnownMutableFunctions.ts`)
+  - Change signature to return void
+  - Record errors on env
+  - Update Pipeline.ts call site (line 303): remove `.unwrap()`
+
+- [x] **4.9 `validateExhaustiveDependencies`** (`src/Validation/ValidateExhaustiveDependencies.ts`)
+  - Change signature to return void
+  - Record errors on env
+  - Update Pipeline.ts call site (line 315): remove `.unwrap()`
+
+- [x] **4.10 `validateMemoizedEffectDependencies`** (`src/Validation/ValidateMemoizedEffectDependencies.ts`)
+  - Change signature to return void (note: operates on `ReactiveFunction`)
+  - Record errors on the function's env
+  - Update Pipeline.ts call site (line 565): remove `.unwrap()`
+
+- [x] **4.11 `validatePreservedManualMemoization`** (`src/Validation/ValidatePreservedManualMemoization.ts`)
+  - Change signature to return void (note: operates on `ReactiveFunction`)
+  - Record errors on the function's env
+  - Update Pipeline.ts call site (line 572): remove `.unwrap()`
+
+- [x] **4.12 `validateSourceLocations`** (`src/Validation/ValidateSourceLocations.ts`)
+  - Change signature to return void
+  - Record errors on env
+  - Update Pipeline.ts call site (line 585): remove `.unwrap()`
+
+#### Pattern B passes (currently use `env.logErrors()`)
+
+These already use a soft-logging pattern and don't block compilation. They can be migrated to `env.recordError()` so all errors are aggregated in one place.
+
+- [ ] **4.13 `validateNoDerivedComputationsInEffects_exp`** — change to record on env directly
+- [ ] **4.14 `validateNoSetStateInEffects`** — change to record on env directly
+- [ ] **4.15 `validateNoJSXInTryStatement`** — change to record on env directly
+- [ ] **4.16 `validateStaticComponents`** — change to record on env directly
+
+#### Pattern D passes (currently throw directly, no Result)
+
+These throw `CompilerError` directly (not via Result). They need the most work.
+
+- [x] **4.17 `validateContextVariableLValues`** (`src/Validation/ValidateContextVariableLValues.ts`)
+  - Currently throws via `CompilerError.throwTodo()` and `CompilerError.invariant()`
+  - Change to record Todo errors on env and continue
+  - Keep invariant throws (those indicate internal bugs)
+
+- [x] **4.18 `validateLocalsNotReassignedAfterRender`** (`src/Validation/ValidateLocalsNotReassignedAfterRender.ts`)
+  - Currently constructs a `CompilerError` and `throw`s it directly
+  - Change to record errors on env
+
+- [x] **4.19 `validateNoDerivedComputationsInEffects`** (`src/Validation/ValidateNoDerivedComputationsInEffects.ts`)
+  - Currently throws directly
+  - Change to record errors on env
+
+### Phase 5: Update Inference Passes
+
+The inference passes are the most critical to handle correctly because they produce side effects (populating effects on instructions, computing mutable ranges) that downstream passes depend on. They must continue producing valid (even if imprecise) output when errors are encountered.
+
+- [x] **5.1 `inferMutationAliasingEffects`** (`src/Inference/InferMutationAliasingEffects.ts`)
+  - Currently returns `Result<void, CompilerError>` — errors are about mutation of frozen/global values
+  - Change to record errors on `fn.env` instead of accumulating internally
+  - **Key recovery strategy**: When a mutation of a frozen value is detected, record the error but treat the operation as a non-mutating read. This way downstream passes see a consistent (if conservative) view
+  - When a mutation of a global is detected, record the error but continue with the global unchanged
+  - Update Pipeline.ts (lines 233-239): remove the conditional `.isErr()` / throw pattern
+
+- [x] **5.2 `inferMutationAliasingRanges`** (`src/Inference/InferMutationAliasingRanges.ts`)
+  - Currently returns `Result<Array<AliasingEffect>, CompilerError>`
+  - This pass has a meaningful success value (the function's external aliasing effects)
+  - Change to: always produce a best-effort effects array, record errors on env
+  - When errors are encountered, produce conservative effects (e.g., assume no external mutation)
+  - Update Pipeline.ts (lines 258-267): remove the conditional throw pattern, call directly
+
+### Phase 6: Update Codegen
+
+- [x] **6.1 `codegenFunction`** (`src/ReactiveScopes/CodegenReactiveFunction.ts`)
+  - Currently returns `Result<CodegenFunction, CompilerError>`
+  - Change to: always produce a `CodegenFunction`, record errors on env
+  - If codegen encounters an error (e.g., an instruction it can't generate code for), it should:
+    - Record the error
+    - For `UnsupportedNode` values: pass through the original AST node (already works this way)
+    - For other error cases: emit a placeholder or the original AST where possible
+  - Update Pipeline.ts (line 575-578): remove `.unwrap()`
+
+### Phase 7: Pipeline.ts Pass-by-Pass Migration
+
+Walk through `runWithEnvironment` and wrap each pass call site. This is the integration work tying Phases 3-6 together.
+
+- [x] **7.1 Wrap `lower()` call** (line 163)
+  - Change from `lower(func, env).unwrap()` to `lower(func, env)` (direct return after Phase 3.1)
+
+- [x] **7.2 Wrap validation calls that use `.unwrap()`** (lines 169-303)
+  - Remove `.unwrap()` from all validation calls after they're updated in Phase 4
+  - For validations guarded by `env.enableValidations`, keep the guard but remove the `.unwrap()`
+
+- [x] **7.3 Wrap inference calls** (lines 233-267)
+  - After Phase 5, `inferMutationAliasingEffects` and `inferMutationAliasingRanges` record errors directly
+  - Remove the `mutabilityAliasingErrors` / `mutabilityAliasingRangeErrors` variables and their conditional throw logic
+
+- [x] **7.4 Wrap `env.logErrors()` calls** (lines 286-331)
+  - After Phase 4.13-4.16, these passes record on env directly
+  - Remove the `env.logErrors()` wrapper calls
+
+- [x] **7.5 Wrap codegen** (lines 575-578)
+  - After Phase 6.1, `codegenFunction` returns directly
+  - Remove the `.unwrap()`
+
+- [x] **7.6 Add final error check** (end of `runWithEnvironment`)
+  - After all passes complete, check `env.hasErrors()`
+  - If no errors: return `Ok(ast)`
+  - If errors: return `Err(env.aggregateErrors())`
+
+- [x] **7.7 Consider wrapping each pass in `env.tryRecord()`** as a safety net
+  - Even after individual passes are updated, wrapping each pass call in `env.tryRecord()` provides defense-in-depth
+  - If a pass unexpectedly throws a CompilerError (e.g., from a code path we missed), it gets caught and recorded rather than aborting the pipeline
+  - Non-CompilerError exceptions and invariants still propagate immediately
+
+### Phase 8: Testing
+
+- [x] **8.1 Update existing `error.todo-*` fixture expectations**
+  - Currently, fixtures with `error.todo-` prefix expect a single error and bailout
+  - After fault tolerance, some of these may now produce multiple errors
+  - Update the `.expect.md` files to reflect the new aggregated error output
+
+- [x] **8.2 Add multi-error test fixtures**
+  - Create test fixtures that contain multiple independent errors (e.g., both a `var` declaration and a mutation of a frozen value)
+  - Verify that all errors are reported, not just the first one
+
+- [x] **8.3 Add test for invariant-still-throws behavior**
+  - Verify that `CompilerError.invariant()` failures still cause immediate abort
+  - Verify that non-CompilerError exceptions still cause immediate abort
+
+- [x] **8.4 Add test for partial HIR codegen**
+  - Verify that when BuildHIR produces partial HIR (with `UnsupportedNode` values), later passes handle it gracefully and codegen produces the original AST for unsupported portions
+
+- [x] **8.5 Verify error severity in aggregated output**
+  - Test that the aggregated `CompilerError` correctly reports `hasErrors()` vs `hasWarning()` vs `hasHints()` based on the mix of accumulated diagnostics
+  - Verify that `panicThreshold` behavior in Program.ts is correct for aggregated errors
+
+- [x] **8.6 Run full test suite**
+  - Run `yarn snap` and `yarn snap -u` to update all fixture expectations
+  - Ensure no regressions in passing tests
+
+### Implementation Notes
+
+**Ordering**: Phases 1 → 2 → 3 → 4/5/6 (parallel) → 7 → 8. Phase 1 (Environment infrastructure) is the foundation. Phase 2 (Pipeline return type) sets up the contract. Phases 3-6 can be done incrementally — each pass can be migrated independently using `env.tryRecord()` as a transitional wrapper. Phase 7 is the integration. Phase 8 validates everything.
+
+**Incremental migration path**: Rather than updating all passes at once, each pass can be individually migrated. During the transition:
+1. First add `env.tryRecord()` in Phase 7.7 around all pass calls in the pipeline — this immediately provides fault tolerance by catching any thrown CompilerError
+2. Then individually update passes (Phases 3-6) to record errors directly on env, which is cleaner but not required for the basic behavior
+3. This means the feature can be landed incrementally: Phase 1 + 2 + 7.7 gives basic fault tolerance, then individual passes can be refined over time
+
+**What NOT to change**:
+- `CompilerError.invariant()` must continue to throw immediately — these represent internal bugs
+- Non-CompilerError exceptions must continue to throw — these are unexpected JS errors
+- The `assertConsistentIdentifiers`, `assertTerminalSuccessorsExist`, `assertTerminalPredsExist`, `assertValidBlockNesting`, `assertValidMutableRanges`, `assertWellFormedBreakTargets`, `assertScopeInstructionsWithinScopes` assertion functions should continue to throw — they are invariant checks on internal data structure consistency
+- The `panicThreshold` mechanism in Program.ts should continue to work — it now operates on the aggregated error from the Result rather than a caught exception, but the behavior is the same
+
+## Key Learnings
+
+* **Phase 2+7 (Pipeline tryRecord wrapping) was sufficient for basic fault tolerance.** Wrapping all passes in `env.tryRecord()` immediately enabled the compiler to continue past errors that previously threw. This caused 52 test fixtures to produce additional errors that were previously masked by the first error bailing out. For example, `error.todo-reassign-const` previously reported only "Support destructuring of context variables" but now also reports the immutability violation.
+* **Lint-only passes (Pattern B: `env.logErrors()`) should not use `tryRecord()`/`recordError()`** because those errors are intentionally non-blocking. They are reported via the logger only and should not cause the pipeline to return `Err`. The `logErrors` pattern was kept for `validateNoDerivedComputationsInEffects_exp`, `validateNoSetStateInEffects`, `validateNoJSXInTryStatement`, and `validateStaticComponents`.
+* **Inference passes that return `Result` with validation errors** (`inferMutationAliasingEffects`, `inferMutationAliasingRanges`) were changed to record errors via `env.recordErrors()` instead of throwing, allowing subsequent passes to proceed.
+* **Value-producing passes** (`memoizeFbtAndMacroOperandsInSameScope`, `renameVariables`, `buildReactiveFunction`) need safe default values when wrapped in `tryRecord()` since the callback can't return values. We initialize with empty defaults (e.g., `new Set()`) before the `tryRecord()` call.
+* **Phase 3 (BuildHIR) revealed that most error sites already used `builder.errors.push()` for accumulation.** The existing lowering code was designed to accumulate errors rather than throw. The main changes were: (1) changing `lower()` return type from `Result` to `HIRFunction`, (2) recording builder errors on env, (3) adding a try/catch around body lowering to catch thrown CompilerErrors from sub-calls like `resolveBinding()`, (4) treating `var` as `let` instead of skipping declarations, and (5) fixing ForStatement init/test handling to produce valid CFG structure.
+* **Partial HIR can trigger downstream invariants.** When lowering skips or partially handles constructs (e.g., unreachable hoisted functions, `var` declarations before the fix), downstream passes like `InferMutationAliasingEffects` may encounter uninitialized identifiers and throw invariants. This is acceptable since the function still correctly bails out of compilation, but error messages may be less specific. The fix for `var` (treating as `let`) demonstrates how to avoid this: continue lowering with a best-effort representation rather than skipping entirely.
+* **Errors accumulated on `env` are lost when an invariant propagates out of the pipeline.** Since invariant CompilerErrors always re-throw through `tryRecord()`, they exit the pipeline as exceptions. The caller only sees the invariant error, not any errors previously recorded on `env`. This is a design limitation that could be addressed by aggregating env errors with caught exceptions in `tryCompileFunction()`.
+* **Dedicated fault tolerance test fixtures** were added in `__tests__/fixtures/compiler/fault-tolerance/`. Each fixture combines two or more errors from different passes to verify the compiler reports all of them rather than short-circuiting on the first. Coverage includes: `var`+props mutation (BuildHIR→InferMutationAliasingEffects), `var`+ref access (BuildHIR→ValidateNoRefAccessInRender), `try/finally`+props mutation (BuildHIR→InferMutationAliasingEffects), `try/finally`+ref access (BuildHIR→ValidateNoRefAccessInRender), and a 3-error test combining try/finally+ref access+props mutation.
+* **Cleanup: consistent `tryRecord()` wrapping in Pipeline.ts.** All validation passes and inference passes are now wrapped in `env.tryRecord()` for defense-in-depth, consistent with the approach used for transform passes. Previously only transform passes were wrapped. Merged duplicate `env.enableValidations` guard blocks. Pattern B lint-only passes (`env.logErrors()`) were intentionally not wrapped since they use a different error recording strategy.
+* **Cleanup: normalized validation error recording pattern.** Four validation passes (`ValidateNoDerivedComputationsInEffects`, `ValidateMemoizedEffectDependencies`, `ValidatePreservedManualMemoization`, `ValidateSourceLocations`) were using `for (const detail of errors.details) { env.recordError(detail); }` instead of the simpler `env.recordErrors(errors)`. Normalized to use the batch method.
+

compiler/package.json

@@ -19,7 +19,8 @@
     "test": "yarn workspaces run test",
     "snap": "yarn workspace babel-plugin-react-compiler run snap",
     "snap:build": "yarn workspace snap run build",
-    "npm:publish": "node scripts/release/publish"
+    "npm:publish": "node scripts/release/publish",
+    "eslint-docs": "yarn workspace babel-plugin-react-compiler build && node scripts/build-eslint-docs.js"
   },
   "dependencies": {
     "fs-extra": "^4.0.2",

compiler/packages/babel-plugin-react-compiler/docs/passes/01-lower.md

@@ -0,0 +1,157 @@
+# lower (BuildHIR)
+
+## File
+`src/HIR/BuildHIR.ts`
+
+## Purpose
+Converts a Babel AST function node into a High-level Intermediate Representation (HIR), which represents code as a control-flow graph (CFG) with basic blocks, instructions, and terminals. This is the first major transformation pass in the React Compiler pipeline, enabling precise expression-level memoization analysis.
+
+## Input Invariants
+- Input must be a valid Babel `NodePath<t.Function>` (FunctionDeclaration, FunctionExpression, or ArrowFunctionExpression)
+- The function must be a component or hook (determined by the environment)
+- Babel scope analysis must be available for binding resolution
+- An `Environment` instance must be provided with compiler configuration
+- Optional `bindings` map for nested function lowering (recursive calls)
+- Optional `capturedRefs` map for context variables captured from outer scope
+
+## Output Guarantees
+- Returns `Result<HIRFunction, CompilerError>` - either a successfully lowered function or compilation errors
+- The HIR function contains:
+  - A complete CFG with basic blocks (`body.blocks: Map<BlockId, BasicBlock>`)
+  - Each block has an array of instructions and exactly one terminal
+  - All control flow is explicit (if/else, loops, switch, logical operators, ternary)
+  - Parameters are converted to `Place` or `SpreadPattern`
+  - Context captures are tracked in `context` array
+  - Function metadata (id, async, generator, directives)
+- All identifiers get unique `IdentifierId` values
+- Instructions have placeholder instruction IDs (set to 0, assigned later)
+- Effects are null (populated by later inference passes)
+
+## Algorithm
+The lowering algorithm uses a recursive descent pattern with a `HIRBuilder` helper class:
+
+1. **Initialization**: Create an `HIRBuilder` with environment and optional bindings. Process captured context variables.
+
+2. **Parameter Processing**: For each function parameter:
+   - Simple identifiers: resolve binding and create Place
+   - Patterns (object/array): create temporary Place, then emit destructuring assignments
+   - Rest elements: wrap in SpreadPattern
+   - Unsupported: emit Todo error
+
+3. **Body Processing**:
+   - Arrow function expressions: lower body expression to temporary, emit implicit return
+   - Block statements: recursively lower each statement
+
+4. **Statement Lowering** (`lowerStatement`): Handle each statement type:
+   - **Control flow**: Create separate basic blocks for branches, loops connect back to conditional blocks
+   - **Variable declarations**: Create `DeclareLocal`/`DeclareContext` or `StoreLocal`/`StoreContext` instructions
+   - **Expressions**: Lower to temporary and discard result
+   - **Hoisting**: Detect forward references and emit `DeclareContext` for hoisted identifiers
+
+5. **Expression Lowering** (`lowerExpression`): Convert expressions to `InstructionValue`:
+   - **Identifiers**: Create `LoadLocal`, `LoadContext`, or `LoadGlobal` based on binding
+   - **Literals**: Create `Primitive` values
+   - **Operators**: Create `BinaryExpression`, `UnaryExpression` etc.
+   - **Calls**: Distinguish `CallExpression` vs `MethodCall` (member expression callee)
+   - **Control flow expressions**: Create separate value blocks for branches (ternary, logical, optional chaining)
+   - **JSX**: Lower to `JsxExpression` with lowered tag, props, and children
+
+6. **Block Management**: The builder maintains:
+   - A current work-in-progress block accumulating instructions
+   - Completed blocks map
+   - Scope stack for break/continue resolution
+   - Exception handler stack for try/catch
+
+7. **Termination**: Add implicit void return at end if no explicit return
+
+## Key Data Structures
+
+### HIRBuilder (from HIRBuilder.ts)
+- `#current: WipBlock` - Work-in-progress block being populated
+- `#completed: Map<BlockId, BasicBlock>` - Finished blocks
+- `#scopes: Array<Scope>` - Stack for break/continue target resolution (LoopScope, LabelScope, SwitchScope)
+- `#exceptionHandlerStack: Array<BlockId>` - Stack of catch handlers for try/catch
+- `#bindings: Bindings` - Map of variable names to their identifiers
+- `#context: Map<t.Identifier, SourceLocation>` - Captured context variables
+- Methods: `push()`, `reserve()`, `enter()`, `terminate()`, `terminateWithContinuation()`
+
+### Core HIR Types
+- **BasicBlock**: Contains `instructions: Array<Instruction>`, `terminal: Terminal`, `preds: Set<BlockId>`, `phis: Set<Phi>`, `kind: BlockKind`
+- **Instruction**: Contains `id`, `lvalue` (Place), `value` (InstructionValue), `effects` (null initially), `loc`
+- **Terminal**: Block terminator - `if`, `branch`, `goto`, `return`, `throw`, `for`, `while`, `switch`, `ternary`, `logical`, etc.
+- **Place**: Reference to a value - `{kind: 'Identifier', identifier, effect, reactive, loc}`
+- **InstructionValue**: The operation - `LoadLocal`, `StoreLocal`, `CallExpression`, `BinaryExpression`, `FunctionExpression`, etc.
+
+### Block Kinds
+- `block` - Regular sequential block
+- `loop` - Loop header/test block
+- `value` - Block that produces a value (ternary/logical branches)
+- `sequence` - Sequence expression block
+- `catch` - Exception handler block
+
+## Edge Cases
+
+1. **Hoisting**: Forward references to `let`/`const`/`function` declarations emit `DeclareContext` before the reference, enabling correct temporal dead zone handling
+
+2. **Context Variables**: Variables captured by nested functions use `LoadContext`/`StoreContext` instead of `LoadLocal`/`StoreLocal`
+
+3. **For-of/For-in Loops**: Synthesize iterator instructions (`GetIterator`, `IteratorNext`, `NextPropertyOf`)
+
+4. **Optional Chaining**: Creates nested `OptionalTerminal` structures with short-circuit branches
+
+5. **Logical Expressions**: Create branching structures where left side stores to temporary, right side only evaluated if needed
+
+6. **Try/Catch**: Adds `MaybeThrowTerminal` after each instruction in try block, modeling potential control flow to handler
+
+7. **JSX in fbt**: Tracks `fbtDepth` counter to handle whitespace differently in fbt/fbs tags
+
+8. **Unsupported Syntax**: `var` declarations, `with` statements, inline `class` declarations, `eval` - emit appropriate errors
+
+## TODOs
+- `returnTypeAnnotation: null, // TODO: extract the actual return type node if present`
+- `TODO(gsn): In the future, we could only pass in the context identifiers that are actually used by this function and its nested functions`
+- Multiple `// TODO remove type cast` in destructuring pattern handling
+- `// TODO: should JSX namespaced names be handled here as well?`
+
+## Example
+Input JavaScript:
+```javascript
+export default function foo(x, y) {
+  if (x) {
+    return foo(false, y);
+  }
+  return [y * 10];
+}
+```
+
+Output HIR (simplified):
+```
+foo(<unknown> x$0, <unknown> y$1): <unknown> $12
+bb0 (block):
+  [1] <unknown> $6 = LoadLocal <unknown> x$0
+  [2] If (<unknown> $6) then:bb2 else:bb1 fallthrough=bb1
+
+bb2 (block):
+  predecessor blocks: bb0
+  [3] <unknown> $2 = LoadGlobal(module) foo
+  [4] <unknown> $3 = false
+  [5] <unknown> $4 = LoadLocal <unknown> y$1
+  [6] <unknown> $5 = Call <unknown> $2(<unknown> $3, <unknown> $4)
+  [7] Return Explicit <unknown> $5
+
+bb1 (block):
+  predecessor blocks: bb0
+  [8] <unknown> $7 = LoadLocal <unknown> y$1
+  [9] <unknown> $8 = 10
+  [10] <unknown> $9 = Binary <unknown> $7 * <unknown> $8
+  [11] <unknown> $10 = Array [<unknown> $9]
+  [12] Return Explicit <unknown> $10
+```
+
+Key observations:
+- The function has 3 basic blocks: entry (bb0), consequent (bb2), alternate/fallthrough (bb1)
+- The if statement creates an `IfTerminal` at the end of bb0
+- Each branch ends with its own `ReturnTerminal`
+- All values are stored in temporaries (`$N`) or named identifiers (`x$0`, `y$1`)
+- Instructions have sequential IDs within blocks
+- Types and effects are `<unknown>` at this stage (populated by later passes)

compiler/packages/babel-plugin-react-compiler/docs/passes/02-enterSSA.md

@@ -0,0 +1,182 @@
+# enterSSA
+
+## File
+`src/SSA/EnterSSA.ts`
+
+## Purpose
+Converts the HIR from a non-SSA form (where variables can be reassigned) into Static Single Assignment (SSA) form, where each variable is defined exactly once and phi nodes are inserted at control flow join points to merge values from different paths.
+
+## Input Invariants
+- The HIR must have blocks in reverse postorder (predecessors visited before successors, except for back-edges)
+- Block predecessor information (`block.preds`) must be populated correctly
+- The function's `context` array must be empty for the root function (outer function declarations)
+- Identifiers may be reused across multiple definitions/assignments (non-SSA form)
+
+## Output Guarantees
+- Each identifier has a unique `IdentifierId` - no identifier is defined more than once
+- All operand references use the SSA-renamed identifiers
+- Phi nodes are inserted at join points where values from different control flow paths converge
+- Function parameters are SSA-renamed
+- Nested functions (FunctionExpression, ObjectMethod) are recursively converted to SSA form
+- Context variables (captured from outer scopes) are handled specially and not redefined
+
+## Algorithm
+The pass uses the Braun et al. algorithm ("Simple and Efficient Construction of Static Single Assignment Form") with adaptations for handling loops and nested functions.
+
+### Key Steps:
+1. **Block Traversal**: Iterate through blocks in order (assumed reverse postorder from previous passes)
+2. **Definition Tracking**: Maintain a per-block `defs` map from original identifiers to their SSA-renamed versions
+3. **Renaming**:
+   - When a value is **defined** (lvalue), create a new SSA identifier with fresh `IdentifierId`
+   - When a value is **used** (operand), look up the current SSA identifier via `getIdAt`
+4. **Phi Node Insertion**: When looking up an identifier at a block with multiple predecessors:
+   - If all predecessors have been visited, create a phi node collecting values from each predecessor
+   - If some predecessors are unvisited (back-edge/loop), create an "incomplete phi" that will be fixed later
+5. **Incomplete Phi Resolution**: When all predecessors of a block are finally visited, fix any incomplete phi nodes by populating their operands
+6. **Nested Function Handling**: Recursively apply SSA transformation to nested functions, temporarily adding a fake predecessor edge to enable identifier lookup from the enclosing scope
+
+### Phi Node Placement Logic (`getIdAt`):
+- If the identifier is defined locally in the current block, return it
+- If at entry block with no predecessors and not found, mark as unknown (global)
+- If some predecessors are unvisited (loop), create incomplete phi
+- If exactly one predecessor, recursively look up in that predecessor
+- If multiple predecessors, create phi node with operands from all predecessors
+
+## Key Data Structures
+- **SSABuilder**: Main class managing the transformation
+  - `#states: Map<BasicBlock, State>` - Per-block state (defs map and incomplete phis)
+  - `unsealedPreds: Map<BasicBlock, number>` - Count of unvisited predecessors per block
+  - `#unknown: Set<Identifier>` - Identifiers assumed to be globals
+  - `#context: Set<Identifier>` - Context variables that should not be redefined
+- **State**: Per-block state containing:
+  - `defs: Map<Identifier, Identifier>` - Maps original identifiers to SSA-renamed versions
+  - `incompletePhis: Array<IncompletePhi>` - Phi nodes waiting for predecessor values
+- **IncompletePhi**: Tracks a phi node created before all predecessors were visited
+  - `oldPlace: Place` - Original place being phi'd
+  - `newPlace: Place` - SSA-renamed phi result place
+- **Phi**: The actual phi node in the HIR
+  - `place: Place` - The result of the phi
+  - `operands: Map<BlockId, Place>` - Maps predecessor block to the place providing the value
+
+## Edge Cases
+- **Loops (back-edges)**: When a variable is used in a loop header before the loop body assigns it, an incomplete phi is created and later fixed when the loop body block is visited
+- **Globals**: If an identifier is used but never defined (reaching the entry block without a definition), it's assumed to be a global and not renamed
+- **Context variables**: Variables captured from an outer function scope are tracked specially and not redefined when reassigned
+- **Nested functions**: Function expressions and object methods are processed recursively with a temporary predecessor edge linking them to the enclosing block
+
+## TODOs
+- `[hoisting] EnterSSA: Expected identifier to be defined before being used` - Handles cases where hoisting causes an identifier to be used before definition (throws a Todo error for graceful bailout)
+
+## Example
+
+### Input (simple reassignment with control flow):
+```javascript
+function foo() {
+  let y = 2;
+  if (y > 1) {
+    y = 1;
+  } else {
+    y = 2;
+  }
+  let x = y;
+}
+```
+
+### Before SSA (HIR):
+```
+bb0 (block):
+  [1] $0 = 2
+  [2] $2 = StoreLocal Let y$1 = $0
+  [3] $7 = LoadLocal y$1
+  [4] $8 = 1
+  [5] $9 = Binary $7 > $8
+  [6] If ($9) then:bb2 else:bb3 fallthrough=bb1
+
+bb2 (block):
+  predecessor blocks: bb0
+  [7] $3 = 1
+  [8] $4 = StoreLocal Reassign y$1 = $3  // Same y$1 reassigned
+  [9] Goto bb1
+
+bb3 (block):
+  predecessor blocks: bb0
+  [10] $5 = 2
+  [11] $6 = StoreLocal Reassign y$1 = $5  // Same y$1 reassigned
+  [12] Goto bb1
+
+bb1 (block):
+  predecessor blocks: bb2 bb3
+  [13] $10 = LoadLocal y$1                // Which y$1?
+  [14] $12 = StoreLocal Let x$11 = $10
+```
+
+### After SSA:
+```
+bb0 (block):
+  [1] $15 = 2
+  [2] $17 = StoreLocal Let y$16 = $15    // y$16: initial definition
+  [3] $18 = LoadLocal y$16
+  [4] $19 = 1
+  [5] $20 = Binary $18 > $19
+  [6] If ($20) then:bb2 else:bb3 fallthrough=bb1
+
+bb2 (block):
+  predecessor blocks: bb0
+  [7] $21 = 1
+  [8] $23 = StoreLocal Reassign y$22 = $21  // y$22: new SSA name
+  [9] Goto bb1
+
+bb3 (block):
+  predecessor blocks: bb0
+  [10] $24 = 2
+  [11] $26 = StoreLocal Reassign y$25 = $24  // y$25: new SSA name
+  [12] Goto bb1
+
+bb1 (block):
+  predecessor blocks: bb2 bb3
+  y$27: phi(bb2: y$22, bb3: y$25)           // PHI NODE: merges y$22 and y$25
+  [13] $28 = LoadLocal y$27                  // Uses phi result
+  [14] $30 = StoreLocal Let x$29 = $28
+```
+
+### Loop Example (while loop with back-edge):
+```javascript
+function foo() {
+  let x = 1;
+  while (x < 10) {
+    x = x + 1;
+  }
+  return x;
+}
+```
+
+### After SSA:
+```
+bb0 (block):
+  [1] $13 = 1
+  [2] $15 = StoreLocal Let x$14 = $13    // x$14: initial definition
+  [3] While test=bb1 loop=bb3 fallthrough=bb2
+
+bb1 (loop):
+  predecessor blocks: bb0 bb3
+  x$16: phi(bb0: x$14, bb3: x$23)        // PHI merges initial and loop-updated values
+  [4] $17 = LoadLocal x$16
+  [5] $18 = 10
+  [6] $19 = Binary $17 < $18
+  [7] Branch ($19) then:bb3 else:bb2
+
+bb3 (block):
+  predecessor blocks: bb1
+  [8] $20 = LoadLocal x$16               // Uses phi result
+  [9] $21 = 1
+  [10] $22 = Binary $20 + $21
+  [11] $24 = StoreLocal Reassign x$23 = $22  // x$23: new SSA name in loop body
+  [12] Goto(Continue) bb1
+
+bb2 (block):
+  predecessor blocks: bb1
+  [13] $25 = LoadLocal x$16              // Uses phi result
+  [14] Return Explicit $25
+```
+
+The phi node at `bb1` (the loop header) is initially created as an "incomplete phi" when first visited because `bb3` (the loop body) hasn't been visited yet. Once `bb3` is processed and its terminal is handled, the incomplete phi is fixed by calling `fixIncompletePhis` to populate the operand from `bb3`.

compiler/packages/babel-plugin-react-compiler/docs/passes/03-eliminateRedundantPhi.md

@@ -0,0 +1,90 @@
+# eliminateRedundantPhi
+
+## File
+`src/SSA/EliminateRedundantPhi.ts`
+
+## Purpose
+Eliminates phi nodes whose operands are trivially the same, replacing all usages of the phi's output identifier with the single source identifier. This simplifies the HIR by removing unnecessary join points that do not actually merge distinct values.
+
+## Input Invariants
+- The function must be in SSA form (i.e., `enterSSA` has already run)
+- Blocks are in reverse postorder (guaranteed by the HIR structure)
+- Phi nodes exist at the start of blocks where control flow merges
+
+## Output Guarantees
+- All redundant phi nodes are removed from the HIR
+- All references to eliminated phi identifiers are rewritten to the source identifier
+- Non-redundant phi nodes (those merging two or more distinct values) are preserved
+- Nested function expressions (FunctionExpression, ObjectMethod) also have their redundant phis eliminated and contexts rewritten
+
+## Algorithm
+A phi node is considered redundant when:
+1. **All operands are the same identifier**: e.g., `x2 = phi(x1, x1, x1)` - the phi is replaced with `x1`
+2. **All operands are either the same identifier OR the phi's output**: e.g., `x2 = phi(x1, x2, x1, x2)` - this handles loop back-edges where the phi references itself
+
+The algorithm works as follows:
+1. Visit blocks in reverse postorder, building a rewrite table (`Map<Identifier, Identifier>`)
+2. For each phi node in a block:
+   - First rewrite operands using any existing rewrites (to handle cascading eliminations)
+   - Check if all operands (excluding self-references) point to the same identifier
+   - If so, add a mapping from the phi's output to that identifier and delete the phi
+3. After processing phis, rewrite all instruction lvalues, operands, and terminal operands
+4. For nested functions, recursively call `eliminateRedundantPhi` with shared rewrites
+5. If the CFG has back-edges (loops) and new rewrites were added, repeat the entire process
+
+The loop termination condition `rewrites.size > size && hasBackEdge` ensures:
+- Without loops: completes in a single pass (reverse postorder guarantees forward propagation)
+- With loops: repeats until no new rewrites are found (fixpoint)
+
+## Key Data Structures
+- **`Phi`** (from `src/HIR/HIR.ts`): Represents a phi node with:
+  - `place: Place` - the output identifier
+  - `operands: Map<BlockId, Place>` - maps predecessor block IDs to source places
+- **`rewrites: Map<Identifier, Identifier>`**: Maps eliminated phi outputs to their replacement identifier
+- **`visited: Set<BlockId>`**: Tracks visited blocks to detect back-edges (loops)
+
+## Edge Cases
+- **Loop back-edges**: When a block has a predecessor that hasn't been visited yet (in reverse postorder), that predecessor is a back-edge. The algorithm handles self-referential phis like `x2 = phi(x1, x2)` by ignoring operands equal to the phi's output.
+- **Cascading eliminations**: When one phi's output is used in another phi's operands, the algorithm rewrites operands before checking redundancy, enabling transitive elimination in a single pass (for non-loop cases).
+- **Nested functions**: FunctionExpression and ObjectMethod values contain nested HIR that may have their own phis. The algorithm recursively processes these with a shared rewrite table, ensuring context captures are also rewritten.
+- **Empty phi check**: The algorithm includes an invariant check that phi operands are never empty (which would be invalid HIR).
+
+## TODOs
+(None found in the source code)
+
+## Example
+
+Consider this fixture from `rewrite-phis-in-lambda-capture-context.js`:
+
+```javascript
+function Component() {
+  const x = 4;
+  const get4 = () => {
+    while (bar()) {
+      if (baz) { bar(); }
+    }
+    return () => x;
+  };
+  return get4;
+}
+```
+
+**After SSA pass**, the inner function has redundant phis due to the loop:
+
+```
+bb2 (loop):
+  predecessor blocks: bb1 bb5
+  x$29: phi(bb1: x$21, bb5: x$30)  // Loop header phi
+  ...
+bb5 (block):
+  predecessor blocks: bb6 bb4
+  x$30: phi(bb6: x$29, bb4: x$29)  // Redundant: both operands are x$29
+  ...
+```
+
+**After EliminateRedundantPhi**:
+- `x$30 = phi(x$29, x$29)` is eliminated because both operands are `x$29`
+- `x$29 = phi(x$21, x$30)` becomes `x$29 = phi(x$21, x$29)` after rewriting, which is also redundant (one operand is the phi itself, the other is `x$21`)
+- Both phis are eliminated, and all uses of `x$29` and `x$30` are rewritten to `x$21`
+
+The result: the context capture `@context[x$29]` becomes `@context[x$21]`, correctly propagating that `x` is never modified inside the loop.

compiler/packages/babel-plugin-react-compiler/docs/passes/04-constantPropagation.md

@@ -0,0 +1,110 @@
+# constantPropagation
+
+## File
+`src/Optimization/ConstantPropagation.ts`
+
+## Purpose
+Applies Sparse Conditional Constant Propagation (SCCP) to fold compile-time evaluable expressions to constant values, propagate those constants through the program, and eliminate unreachable branches when conditionals have known constant values.
+
+## Input Invariants
+- HIR must be in SSA form (runs after `enterSSA`)
+- Redundant phi nodes should be eliminated (runs after `eliminateRedundantPhi`)
+- Consistent identifiers must be ensured (`assertConsistentIdentifiers`)
+- Terminal successors must exist (`assertTerminalSuccessorsExist`)
+
+## Output Guarantees
+- Instructions with compile-time evaluable operands are replaced with `Primitive` constants
+- `ComputedLoad`/`ComputedStore` with constant string/number properties are converted to `PropertyLoad`/`PropertyStore`
+- `LoadLocal` and `StoreLocal` propagate known constant values
+- `IfTerminal` with constant boolean test values are replaced with `goto` terminals
+- Unreachable blocks are removed and the CFG is minimized
+- Phi nodes with unreachable predecessor operands are pruned
+- Nested functions (`FunctionExpression`, `ObjectMethod`) are recursively processed
+
+## Algorithm
+The pass uses Sparse Conditional Constant Propagation (SCCP) with fixpoint iteration:
+
+1. **Data Structure**: A `Constants` map (`Map<IdentifierId, Constant>`) tracks known constant values (either `Primitive` or `LoadGlobal`)
+
+2. **Single Pass per Iteration**: Visits all blocks in order:
+   - Evaluates phi nodes - if all operands have the same constant value, the phi result is constant
+   - Evaluates instructions - replaces evaluable expressions with constants
+   - Evaluates terminals - if an `IfTerminal` test is a constant, replaces it with a `goto`
+
+3. **Fixpoint Loop**: If any terminals changed (branch elimination):
+   - Recomputes block ordering (`reversePostorderBlocks`)
+   - Removes unreachable code (`removeUnreachableForUpdates`, `removeDeadDoWhileStatements`, `removeUnnecessaryTryCatch`)
+   - Renumbers instructions (`markInstructionIds`)
+   - Updates predecessors (`markPredecessors`)
+   - Prunes phi operands from unreachable predecessors
+   - Eliminates newly-redundant phis (`eliminateRedundantPhi`)
+   - Merges consecutive blocks (`mergeConsecutiveBlocks`)
+   - Repeats until no more changes
+
+4. **Instruction Evaluation**: Handles various instruction types:
+   - **Primitives/LoadGlobal**: Directly constant
+   - **BinaryExpression**: Folds arithmetic (`+`, `-`, `*`, `/`, `%`, `**`), bitwise (`|`, `&`, `^`, `<<`, `>>`, `>>>`), and comparison (`<`, `<=`, `>`, `>=`, `==`, `===`, `!=`, `!==`) operators
+   - **UnaryExpression**: Folds `!` (boolean negation) and `-` (numeric negation)
+   - **PostfixUpdate/PrefixUpdate**: Folds `++`/`--` on constant numbers
+   - **PropertyLoad**: Folds `.length` on constant strings
+   - **TemplateLiteral**: Folds template strings with constant interpolations
+   - **ComputedLoad/ComputedStore**: Converts to property access when property is constant string/number
+
+## Key Data Structures
+- `Constant = Primitive | LoadGlobal` - The lattice values (no top/bottom, absence means unknown)
+- `Constants = Map<IdentifierId, Constant>` - Maps identifier IDs to their known constant values
+- Uses HIR types: `Instruction`, `Phi`, `Place`, `Primitive`, `LoadGlobal`, `InstructionValue`
+
+## Edge Cases
+- **Last instruction of sequence blocks**: Skipped to preserve evaluation order
+- **Phi nodes with back-edges**: Single-pass analysis means loop back-edges won't have constant values propagated
+- **Template literals with Symbol**: Not folded (would throw at runtime)
+- **Template literals with objects/arrays**: Not folded (custom toString behavior)
+- **Division results**: Computed at compile time (may produce `NaN`, `Infinity`, etc.)
+- **LoadGlobal in phis**: Only propagated if all operands reference the same global name
+- **Nested functions**: Constants from outer scope are propagated into nested function expressions
+
+## TODOs
+- `// TODO: handle more cases` - The default case in `evaluateInstruction` has room for additional instruction types
+
+## Example
+
+**Input:**
+```javascript
+function Component() {
+  let a = 1;
+
+  let b;
+  if (a === 1) {
+    b = true;
+  } else {
+    b = false;
+  }
+
+  let c;
+  if (b) {
+    c = 'hello';
+  } else {
+    c = null;
+  }
+
+  return c;
+}
+```
+
+**After ConstantPropagation:**
+- `a === 1` evaluates to `true`
+- The `if (a === 1)` branch is eliminated, only consequent remains
+- `b` is known to be `true`
+- `if (b)` branch is eliminated, only consequent remains
+- `c` is known to be `'hello'`
+- All intermediate blocks are merged
+
+**Output:**
+```javascript
+function Component() {
+  return "hello";
+}
+```
+
+The pass performs iterative simplification: first iteration determines `a === 1` is `true` and eliminates that branch. The CFG is updated, phi for `b` is pruned to single operand making `b = true`. Second iteration uses `b = true` to eliminate the next branch. This continues until no more branches can be eliminated.

compiler/packages/babel-plugin-react-compiler/docs/passes/05-deadCodeElimination.md

@@ -0,0 +1,109 @@
+# deadCodeElimination
+
+## File
+`src/Optimization/DeadCodeElimination.ts`
+
+## Purpose
+Eliminates instructions whose values are unused, reducing generated code size. The pass performs mark-and-sweep analysis to identify and remove dead code while preserving side effects and program semantics.
+
+## Input Invariants
+- Must run after `InferMutationAliasingEffects` because "dead" code may still affect effect inference
+- HIR is in SSA form with phi nodes
+- Unreachable blocks are already pruned during HIR construction
+
+## Output Guarantees
+- All instructions with unused lvalues (that are safe to prune) are removed
+- Unused phi nodes are deleted
+- Unused context variables are removed from `fn.context`
+- Destructuring patterns are rewritten to remove unused bindings
+- `StoreLocal` instructions with unused initializers are converted to `DeclareLocal`
+
+## Algorithm
+Two-phase mark-and-sweep with fixed-point iteration for loops:
+
+**Phase 1: Mark (findReferencedIdentifiers)**
+1. Detect if function has back-edges (loops)
+2. Iterate blocks in reverse postorder (successors before predecessors) to visit usages before declarations
+3. For each block:
+   - Mark all terminal operands as referenced
+   - Process instructions in reverse order:
+     - If lvalue is used OR instruction is not pruneable, mark the lvalue and all operands as referenced
+     - Special case for `StoreLocal`: only mark initializer if the SSA lvalue is actually read
+   - Mark phi operands if the phi result is used
+4. If loops exist and new identifiers were marked, repeat until fixed point
+
+**Phase 2: Sweep**
+1. Remove unused phi nodes from each block
+2. Remove instructions with unused lvalues using `retainWhere`
+3. Rewrite retained instructions:
+   - **Array destructuring**: Replace unused elements with holes, truncate trailing holes
+   - **Object destructuring**: Remove unused properties (only if rest element is unused or absent)
+   - **StoreLocal**: Convert to `DeclareLocal` if initializer value is never read
+4. Remove unused context variables
+
+## Key Data Structures
+- **State class**: Tracks referenced identifiers
+  - `identifiers: Set<IdentifierId>` - SSA-specific usages
+  - `named: Set<string>` - Named variable usages (any version)
+  - `isIdOrNameUsed()` - Checks if identifier or any version of named variable is used
+  - `isIdUsed()` - Checks if specific SSA id is used
+- **hasBackEdge/findBlocksWithBackEdges**: Detect loops requiring fixed-point iteration
+
+## Edge Cases
+- **Preserved even if unused:**
+  - `debugger` statements (to not break debugging workflows)
+  - Call expressions and method calls (may have side effects)
+  - Await expressions
+  - Store operations (ComputedStore, PropertyStore, StoreGlobal)
+  - Delete operations (ComputedDelete, PropertyDelete)
+  - Iterator operations (GetIterator, IteratorNext, NextPropertyOf)
+  - Context operations (LoadContext, DeclareContext, StoreContext)
+  - Memoization markers (StartMemoize, FinishMemoize)
+
+- **SSR mode special case:**
+  - In SSR mode, unused `useState`, `useReducer`, and `useRef` hooks can be removed
+
+- **Object destructuring with rest:**
+  - Cannot remove unused properties if rest element is used (would change rest's value)
+
+- **Block value instructions:**
+  - Last instruction of value blocks (not 'block' kind) is never pruned as it's the block's value
+
+## TODOs
+- "TODO: we could be more precise and make this conditional on whether any arguments are actually modified" (for mutating instructions)
+
+## Example
+
+**Input:**
+```javascript
+function Component(props) {
+  const _ = 42;
+  return props.value;
+}
+```
+
+**After DeadCodeElimination:**
+The `const _ = 42` assignment is removed since `_` is never used:
+```javascript
+function Component(props) {
+  return props.value;
+}
+```
+
+**Array destructuring example:**
+
+Input:
+```javascript
+function foo(props) {
+  const [x, unused, y] = props.a;
+  return x + y;
+}
+```
+
+Output (middle element becomes a hole):
+```javascript
+function foo(props) {
+  const [x, , y] = props.a;
+  return x + y;
+}
+```

compiler/packages/babel-plugin-react-compiler/docs/passes/06-inferTypes.md

@@ -0,0 +1,124 @@
+# inferTypes
+
+## File
+`src/TypeInference/InferTypes.ts`
+
+## Purpose
+Infers types for all identifiers in the HIR by generating type equations and solving them using unification. This pass annotates identifiers with concrete types (Primitive, Object, Function) based on the operations performed on them and the types of globals/hooks they interact with.
+
+## Input Invariants
+- The HIR must be in SSA form (the pass runs after `enterSSA` and `eliminateRedundantPhi`)
+- Constant propagation has already run
+- Global declarations and hook shapes are available via the Environment
+
+## Output Guarantees
+- All identifier types are resolved from type variables (`Type`) to concrete types where possible
+- Phi nodes have their operand types unified to produce a single result type
+- Function return types are inferred from the unified types of all return statements
+- Property accesses on known objects/hooks resolve to the declared property types
+- Component props parameters are typed as `TObject<BuiltInProps>`
+- Component ref parameters are typed as `TObject<BuiltInUseRefId>`
+
+## Algorithm
+The pass uses a classic constraint-based type inference approach with three phases:
+
+1. **Constraint Generation (`generate`)**: Traverses all instructions and generates type equations:
+   - Primitives, literals, unary/binary operations -> `Primitive` type
+   - Hook/function calls -> Function type with fresh return type variable
+   - Property loads -> `Property` type that defers to object shape lookup
+   - Destructuring -> Property types for each extracted element
+   - Phi nodes -> `Phi` type with all operand types as candidates
+   - JSX -> `Object<BuiltInJsx>`
+   - Arrays -> `Object<BuiltInArray>`
+   - Objects -> `Object<BuiltInObject>`
+
+2. **Unification (`Unifier.unify`)**: Solves constraints by unifying type equations:
+   - Type variables are bound to concrete types via substitution
+   - Property types are resolved by looking up the object's shape
+   - Phi types are resolved by finding a common type among operands (or falling back to `Phi` if incompatible)
+   - Function types are unified by unifying their return types
+   - Occurs check prevents infinite types (cycles in type references)
+
+3. **Application (`apply`)**: Applies the computed substitutions to all identifiers in the HIR, replacing type variables with their resolved types.
+
+## Key Data Structures
+- **TypeVar** (`kind: 'Type'`): A type variable with a unique TypeId, used for unknowns
+- **Unifier**: Maintains a substitution map from TypeId to Type, with methods for unification and cycle detection
+- **TypeEquation**: A pair of types that should be equal, used as constraints
+- **PhiType** (`kind: 'Phi'`): Represents the join of multiple types from control flow merge points
+- **PropType** (`kind: 'Property'`): Deferred property lookup that resolves based on object shape
+- **FunctionType** (`kind: 'Function'`): Callable type with optional shapeId and return type
+- **ObjectType** (`kind: 'Object'`): Object with optional shapeId for shape lookup
+
+## Edge Cases
+
+### Phi Type Resolution
+When phi operands have incompatible types, the pass attempts to find a union:
+- `Union(Primitive | MixedReadonly) = MixedReadonly`
+- `Union(Array | MixedReadonly) = Array`
+- If no union is possible, the type remains as `Phi`
+
+### Ref-like Name Inference
+When `enableTreatRefLikeIdentifiersAsRefs` is enabled, property access on variables matching the pattern `/^(?:[a-zA-Z$_][a-zA-Z$_0-9]*)Ref$|^ref$/` with property name `current` infers:
+- Object type as `TObject<BuiltInUseRefId>`
+- Property type as `TObject<BuiltInRefValue>`
+
+### Cycle Detection
+The `occursCheck` method prevents infinite types by detecting when a type variable appears in its own substitution. When a cycle is detected, `tryResolveType` removes the cyclic reference from Phi operands.
+
+### Context Variables
+- `DeclareContext` and `LoadContext` generate no type equations (intentionally untyped)
+- `StoreContext` with `Const` kind does propagate the rvalue type to enable ref inference through context variables
+
+## TODOs
+1. **Hook vs Function type ambiguity**:
+   > "TODO: callee could be a hook or a function, so this type equation isn't correct. We should change Hook to a subtype of Function or change unifier logic."
+
+2. **PropertyStore rvalue inference**:
+   > "TODO: consider using the rvalue type here" - Currently uses a dummy type for PropertyStore to avoid inferring rvalue types from lvalue assignments.
+
+## Example
+
+**Input (infer-phi-primitive.js):**
+```javascript
+function foo(a, b) {
+  let x;
+  if (a) {
+    x = 1;
+  } else {
+    x = 2;
+  }
+  let y = x;
+  return y;
+}
+```
+
+**Before InferTypes (SSA form):**
+```
+<unknown> x$26: phi(bb2: <unknown> x$21, bb3: <unknown> x$24)
+[10] <unknown> $27 = LoadLocal <unknown> x$26
+[11] <unknown> $29 = StoreLocal Let <unknown> y$28 = <unknown> $27
+```
+
+**After InferTypes:**
+```
+<unknown> x$26:TPrimitive: phi(bb2: <unknown> x$21:TPrimitive, bb3: <unknown> x$24:TPrimitive)
+[10] <unknown> $27:TPrimitive = LoadLocal <unknown> x$26:TPrimitive
+[11] <unknown> $29:TPrimitive = StoreLocal Let <unknown> y$28:TPrimitive = <unknown> $27:TPrimitive
+```
+
+The pass infers that:
+- Literals `1` and `2` are `TPrimitive`
+- The phi of two primitives is `TPrimitive`
+- Variables `x` and `y` are `TPrimitive`
+- The function return type is `TPrimitive`
+
+**Hook type inference example (useState):**
+```javascript
+const [x, setX] = useState(initialValue);
+```
+
+After InferTypes:
+- `useState` -> `TFunction<BuiltInUseState>:TObject<BuiltInUseState>`
+- Return value `$27` -> `TObject<BuiltInUseState>`
+- Destructured `setX` -> `TFunction<BuiltInSetState>:TPrimitive`

compiler/packages/babel-plugin-react-compiler/docs/passes/07-analyseFunctions.md

@@ -0,0 +1,84 @@
+# analyseFunctions
+
+## File
+`src/Inference/AnalyseFunctions.ts`
+
+## Purpose
+Recursively analyzes all nested function expressions and object methods in a function to infer their aliasing effect signatures, which describe how the function affects its captured variables when invoked.
+
+## Input Invariants
+- The HIR has been through SSA conversion and type inference
+- FunctionExpression and ObjectMethod instructions have an empty `aliasingEffects` array (`@aliasingEffects=[]`)
+- Context variables (captured variables from outer scope) exist on `fn.context` but do not have their effect populated
+
+## Output Guarantees
+- Every FunctionExpression and ObjectMethod has its `aliasingEffects` array populated with the effects the function performs when called (mutations, captures, aliasing to return value, etc.)
+- Each context variable's `effect` property is set to either `Effect.Capture` (if the variable is captured or mutated by the inner function) or `Effect.Read` (if only read)
+- Context variable mutable ranges are reset to `{start: 0, end: 0}` and scopes are set to `null` to prepare for the outer function's subsequent `inferMutationAliasingRanges` pass
+
+## Algorithm
+1. **Recursive traversal**: Iterates through all blocks and instructions looking for `FunctionExpression` or `ObjectMethod` instructions
+2. **Depth-first processing**: For each function expression found, calls `lowerWithMutationAliasing()` which:
+   - Recursively calls `analyseFunctions()` on the inner function (handles nested functions)
+   - Runs `inferMutationAliasingEffects()` on the inner function to determine effects
+   - Runs `deadCodeElimination()` to clean up
+   - Runs `inferMutationAliasingRanges()` to compute mutable ranges and extract externally-visible effects
+   - Runs `rewriteInstructionKindsBasedOnReassignment()` and `inferReactiveScopeVariables()`
+   - Stores the computed effects in `fn.aliasingEffects`
+3. **Context variable effect classification**: Scans the computed effects to determine which context variables are captured/mutated vs only read:
+   - Effects like `Capture`, `Alias`, `Assign`, `MaybeAlias`, `CreateFrom` mark the source as captured
+   - Mutation effects (`Mutate`, `MutateTransitive`, etc.) mark the target as captured
+   - Sets `operand.effect = Effect.Capture` or `Effect.Read` accordingly
+4. **Range reset**: Resets mutable ranges and scopes on context variables to prepare for outer function analysis
+
+## Key Data Structures
+- **HIRFunction.aliasingEffects**: Array of `AliasingEffect` storing the externally-visible behavior of a function when called
+- **Place.effect**: Effect enum value (`Capture` or `Read`) describing how a context variable is used
+- **AliasingEffect**: Union type describing data flow (Capture, Alias, Assign, etc.) and mutations (Mutate, MutateTransitive, etc.)
+- **FunctionExpression/ObjectMethod.loweredFunc.func**: The inner HIRFunction to analyze
+
+## Edge Cases
+- **Nested functions**: Handled via recursive call to `analyseFunctions()` before processing the current function - innermost functions are analyzed first
+- **ObjectMethod**: Treated identically to FunctionExpression
+- **Apply effects invariant**: The pass asserts that no `Apply` effects remain in the function's signature - these should have been resolved to more precise effects by `inferMutationAliasingRanges()`
+- **Conditional mutations**: Effects like `MutateTransitiveConditionally` are tracked - a function that conditionally mutates a captured variable will have that effect in its signature
+- **Immutable captures**: `ImmutableCapture`, `Freeze`, `Create`, `Impure`, `Render` effects do not contribute to marking context variables as `Capture`
+
+## TODOs
+- No TODO comments in the pass itself
+
+## Example
+Consider a function that captures and conditionally mutates a variable:
+
+```javascript
+function useHook(a, b) {
+  let z = {a};
+  let y = b;
+  let x = function () {
+    if (y) {
+      maybeMutate(z);  // Unknown function, may mutate z
+    }
+  };
+  return x;
+}
+```
+
+**Before AnalyseFunctions:**
+```
+Function @context[y$28, z$25] @aliasingEffects=[]
+```
+
+**After AnalyseFunctions:**
+```
+Function @context[read y$28, capture z$25] @aliasingEffects=[
+  MutateTransitiveConditionally z$25,
+  Create $14 = primitive
+]
+```
+
+The pass infers:
+- `y` is only read (used in the condition)
+- `z` is captured into the function and conditionally mutated transitively (because `maybeMutate()` is unknown)
+- The inner function's signature includes `MutateTransitiveConditionally z$25` to indicate this potential mutation
+
+This signature is then used by `InferMutationAliasingEffects` on the outer function to understand that creating this function captures `z`, and calling the function may mutate `z`.

compiler/packages/babel-plugin-react-compiler/docs/passes/08-inferMutationAliasingEffects.md

@@ -0,0 +1,144 @@
+# inferMutationAliasingEffects
+
+## File
+`src/Inference/InferMutationAliasingEffects.ts`
+
+## Purpose
+Infers the mutation and aliasing effects for all instructions and terminals in the HIR, making the effects of built-in instructions/functions as well as user-defined functions explicit. These effects form the basis for subsequent analysis to determine the mutable range of each value in the program and for validation against invalid code patterns like mutating frozen values.
+
+## Input Invariants
+- HIR must be in SSA form (run after SSA pass)
+- Types must be inferred (run after InferTypes pass)
+- Functions must be analyzed (run after AnalyseFunctions pass) - this provides `aliasingEffects` on FunctionExpressions
+- Each instruction must have an lvalue (destination place)
+
+## Output Guarantees
+- Every instruction has an `effects` array (or null if no effects) containing `AliasingEffect` objects
+- Terminals that affect data flow (return, try/catch) have their `effects` populated
+- Each instruction's lvalue is guaranteed to be defined in the inference state after visiting
+- Effects describe: creation of values, data flow (Assign, Alias, Capture), mutations (Mutate, MutateTransitive), freezing, and errors (MutateFrozen, MutateGlobal, Impure)
+
+## Algorithm
+The pass uses abstract interpretation with the following key phases:
+
+1. **Initialization**:
+   - Create initial `InferenceState` mapping identifiers to abstract values
+   - Initialize context variables as `ValueKind.Context`
+   - Initialize parameters as `ValueKind.Frozen` (for top-level components/hooks) or `ValueKind.Mutable` (for function expressions)
+
+2. **Two-Phase Effect Processing**:
+   - **Phase 1 - Signature Computation**: For each instruction, compute a "candidate signature" based purely on instruction semantics and types (cached per instruction via `computeSignatureForInstruction`)
+   - **Phase 2 - Effect Application**: Apply the signature to the current abstract state via `applySignature`, which refines effects based on the actual runtime kinds of values
+
+3. **Fixed-Point Iteration**:
+   - Process blocks in a worklist, queuing successors after each block
+   - Merge states at control flow join points using lattice operations
+   - Iterate until no changes occur (max 100 iterations as safety limit)
+   - Phi nodes are handled by unioning the abstract values from all predecessors
+
+4. **Effect Refinement** (in `applyEffect`):
+   - `MutateConditionally` effects are dropped if value is not mutable
+   - `Capture` effects are downgraded to `ImmutableCapture` if source is frozen
+   - `Mutate` on frozen values becomes `MutateFrozen` error
+   - `Assign` from primitives/globals creates new values rather than aliasing
+
+## Key Data Structures
+
+### InferenceState
+Maintains two maps:
+- `#values: Map<InstructionValue, AbstractValue>` - Maps allocation sites to their abstract kind
+- `#variables: Map<IdentifierId, Set<InstructionValue>>` - Maps identifiers to the set of values they may point to (set to handle phi joins)
+
+### AbstractValue
+```typescript
+type AbstractValue = {
+  kind: ValueKind;
+  reason: ReadonlySet<ValueReason>;
+};
+```
+
+### ValueKind (lattice)
+```
+MaybeFrozen    <- top (unknown if frozen or mutable)
+    |
+  Frozen       <- immutable, cannot be mutated
+  Mutable      <- can be mutated locally
+  Context      <- mutable box (context variables)
+    |
+  Global       <- global value
+  Primitive    <- copy-on-write semantics
+```
+
+The `mergeValueKinds` function implements the lattice join:
+- `Frozen | Mutable -> MaybeFrozen`
+- `Context | Mutable -> Context`
+- `Context | Frozen -> MaybeFrozen`
+
+### AliasingEffect Types
+Key effect kinds handled:
+- **Create**: Creates a new value at a place
+- **Assign**: Direct assignment (pointer copy)
+- **Alias**: Mutation of destination implies mutation of source
+- **Capture**: Information flow (MutateTransitive propagates through)
+- **MaybeAlias**: Possible aliasing for unknown function returns
+- **Mutate/MutateTransitive**: Direct/transitive mutation
+- **MutateConditionally/MutateTransitiveConditionally**: Conditional versions
+- **Freeze**: Marks value as immutable
+- **Apply**: Function call with complex data flow
+
+## Edge Cases
+
+1. **Spread Destructuring from Props**: The `findNonMutatedDestructureSpreads` pre-pass identifies spread patterns from frozen values that are never mutated, allowing them to be treated as frozen.
+
+2. **Hoisted Context Declarations**: Special handling for variables declared with hoisting (`HoistedConst`, `HoistedFunction`, `HoistedLet`) to detect access before declaration.
+
+3. **Try-Catch Aliasing**: When a `maybe-throw` terminal is reached, call return values are aliased into the catch binding since exceptions can throw return values.
+
+4. **Function Expressions**: Functions are considered mutable only if they have mutable captures or tracked side effects (MutateFrozen, MutateGlobal, Impure).
+
+5. **Iterator Mutation**: Non-builtin iterators may alias their collection and mutation of the iterator is conditional.
+
+6. **Array.push and Similar**: Uses legacy signature system with `Store` effect on receiver and `Capture` of arguments.
+
+## TODOs
+- `// TODO: using InstructionValue as a bit of a hack, but it's pragmatic` - context variable initialization
+- `// TODO: call applyEffect() instead` - try-catch aliasing
+- `// TODO: make sure we're also validating against global mutations somewhere` - global mutation validation for effects/event handlers
+- `// TODO; include "render" here?` - whether to track Render effects in function hasTrackedSideEffects
+- `// TODO: consider using persistent data structures to make clone cheaper` - performance optimization for state cloning
+- `// TODO check this` and `// TODO: what kind here???` - DeclareLocal value kinds
+
+## Example
+
+For the code:
+```javascript
+const arr = [];
+arr.push({});
+arr.push(x, y);
+```
+
+After `InferMutationAliasingEffects`, the effects are:
+
+```
+[10] $39 = Array []
+    Create $39 = mutable           // Array literal creates mutable value
+
+[11] $41 = StoreLocal arr$40 = $39
+    Assign arr$40 = $39            // arr points to the array value
+    Assign $41 = $39
+
+[15] $45 = MethodCall $42.push($44)
+    Apply $45 = $42.$43($44)       // Records the call
+    Mutate $42                      // push mutates the array
+    Capture $42 <- $44             // {} is captured into array
+    Create $45 = primitive         // push returns number (length)
+
+[20] $50 = MethodCall $46.push($48, $49)
+    Apply $50 = $46.$47($48, $49)
+    Mutate $46                      // push mutates the array
+    Capture $46 <- $48             // x captured into array
+    Capture $46 <- $49             // y captured into array
+    Create $50 = primitive
+```
+
+The key insight is that `Mutate` effects extend the mutable range of the array, and `Capture` effects record data flow so that if the array is later frozen (e.g., returned from a component), the captured values are also considered frozen for validation purposes.

compiler/packages/babel-plugin-react-compiler/docs/passes/09-inferMutationAliasingRanges.md

@@ -0,0 +1,149 @@
+# inferMutationAliasingRanges
+
+## File
+`src/Inference/InferMutationAliasingRanges.ts`
+
+## Purpose
+This pass builds an abstract model of the heap and interprets the effects of the given function to determine: (1) the mutable ranges of all identifiers, (2) the externally-visible effects of the function (mutations of params/context-vars, aliasing relationships), and (3) the legacy `Effect` annotation for each Place.
+
+## Input Invariants
+- InferMutationAliasingEffects must have already run, populating `instr.effects` on each instruction with aliasing/mutation effects
+- SSA form must be established (identifiers are in SSA)
+- Type inference has been run (InferTypes)
+- Functions have been analyzed (AnalyseFunctions)
+- Dead code elimination has been performed
+
+## Output Guarantees
+- Every identifier has a populated `mutableRange` (start:end instruction IDs)
+- Every Place has a legacy `Effect` annotation (Read, Capture, Store, Freeze, etc.)
+- The function's `aliasingEffects` array is populated with externally-visible effects (mutations of params/context-vars, aliasing between params/context-vars/return)
+- Validation errors are collected for invalid effects like `MutateFrozen` or `MutateGlobal`
+
+## Algorithm
+The pass operates in three main phases:
+
+**Part 1: Build Data Flow Graph and Infer Mutable Ranges**
+1. Creates an `AliasingState` which maintains a `Node` for each identifier
+2. Iterates through all blocks and instructions, processing effects in program order
+3. For each effect:
+   - `Create`/`CreateFunction`: Creates a new node in the graph
+   - `CreateFrom`/`Assign`/`Alias`: Adds alias edges between nodes (with ordering index)
+   - `MaybeAlias`: Adds conditional alias edges
+   - `Capture`: Adds capture edges (for transitive mutations)
+   - `Mutate*`: Queues mutations for later processing
+   - `Render`: Queues render effects for later processing
+4. Phi node operands are connected once their predecessor blocks have been visited
+5. After the graph is built, mutations are processed:
+   - Mutations propagate both forward (via edges) and backward (via aliases/captures)
+   - Each mutation extends the `mutableRange.end` of affected identifiers
+   - Transitive mutations also traverse capture edges backward
+   - `MaybeAlias` edges downgrade mutations to `Conditional`
+6. Render effects are processed to mark values as rendered
+
+**Part 2: Populate Legacy Per-Place Effects**
+- Sets legacy effects on lvalues and operands based on instruction effects and mutable ranges
+- Fixes up mutable range start values for identifiers that are mutated after creation
+
+**Part 3: Infer Externally-Visible Function Effects**
+- Creates a `Create` effect for the return value
+- Simulates transitive mutations of each param/context-var/return to detect capture relationships
+- Produces `Alias`/`Capture` effects showing data flow between params/context-vars/return
+
+## Key Data Structures
+
+### `AliasingState`
+The main state class maintaining the data flow graph:
+- `nodes: Map<Identifier, Node>` - Maps identifiers to their graph nodes
+
+### `Node`
+Represents an identifier in the data flow graph:
+```typescript
+type Node = {
+  id: Identifier;
+  createdFrom: Map<Identifier, number>;   // CreateFrom edges (source -> index)
+  captures: Map<Identifier, number>;       // Capture edges (source -> index)
+  aliases: Map<Identifier, number>;        // Alias/Assign edges (source -> index)
+  maybeAliases: Map<Identifier, number>;   // MaybeAlias edges (source -> index)
+  edges: Array<{index, node, kind}>;       // Forward edges to other nodes
+  transitive: {kind: MutationKind; loc} | null;  // Transitive mutation info
+  local: {kind: MutationKind; loc} | null;       // Local mutation info
+  lastMutated: number;                     // Index of last mutation affecting this node
+  mutationReason: MutationReason | null;   // Reason for mutation
+  value: {kind: 'Object'} | {kind: 'Phi'} | {kind: 'Function'; function: HIRFunction};
+  render: Place | null;                    // Render context if used in JSX
+};
+```
+
+### `MutationKind`
+Enum describing mutation certainty:
+```typescript
+enum MutationKind {
+  None = 0,
+  Conditional = 1,  // May mutate (e.g., via MaybeAlias or MutateConditionally)
+  Definite = 2,     // Definitely mutates
+}
+```
+
+## Edge Cases
+
+### Phi Nodes
+- Phi nodes are created as special `{kind: 'Phi'}` nodes
+- Phi operands from predecessor blocks are processed with pending edges until the predecessor is visited
+- When traversing "forwards" through edges and encountering a phi, backward traversal is stopped (prevents mutation from one phi input affecting other inputs)
+
+### Transitive vs Local Mutations
+- Local mutations (`Mutate`) only affect alias/assign edges backward
+- Transitive mutations (`MutateTransitive`) also affect capture edges backward
+- Both affect all forward edges
+
+### MaybeAlias
+- Mutations through MaybeAlias edges are downgraded to `Conditional`
+- This prevents false positive errors when we cannot be certain about aliasing
+
+### Function Values
+- Functions are tracked specially as `{kind: 'Function'}` nodes
+- When a function is mutated (transitively), errors from the function body are propagated
+- This handles cases where mutating a captured value in a function affects render safety
+
+### Render Effect Propagation
+- Render effects traverse backward through alias/capture/createFrom edges
+- Functions that have not been mutated are skipped during render traversal (except for JSX-returning functions)
+- Ref types (`isUseRefType`) stop render traversal
+
+## TODOs
+1. Assign effects should have an invariant that the node is not initialized yet. Currently `InferFunctionExpressionAliasingEffectSignatures` infers Assign effects that should be Alias, causing reinitialization.
+
+2. Phi place effects are not properly set today.
+
+3. Phi mutable range start calculation is imprecise - currently just sets it to the instruction before the block rather than computing the exact start.
+
+## Example
+
+Consider the following code:
+```javascript
+function foo() {
+  let a = {};   // Create a (instruction 1)
+  let b = {};   // Create b (instruction 3)
+  a = b;        // Assign a <- b (instruction 8)
+  mutate(a, b); // MutateTransitiveConditionally a, b (instruction 16)
+  return a;
+}
+```
+
+The pass builds a graph:
+1. Creates node for `{}` at instruction 1 (initially assigned to `a`)
+2. Creates node for `{}` at instruction 3 (initially assigned to `b`)
+3. At instruction 8, creates alias edge: `b -> a` with index 8
+4. At instruction 16, mutations are queued for `a` and `b`
+
+When processing the mutation of `a` at instruction 16:
+- Extends `a`'s mutableRange.end to 17
+- Traverses backward through alias edge to `b`, extends `b`'s mutableRange.end to 17
+- Since `a = b`, both objects must be considered mutable until instruction 17
+
+The output shows identifiers with range annotations like `$25[3:17]` meaning:
+- `$25` is the identifier
+- `3` is the instruction where it was created
+- `17` is the instruction after which it is no longer mutated
+
+For aliased values, the ranges are unified - all values that could be affected by a mutation have their ranges extended to include that mutation point.

compiler/packages/babel-plugin-react-compiler/docs/passes/10-inferReactivePlaces.md

@@ -0,0 +1,169 @@
+# inferReactivePlaces
+
+## File
+`src/Inference/InferReactivePlaces.ts`
+
+## Purpose
+Determines which `Place`s (identifiers and temporaries) in the HIR are **reactive** - meaning they may *semantically* change over the course of the component or hook's lifetime. This information is critical for memoization: reactive places form the dependencies that, when changed, should invalidate cached values.
+
+A place is reactive if it derives from any source of reactivity:
+1. **Props** - Component parameters may change between renders
+2. **Hooks** - Hooks can access state or context which can change
+3. **`use` operator** - Can access context which may change
+4. **Mutation with reactive operands** - Values mutated in instructions that have reactive operands become reactive themselves
+5. **Conditional assignment based on reactive control flow** - Values assigned in branches controlled by reactive conditions become reactive
+
+## Input Invariants
+- HIR is in SSA form with phi nodes at join points
+- `inferMutationAliasingEffects` and `inferMutationAliasingRanges` have run, establishing:
+  - Effect annotations on operands (Effect.Capture, Effect.Store, Effect.Mutate, etc.)
+  - Mutable ranges on identifiers
+  - Aliasing relationships captured by `findDisjointMutableValues`
+- All operands have known effects (asserts on `Effect.Unknown`)
+
+## Output Guarantees
+- Every reactive Place has `place.reactive = true`
+- Reactivity is transitively complete (derived from reactive → reactive)
+- All identifiers in a mutable alias group share reactivity
+- Reactivity is propagated to operands used within nested function expressions
+
+## Algorithm
+The algorithm uses **fixpoint iteration** to propagate reactivity forward through the control-flow graph:
+
+### Initialization
+1. Create a `ReactivityMap` backed by disjoint sets of mutably-aliased identifiers
+2. Mark all function parameters as reactive (props are reactive by definition)
+3. Create a `ControlDominators` helper to identify blocks controlled by reactive conditions
+
+### Fixpoint Loop
+Iterate until no changes occur:
+
+For each block:
+1. **Phi Nodes**: Mark phi nodes reactive if:
+   - Any operand is reactive, OR
+   - Any predecessor block is controlled by a reactive condition (control-flow dependency)
+
+2. **Instructions**: For each instruction:
+   - Track stable identifier sources (for hooks like `useRef`, `useState` dispatch)
+   - Check if any operand is reactive
+   - Hook calls and `use` operator are sources of reactivity
+   - If instruction has reactive input:
+     - Mark lvalues reactive (unless they are known-stable like `setState` functions)
+   - If instruction has reactive input OR is in reactive-controlled block:
+     - Mark mutable operands (Capture, Store, Mutate effects) as reactive
+
+3. **Terminals**: Check terminal operands for reactivity
+
+### Post-processing
+Propagate reactivity to inner functions (nested `FunctionExpression` and `ObjectMethod`).
+
+## Key Data Structures
+
+### ReactivityMap
+```typescript
+class ReactivityMap {
+  hasChanges: boolean = false;           // Tracks if fixpoint changed
+  reactive: Set<IdentifierId> = new Set(); // Set of reactive identifiers
+  aliasedIdentifiers: DisjointSet<Identifier>; // Mutable alias groups
+}
+```
+- Uses disjoint sets so that when one identifier in an alias group becomes reactive, they all are effectively reactive
+- `isReactive(place)` checks and marks `place.reactive = true` as a side effect
+- `snapshot()` resets change tracking and returns whether changes occurred
+
+### StableSidemap
+```typescript
+class StableSidemap {
+  map: Map<IdentifierId, {isStable: boolean}> = new Map();
+}
+```
+Tracks sources of stability (e.g., `useState()[1]` dispatch function). Forward data-flow analysis that:
+- Records hook calls that return stable types
+- Propagates stability through PropertyLoad and Destructure from stable containers
+- Propagates through LoadLocal and StoreLocal
+
+### ControlDominators
+Uses post-dominator frontier analysis to determine which blocks are controlled by reactive branch conditions.
+
+## Edge Cases
+
+### Backward Reactivity Propagation via Mutable Aliasing
+```javascript
+const x = [];
+const z = [x];
+x.push(props.input);
+return <div>{z}</div>;
+```
+Here `z` aliases `x` which is later mutated with reactive data. The disjoint set ensures `z` becomes reactive even though the mutation happens after its creation.
+
+### Stable Types Are Not Reactive
+```javascript
+const [state, setState] = useState();
+// setState is stable - not marked reactive despite coming from reactive hook
+```
+The `StableSidemap` tracks these and skips marking them reactive.
+
+### Ternary with Stable Values Still Reactive
+```javascript
+props.cond ? setState1 : setState2
+```
+Even though both branches are stable types, the result depends on reactive control flow, so it cannot be marked non-reactive just based on type.
+
+### Phi Nodes with Reactive Predecessors
+When a phi's predecessor block is controlled by a reactive condition, the phi becomes reactive even if its operands are all non-reactive constants.
+
+## TODOs
+No explicit TODO comments are present in the source file. However, comments note:
+
+- **ComputedLoads not handled for stability**: Only PropertyLoad propagates stability from containers, not ComputedLoad. The comment notes this is safe because stable containers have differently-typed elements, but ComputedLoad handling could be added.
+
+## Example
+
+### Fixture: `reactive-dependency-fixpoint.js`
+
+**Input:**
+```javascript
+function Component(props) {
+  let x = 0;
+  let y = 0;
+  while (x === 0) {
+    x = y;
+    y = props.value;
+  }
+  return [x];
+}
+```
+
+**Before InferReactivePlaces:**
+```
+bb1 (loop):
+  store x$26:TPhi:TPhi: phi(bb0: read x$21:TPrimitive, bb3: read x$32:TPhi)
+  store y$30:TPhi:TPhi: phi(bb0: read y$24:TPrimitive, bb3: read y$37)
+  ...
+bb3 (block):
+  [12] mutate? $35 = LoadLocal read props$19
+  [13] mutate? $36 = PropertyLoad read $35.value
+  [14] mutate? $38 = StoreLocal Reassign mutate? y$37 = read $36
+```
+
+**After InferReactivePlaces:**
+```
+bb1 (loop):
+  store x$26:TPhi{reactive}:TPhi: phi(bb0: read x$21:TPrimitive, bb3: read x$32:TPhi{reactive})
+  store y$30:TPhi{reactive}:TPhi: phi(bb0: read y$24:TPrimitive, bb3: read y$37{reactive})
+  [6] mutate? $27:TPhi{reactive} = LoadLocal read x$26:TPhi{reactive}
+  ...
+bb3 (block):
+  [12] mutate? $35{reactive} = LoadLocal read props$19{reactive}
+  [13] mutate? $36{reactive} = PropertyLoad read $35{reactive}.value
+  [14] mutate? $38{reactive} = StoreLocal Reassign mutate? y$37{reactive} = read $36{reactive}
+```
+
+**Key observations:**
+- `props$19` is marked `{reactive}` as a function parameter
+- The reactivity propagates through the loop:
+  - First iteration: `y$37` becomes reactive from `props.value`
+  - Second iteration: `x$32` becomes reactive from `y$30` (which is reactive via the phi from `y$37`)
+  - The phi nodes `x$26` and `y$30` become reactive because their bb3 operands are reactive
+- The fixpoint algorithm handles this backward propagation through the loop correctly
+- The final output `$40` is reactive, so the array `[x]` will be memoized with `x` as a dependency

compiler/packages/babel-plugin-react-compiler/docs/passes/11-inferReactiveScopeVariables.md

@@ -0,0 +1,176 @@
+# inferReactiveScopeVariables
+
+## File
+`src/ReactiveScopes/InferReactiveScopeVariables.ts`
+
+## Purpose
+This is the **1st of 4 passes** that determine how to break a React function into discrete reactive scopes (independently memoizable units of code). Its specific responsibilities are:
+
+1. **Identify operands that mutate together** - Variables that are mutated in the same instruction must be placed in the same reactive scope
+2. **Assign a unique ReactiveScope to each group** - Each disjoint set of co-mutating identifiers gets assigned a unique ScopeId
+3. **Compute the mutable range** - The scope's range is computed as the union of all member identifiers' mutable ranges
+
+The pass does NOT determine which instructions compute each scope, only which variables belong together.
+
+## Input Invariants
+- `InferMutationAliasingEffects` has run - Effects describe mutations, captures, and aliasing
+- `InferMutationAliasingRanges` has run - Each identifier has a valid `mutableRange` property
+- `InferReactivePlaces` has run - Places are marked as reactive or not
+- `RewriteInstructionKindsBasedOnReassignment` has run - Let/Const properly determined
+- All instructions have been numbered with valid `InstructionId` values
+- Phi nodes are properly constructed at block join points
+
+## Output Guarantees
+- Each identifier that is part of a mutable group has its `identifier.scope` property set to a `ReactiveScope` object
+- All identifiers in the same scope share the same `ReactiveScope` reference
+- The scope's `range` is the union (min start, max end) of all member mutable ranges
+- The scope's `range` is validated to be within [1, maxInstruction+1]
+- Identifiers that only have single-instruction lifetimes (read once) may not be assigned to a scope unless they allocate
+
+## Algorithm
+
+### Phase 1: Find Disjoint Mutable Values (`findDisjointMutableValues`)
+
+Uses a Union-Find (Disjoint Set) data structure to group identifiers that mutate together:
+
+1. **Handle Phi Nodes**: For each phi in each block:
+   - If the phi's result is mutated after creation (mutableRange.end > first instruction in block), union the phi with all its operands
+   - This ensures values that flow through control flow and are later mutated are grouped together
+
+2. **Handle Instructions**: For each instruction:
+   - Collect mutable operands based on instruction type:
+     - If lvalue has extended mutable range OR instruction may allocate, include lvalue
+     - For StoreLocal/StoreContext: Include lvalue if it has extended mutable range, include value if mutable
+     - For Destructure: Include each pattern operand with extended range, include source if mutable
+     - For MethodCall: Include all mutable operands plus the computed property (to keep method resolution in same scope)
+     - For other instructions: Include all mutable operands
+   - Exclude global variables (mutableRange.start === 0) since they cannot be recreated
+   - Union all collected operands together
+
+### Phase 2: Assign Scopes
+
+1. Iterate over all identifiers in the disjoint set using `forEach(item, groupIdentifier)`
+2. For each unique group, create a new ReactiveScope:
+   - Generate a unique ScopeId from the environment
+   - Initialize range from the first member's mutableRange
+   - Set up empty dependencies, declarations, reassignments sets
+3. For subsequent members of the same group:
+   - Expand the scope's range to encompass the member's mutableRange
+   - Merge source locations
+4. Assign the scope to each identifier: `identifier.scope = scope`
+5. Update each identifier's mutableRange to match the scope's range
+
+**Validation**: After scope assignment, validate that all scopes have valid ranges within [1, maxInstruction+1].
+
+## Key Data Structures
+
+### DisjointSet<Identifier>
+A Union-Find data structure optimized for grouping items into disjoint sets:
+
+```typescript
+class DisjointSet<T> {
+  #entries: Map<T, T>;  // Maps each item to its parent (root points to self)
+
+  union(items: Array<T>): void;     // Merge items into one set
+  find(item: T): T | null;          // Find the root of item's set (with path compression)
+  forEach(fn: (item, group) => void): void;  // Iterate all items with their group root
+}
+```
+
+Path compression is used during `find()` to flatten the tree structure, improving subsequent lookup performance.
+
+### ReactiveScope
+```typescript
+type ReactiveScope = {
+  id: ScopeId;
+  range: MutableRange;              // [start, end) instruction range
+  dependencies: Set<ReactiveScopeDependency>;  // Inputs (populated later)
+  declarations: Map<IdentifierId, ReactiveScopeDeclaration>;  // Outputs (populated later)
+  reassignments: Set<Identifier>;   // Reassigned variables (populated later)
+  earlyReturnValue: {...} | null;   // For scopes with early returns
+  merged: Set<ScopeId>;             // IDs of scopes merged into this one
+  loc: SourceLocation;
+};
+```
+
+## Edge Cases
+
+### Global Variables
+Excluded from scopes (mutableRange.start === 0) since they cannot be recreated during memoization.
+
+### Phi Nodes After Mutation
+When a phi's result is mutated after the join point, all phi operands must be in the same scope to ensure the mutation can be recomputed correctly.
+
+### MethodCall Property Resolution
+The computed property load for a method call is explicitly added to the same scope as the call itself.
+
+### Allocating Instructions
+Instructions that allocate (Array, Object, JSX, etc.) add their lvalue to the scope even if the lvalue has a single-instruction range.
+
+### Single-Instruction Ranges
+Values with range `[n, n+1)` (used exactly once) are only included if they allocate, otherwise they're just read.
+
+### enableForest Config
+When enabled, phi operands are unconditionally unioned with the phi result (even without mutation after the phi).
+
+## TODOs
+1. `// TODO: improve handling of module-scoped variables and globals` - The current approach excludes globals entirely, but a more nuanced handling could be beneficial.
+
+2. Known issue with aliasing and mutable lifetimes (from header comments):
+```javascript
+let x = {};
+let y = [];
+x.y = y; // RHS is not considered mutable here bc not further mutation
+mutate(x); // bc y is aliased here, it should still be considered mutable above
+```
+This suggests the pass may miss some co-mutation relationships when aliasing is involved.
+
+## Example
+
+### Fixture: `reactive-scope-grouping.js`
+
+**Input:**
+```javascript
+function foo() {
+  let x = {};
+  let y = [];
+  let z = {};
+  y.push(z);  // y and z co-mutate (z captured into y)
+  x.y = y;    // x and y co-mutate (y captured into x)
+  return x;
+}
+```
+
+**After InferReactiveScopeVariables:**
+```
+[1] mutate? $19_@0[1:14] = Object { }     // x's initial object, scope @0
+[2] store $21_@0[1:14] = StoreLocal x     // x in scope @0
+[3] mutate? $22_@1[3:11] = Array []       // y's array, scope @1
+[4] store $24_@1[3:11] = StoreLocal y     // y in scope @1
+[5] mutate? $25_@2 = Object { }           // z's object, scope @2
+[10] MethodCall y.push(z)                 // Mutates y, captures z
+[13] PropertyStore x.y = y                // Mutates x, captures y
+```
+
+The `y.push(z)` joins y and z into scope @1, and `x.y = y` joins x and y into scope @0. Because y is now in @0, and z was captured into y, ultimately x, y, and z all end up in the same scope @0.
+
+**Compiled Output:**
+```javascript
+function foo() {
+  const $ = _c(1);
+  let x;
+  if ($[0] === Symbol.for("react.memo_cache_sentinel")) {
+    x = {};
+    const y = [];
+    const z = {};
+    y.push(z);
+    x.y = y;
+    $[0] = x;
+  } else {
+    x = $[0];
+  }
+  return x;
+}
+```
+
+All three objects (x, y, z) are created within the same memoization block because they co-mutate and could potentially alias each other.

compiler/packages/babel-plugin-react-compiler/docs/passes/12-rewriteInstructionKindsBasedOnReassignment.md

@@ -0,0 +1,151 @@
+# rewriteInstructionKindsBasedOnReassignment
+
+## File
+`src/SSA/RewriteInstructionKindsBasedOnReassignment.ts`
+
+## Purpose
+Rewrites the `InstructionKind` of variable declaration and assignment instructions to correctly reflect whether variables should be declared as `const` or `let` in the final output. It determines this based on whether a variable is subsequently reassigned after its initial declaration.
+
+The key insight is that this pass runs **after dead code elimination (DCE)**, so a variable that was originally declared with `let` in the source (because it was reassigned) may be converted to `const` if the reassignment was removed by DCE. However, variables originally declared as `const` cannot become `let`.
+
+## Input Invariants
+- SSA form: Each identifier has a unique `IdentifierId` and `DeclarationId`
+- Dead code elimination has run: Unused assignments have been removed
+- Mutation/aliasing inference complete: Runs after `InferMutationAliasingRanges` and `InferReactivePlaces` in the main pipeline
+- All instruction kinds are initially set (typically `Let` for variables that may be reassigned)
+
+## Output Guarantees
+- **First declaration gets `Const` or `Let`**: The first `StoreLocal` for a named variable is marked as:
+  - `InstructionKind.Const` if the variable is never reassigned after
+  - `InstructionKind.Let` if the variable has subsequent reassignments
+- **Reassignments marked as `Reassign`**: Any subsequent `StoreLocal` to the same `DeclarationId` is marked as `InstructionKind.Reassign`
+- **Destructure consistency**: All places in a destructuring pattern must have consistent kinds (all Const or all Reassign)
+- **Update operations trigger Let**: `PrefixUpdate` and `PostfixUpdate` operations (like `++x` or `x--`) mark the original declaration as `Let`
+
+## Algorithm
+
+1. **Initialize declarations map**: Create a `Map<DeclarationId, LValue | LValuePattern>` to track declared variables.
+
+2. **Seed with parameters and context**: Add all named function parameters and captured context variables to the map with kind `Let` (since they're already "declared" outside the function body).
+
+3. **Process blocks in order**: Iterate through all blocks and instructions:
+
+   - **DeclareLocal**: Record the declaration in the map (invariant: must not already exist)
+
+   - **StoreLocal**:
+     - If not in map: This is the first store, add to map with `kind = Const`
+     - If already in map: This is a reassignment. Update original declaration to `Let`, set current instruction to `Reassign`
+
+   - **Destructure**:
+     - For each operand in the pattern, check if it's already declared
+     - All operands must be consistent (all new declarations OR all reassignments)
+     - Set pattern kind to `Const` for new declarations, `Reassign` for existing ones
+
+   - **PrefixUpdate / PostfixUpdate**: Look up the declaration and mark it as `Let` (these always imply reassignment)
+
+## Key Data Structures
+
+```typescript
+// Main tracking structure
+const declarations = new Map<DeclarationId, LValue | LValuePattern>();
+
+// InstructionKind enum (from HIR.ts)
+enum InstructionKind {
+  Const = 'Const',      // const declaration
+  Let = 'Let',          // let declaration
+  Reassign = 'Reassign', // reassignment to existing binding
+  Catch = 'Catch',      // catch clause binding
+  HoistedLet = 'HoistedLet',     // hoisted let
+  HoistedConst = 'HoistedConst', // hoisted const
+  HoistedFunction = 'HoistedFunction', // hoisted function
+  Function = 'Function', // function declaration
+}
+```
+
+## Edge Cases
+
+### DCE Removes Reassignment
+A `let x = 0; x = 1;` where `x = 1` is unused becomes `const x = 0;` after DCE.
+
+### Destructuring with Mixed Operands
+The invariant checks ensure all operands in a destructure pattern are either all new declarations or all reassignments. Mixed cases cause a compiler error.
+
+### Value Blocks with DCE
+There's a TODO for handling reassignment in value blocks where the original declaration was removed by DCE.
+
+### Parameters and Context Variables
+These are pre-seeded as `Let` in the declarations map since they're conceptually "declared" at function entry.
+
+### Update Expressions
+`++x` and `x--` always mark the variable as `Let`, even if used inline.
+
+## TODOs
+```typescript
+CompilerError.invariant(block.kind !== 'value', {
+  reason: `TODO: Handle reassignment in a value block where the original
+           declaration was removed by dead code elimination (DCE)`,
+  ...
+});
+```
+
+This indicates an edge case where a destructuring reassignment occurs in a value block but the original declaration was eliminated by DCE. This is currently an invariant violation rather than handled gracefully.
+
+## Example
+
+### Fixture: `reassignment.js`
+
+**Input Source:**
+```javascript
+function Component(props) {
+  let x = [];
+  x.push(props.p0);
+  let y = x;
+
+  x = [];
+  let _ = <Component x={x} />;
+
+  y.push(props.p1);
+
+  return <Component x={x} y={y} />;
+}
+```
+
+**Before Pass (InferReactivePlaces output):**
+```
+[2] StoreLocal Let x$32 = $31      // x is initially marked Let
+[9] StoreLocal Let y$40 = $39      // y is initially marked Let
+[11] StoreLocal Reassign x$43 = $42  // reassignment already marked
+```
+
+**After Pass:**
+```
+[2] StoreLocal Let x$32 = $31      // x stays Let (has reassignment at line 11)
+[9] StoreLocal Const y$40 = $39    // y becomes Const (never reassigned)
+[11] StoreLocal Reassign x$43 = $42  // stays Reassign
+```
+
+**Final Generated Code:**
+```javascript
+function Component(props) {
+  const $ = _c(4);
+  let t0;
+  if ($[0] !== props.p0 || $[1] !== props.p1) {
+    let x = [];              // let because reassigned
+    x.push(props.p0);
+    const y = x;             // const because never reassigned
+    // ... x = t1; (reassignment)
+    y.push(props.p1);
+    t0 = <Component x={x} y={y} />;
+    // ...
+  }
+  return t0;
+}
+```
+
+The pass correctly identified that `x` needs `let` (since it's reassigned on line 6 of the source) while `y` can use `const` (it's never reassigned after initialization).
+
+## Where This Pass is Called
+
+1. **Main Pipeline** (`src/Entrypoint/Pipeline.ts:322`): Called after `InferReactivePlaces` and before `InferReactiveScopeVariables`.
+
+2. **AnalyseFunctions** (`src/Inference/AnalyseFunctions.ts:58`): Called when lowering inner function expressions as part of the function analysis phase.

compiler/packages/babel-plugin-react-compiler/docs/passes/13-alignMethodCallScopes.md

@@ -0,0 +1,131 @@
+# alignMethodCallScopes
+
+## File
+`src/ReactiveScopes/AlignMethodCallScopes.ts`
+
+## Purpose
+Ensures that `MethodCall` instructions and their associated `PropertyLoad` instructions (which load the method being called) have consistent scope assignments. The pass enforces one of two invariants:
+1. Both the MethodCall lvalue and the property have the **same** reactive scope
+2. **Neither** has a reactive scope
+
+This alignment is critical because the PropertyLoad and MethodCall are semantically a single operation (`receiver.method(args)`) and must be memoized together as a unit. If they had different scopes, the generated code would incorrectly try to memoize the property load separately from the method call, which could break correctness.
+
+## Input Invariants
+- The function has been converted to HIR form
+- `inferReactiveScopeVariables` has already run, assigning initial reactive scopes to identifiers based on mutation analysis
+- Each instruction's lvalue has an `identifier.scope` that is either a `ReactiveScope` or `null`
+- For `MethodCall` instructions, the `value.property` field contains a `Place` referencing the loaded method
+
+## Output Guarantees
+After this pass runs:
+- For every `MethodCall` instruction in the function:
+  - If the lvalue has a scope AND the property has a scope, they point to the **same merged scope**
+  - If only the lvalue has a scope, the property's scope is set to match the lvalue's scope
+  - If only the property has a scope, the property's scope is set to `null` (so neither has a scope)
+- Merged scopes have their `range` extended to cover the union of the original scopes' ranges
+- Nested functions (FunctionExpression, ObjectMethod) are recursively processed
+
+## Algorithm
+
+### Phase 1: Collect Scope Relationships
+```
+For each instruction in all blocks:
+  If instruction is a MethodCall:
+    lvalueScope = instruction.lvalue.identifier.scope
+    propertyScope = instruction.value.property.identifier.scope
+
+    If both have scopes:
+      Record that these scopes should be merged (using DisjointSet.union)
+    Else if only lvalue has scope:
+      Record that property should be assigned to lvalueScope
+    Else if only property has scope:
+      Record that property should be assigned to null (no scope)
+
+  If instruction is FunctionExpression or ObjectMethod:
+    Recursively process the nested function
+```
+
+### Phase 2: Merge Scopes
+```
+For each merged scope group:
+  Pick a "root" scope
+  Extend root's range to cover all merged scopes:
+    root.range.start = min(all scope start points)
+    root.range.end = max(all scope end points)
+```
+
+### Phase 3: Apply Changes
+```
+For each instruction:
+  If lvalue was recorded for remapping:
+    Set identifier.scope to the mapped value
+  Else if identifier has a scope that was merged:
+    Set identifier.scope to the merged root scope
+```
+
+## Key Data Structures
+
+1. **`scopeMapping: Map<IdentifierId, ReactiveScope | null>`**
+   - Maps property identifier IDs to their new scope assignment
+   - Value of `null` means the scope should be removed
+
+2. **`mergedScopes: DisjointSet<ReactiveScope>`**
+   - Union-find data structure tracking scopes that need to be merged
+   - Used when both MethodCall and property have different scopes
+
+3. **`ReactiveScope`** (from HIR)
+   - Contains `range: { start: InstructionId, end: InstructionId }`
+   - The range defines which instructions are part of the scope
+
+## Edge Cases
+
+### Both Have the Same Scope Already
+No action needed (implicit in the logic).
+
+### Nested Functions
+The pass recursively processes `FunctionExpression` and `ObjectMethod` instructions to handle closures.
+
+### Multiple MethodCalls Sharing Scopes
+The DisjointSet handles transitive merging - if A merges with B, and B merges with C, all three end up in the same scope.
+
+### Property Without Scope, MethodCall Without Scope
+No action needed (both already aligned at `null`).
+
+## TODOs
+There are no explicit TODO comments in the source code.
+
+## Example
+
+### Fixture: `alias-capture-in-method-receiver.js`
+
+**Source code:**
+```javascript
+function Component() {
+  let a = someObj();
+  let x = [];
+  x.push(a);
+  return [x, a];
+}
+```
+
+**Before AlignMethodCallScopes:**
+```
+[7] store $24_@1[4:10]:TFunction = PropertyLoad capture $23_@1.push
+[9] mutate? $26:TPrimitive = MethodCall store $23_@1.read $24_@1(capture $25)
+```
+- PropertyLoad result `$24_@1` has scope `@1`
+- MethodCall result `$26` has no scope (`null`)
+
+**After AlignMethodCallScopes:**
+```
+[7] store $24[4:10]:TFunction = PropertyLoad capture $23_@1.push
+[9] mutate? $26:TPrimitive = MethodCall store $23_@1.read $24(capture $25)
+```
+- PropertyLoad result `$24` now has **no scope** (the `_@1` suffix removed)
+- MethodCall result `$26` still has no scope
+
+**Why this matters:**
+Without this alignment, later passes might try to memoize the `.push` property load separately from the actual `push()` call. This would be incorrect because:
+1. Reading a method from an object and calling it are semantically one operation
+2. The property load's value (the bound method) is only valid immediately when called on the same receiver
+3. Separate memoization could lead to stale method references or incorrect this-binding

compiler/packages/babel-plugin-react-compiler/docs/passes/14-alignObjectMethodScopes.md

@@ -0,0 +1,128 @@
+# alignObjectMethodScopes
+
+## File
+`src/ReactiveScopes/AlignObjectMethodScopes.ts`
+
+## Purpose
+Ensures that object method values and their enclosing object expressions share the same reactive scope. This is critical for code generation because JavaScript requires object method definitions to be inlined within their containing object literals. If the object method and object expression were in different reactive scopes (which map to different memoization blocks), the generated code would be invalid since you cannot reference an object method defined in one block from an object literal in a different block.
+
+From the file's documentation:
+> "To produce a well-formed JS program in Codegen, object methods and object expressions must be in the same ReactiveBlock as object method definitions must be inlined."
+
+## Input Invariants
+- Reactive scopes have been inferred: This pass runs after `InferReactiveScopeVariables`
+- ObjectMethod and ObjectExpression have non-null scopes: The pass asserts this with an invariant check
+- Scopes are disjoint across functions: The pass assumes that scopes do not overlap between parent and nested functions
+
+## Output Guarantees
+- ObjectMethod and ObjectExpression share the same scope: Any ObjectMethod used as a property in an ObjectExpression will have its scope merged with the ObjectExpression's scope
+- Merged scope covers both ranges: The resulting merged scope's range is expanded to cover the minimum start and maximum end of all merged scopes
+- All identifiers are repointed: All identifiers whose scopes were merged are updated to point to the canonical root scope
+- Inner functions are also processed: The pass recursively handles nested ObjectMethod and FunctionExpression values
+
+## Algorithm
+
+### Phase 1: Find Scopes to Merge (`findScopesToMerge`)
+1. Iterate through all blocks and instructions in the function
+2. Track all ObjectMethod declarations in a set by their lvalue identifier
+3. When encountering an ObjectExpression, check each operand:
+   - If an operand's identifier was previously recorded as an ObjectMethod declaration
+   - Get the scope of both the ObjectMethod operand and the ObjectExpression lvalue
+   - Assert both scopes are non-null
+   - Union these two scopes together in a DisjointSet data structure
+
+### Phase 2: Merge and Repoint Scopes (`alignObjectMethodScopes`)
+1. Recursively process inner functions first (ObjectMethod and FunctionExpression values)
+2. Canonicalize the DisjointSet to get a mapping from each scope to its root
+3. **Step 1 - Merge ranges**: For each scope that maps to a different root:
+   - Expand the root's range to encompass both the original range and the merged scope's range
+   - `root.range.start = min(scope.range.start, root.range.start)`
+   - `root.range.end = max(scope.range.end, root.range.end)`
+4. **Step 2 - Repoint identifiers**: For each instruction's lvalue:
+   - If the identifier has a scope that was merged
+   - Update the identifier's scope reference to point to the canonical root
+
+## Key Data Structures
+
+1. **DisjointSet<ReactiveScope>** - A union-find data structure that tracks which scopes should be merged together. Uses path compression for efficient `find()` operations.
+
+2. **Set<Identifier>** - Tracks which identifiers are ObjectMethod declarations, used to identify when an ObjectExpression operand is an object method.
+
+3. **ReactiveScope** - Contains:
+   - `id: ScopeId` - Unique identifier
+   - `range: MutableRange` - Start and end instruction IDs
+   - `dependencies` - Inputs to the scope
+   - `declarations` - Values produced by the scope
+
+4. **MutableRange** - Has `start` and `end` InstructionId fields that define the scope's extent.
+
+## Edge Cases
+
+### Nested Object Methods
+When an object method itself contains another object with methods, the pass recursively processes inner functions first before handling the outer function's scopes.
+
+### Multiple Object Methods in Same Object
+If an object has multiple method properties, all their scopes will be merged with the object's scope through the DisjointSet.
+
+### Object Methods in Conditional Expressions
+Object methods inside ternary expressions still need scope alignment to ensure the method and its containing object are in the same reactive block.
+
+### Method Call After Object Creation
+The pass works in conjunction with `AlignMethodCallScopes` (which runs immediately before) to ensure that method calls on objects with object methods are also properly scoped.
+
+## TODOs
+None explicitly marked in the source file.
+
+## Example
+
+### Fixture: `object-method-shorthand.js`
+
+**Input:**
+```javascript
+function Component() {
+  let obj = {
+    method() {
+      return 1;
+    },
+  };
+  return obj.method();
+}
+```
+
+**Before AlignObjectMethodScopes:**
+```
+InferReactiveScopeVariables:
+  [1] mutate? $12_@0:TObjectMethod = ObjectMethod ...    // scope @0
+  [2] mutate? $14_@1[2:7]:TObject = Object { method: ... } // scope @1 (range 2:7)
+```
+The ObjectMethod `$12` is in scope `@0` while the ObjectExpression `$14` is in scope `@1` with range `[2:7]`.
+
+**After AlignObjectMethodScopes:**
+```
+AlignObjectMethodScopes:
+  [1] mutate? $12_@0[1:7]:TObjectMethod = ObjectMethod ... // scope @0, range now 1:7
+  [2] mutate? $14_@0[1:7]:TObject = Object { method: ... } // also scope @0, range 1:7
+```
+Both identifiers are in the same scope `@0`, and the scope's range has been expanded to `[1:7]` to cover both instructions.
+
+**Final Generated Code:**
+```javascript
+function Component() {
+  const $ = _c(1);
+  let t0;
+  if ($[0] === Symbol.for("react.memo_cache_sentinel")) {
+    const obj = {
+      method() {
+        return 1;
+      },
+    };
+    t0 = obj.method();
+    $[0] = t0;
+  } else {
+    t0 = $[0];
+  }
+  return t0;
+}
+```
+
+The object literal with its method and the subsequent method call are all inside the same memoization block, producing valid JavaScript where the method definition is inlined within the object literal.

compiler/packages/babel-plugin-react-compiler/docs/passes/15-alignReactiveScopesToBlockScopesHIR.md

@@ -0,0 +1,177 @@
+# alignReactiveScopesToBlockScopesHIR
+
+## File
+`src/ReactiveScopes/AlignReactiveScopesToBlockScopesHIR.ts`
+
+## Purpose
+This is the **2nd of 4 passes** that determine how to break a function into discrete reactive scopes (independently memoizable units of code). The pass aligns reactive scope boundaries to control flow (block scope) boundaries.
+
+The problem it solves: Prior inference passes assign reactive scopes to operands based on mutation ranges at arbitrary instruction points in the control-flow graph. However, to generate memoization blocks around instructions, scopes must be aligned to block-scope boundaries -- you cannot memoize half of a loop or half of an if-block.
+
+**Example from the source code comments:**
+```javascript
+function foo(cond, a) {
+                    // original scope end
+                         // expanded scope end
+   const x = [];    |    |
+   if (cond) {      |    |
+     ...            |    |
+     x.push(a);     <--- original scope ended here
+     ...                 |
+   }                     <--- scope must extend to here
+}
+```
+
+## Input Invariants
+- `InferReactiveScopeVariables` has run: Each identifier has been assigned a `ReactiveScope` with a `range` (start/end instruction IDs) based on mutation analysis
+- The HIR is in SSA form: Blocks have unique IDs, instructions have unique IDs, and control flow is represented with basic blocks
+- Each block has a terminal with possible successors and fallthroughs
+- Each scope has a mutable range `{start: InstructionId, end: InstructionId}` indicating when the scope is active
+
+## Output Guarantees
+- **Scopes end at valid block boundaries**: A reactive scope may only end at the same block scope level as it began. The scope's `range.end` is updated to the first instruction of the fallthrough block after any control flow structure that the scope overlaps
+- **Scopes start at valid block boundaries**: For labeled breaks (gotos to a label), scopes that extend beyond the goto have their `range.start` extended back to include the label
+- **Value blocks (ternary, logical, optional) are handled specially**: Scopes inside value blocks are extended to align with the outer block scope's instruction range
+
+## Algorithm
+
+The pass performs a single forward traversal over all blocks:
+
+### 1. Tracking Active Scopes
+- Maintains `activeScopes: Set<ReactiveScope>` - scopes whose range overlaps the current block
+- Maintains `activeBlockFallthroughRanges: Array<{range, fallthrough}>` - stack of pending block-fallthrough ranges
+
+### 2. Per-Block Processing
+For each block:
+- Prune `activeScopes` to only those that extend past the current block's start
+- If this block is a fallthrough target, pop the range from the stack and extend all active scopes' start to the range start
+
+### 3. Recording Places
+For each instruction lvalue and operand:
+- If the place has a scope, add it to `activeScopes`
+- If inside a value block, extend the scope's range to match the value block's outer range
+
+### 4. Handling Block Fallthroughs
+When a terminal has a fallthrough (not a simple branch):
+- Extend all active scopes whose `range.end > terminal.id` to at least the first instruction of the fallthrough block
+- Push the fallthrough range onto the stack for future scopes
+
+### 5. Handling Labeled Breaks (Goto)
+When encountering a goto to a label (not the natural fallthrough):
+- Find the corresponding fallthrough range on the stack
+- Extend all active scopes to span from the label start to its fallthrough end
+
+### 6. Value Block Handling
+For ternary, logical, and optional terminals:
+- Create `ValueBlockNode` to track the outer block's instruction range
+- Scopes inside value blocks inherit this range, ensuring they align to the outer block scope
+
+## Key Data Structures
+
+```typescript
+type ValueBlockNode = {
+  kind: 'node';
+  id: InstructionId;
+  valueRange: MutableRange;  // Range of outer block scope
+  children: Array<ValueBlockNode | ReactiveScopeNode>;
+};
+
+type ReactiveScopeNode = {
+  kind: 'scope';
+  id: InstructionId;
+  scope: ReactiveScope;
+};
+
+// Tracked during traversal:
+activeBlockFallthroughRanges: Array<{
+  range: InstructionRange;
+  fallthrough: BlockId;
+}>;
+activeScopes: Set<ReactiveScope>;
+valueBlockNodes: Map<BlockId, ValueBlockNode>;
+```
+
+## Edge Cases
+
+### Labeled Breaks
+When a `goto` jumps to a label (not the natural fallthrough), scopes must be extended to include the entire labeled block range, preventing the break from jumping out of the scope.
+
+### Value Blocks (Ternary/Logical/Optional)
+These create nested "value" contexts. Scopes inside must be aligned to the outer block scope's boundaries, not the value block's boundaries.
+
+### Nested Control Flow
+Deeply nested if-statements require the scope to be extended through all levels back to the outermost block where the scope started.
+
+### do-while and try/catch
+The terminal's successor might be a block (not value block), which is handled specially.
+
+## TODOs
+1. `// TODO: consider pruning activeScopes per instruction` - Currently, `activeScopes` is only pruned at block start points. Some scopes may no longer be active by the time a goto is encountered.
+
+2. `// TODO: add a variant of eachTerminalSuccessor() that visits _all_ successors, not just those that are direct successors for normal control-flow ordering.` - The current implementation uses `mapTerminalSuccessors` which may not visit all successors in all cases.
+
+## Example
+
+### Fixture: `extend-scopes-if.js`
+
+**Input:**
+```javascript
+function foo(a, b, c) {
+  let x = [];
+  if (a) {
+    if (b) {
+      if (c) {
+        x.push(0);  // Mutation of x ends here (instruction 12-13)
+      }
+    }
+  }
+  if (x.length) {  // instruction 16
+    return x;
+  }
+  return null;
+}
+```
+
+**Before AlignReactiveScopesToBlockScopesHIR:**
+```
+x$23_@0[1:13]  // Scope range 1-13
+```
+The scope for `x` ends at instruction 13 (inside the innermost if block).
+
+**After AlignReactiveScopesToBlockScopesHIR:**
+```
+x$23_@0[1:16]  // Scope range extended to 1-16
+```
+The scope is extended to instruction 16 (the first instruction after all the nested if-blocks), aligning to the block scope boundary.
+
+**Generated Code:**
+```javascript
+function foo(a, b, c) {
+  const $ = _c(4);
+  let x;
+  if ($[0] !== a || $[1] !== b || $[2] !== c) {
+    x = [];
+    if (a) {
+      if (b) {
+        if (c) {
+          x.push(0);
+        }
+      }
+    }
+    // Scope ends here, after ALL the if-blocks
+    $[0] = a;
+    $[1] = b;
+    $[2] = c;
+    $[3] = x;
+  } else {
+    x = $[3];
+  }
+  // Code outside the scope
+  if (x.length) {
+    return x;
+  }
+  return null;
+}
+```
+
+The memoization block correctly wraps the entire nested if-structure, not just part of it.

compiler/packages/babel-plugin-react-compiler/docs/passes/16-mergeOverlappingReactiveScopesHIR.md

@@ -0,0 +1,134 @@
+# mergeOverlappingReactiveScopesHIR
+
+## File
+`src/HIR/MergeOverlappingReactiveScopesHIR.ts`
+
+## Purpose
+This pass ensures that reactive scope ranges form valid, non-overlapping blocks in the output JavaScript program. It merges reactive scopes that would otherwise be inconsistent with each other due to:
+
+1. **Overlapping ranges**: Scopes whose instruction ranges partially overlap (not disjoint and not nested) must be merged because the compiler cannot produce valid `if-else` memo blocks for overlapping scopes.
+
+2. **Cross-scope mutations**: When an instruction within one scope mutates a value belonging to a different (outer) scope, those scopes must be merged to maintain correctness.
+
+The pass guarantees that after execution, any two reactive scopes are either:
+- Entirely disjoint (no common instructions)
+- Properly nested (one scope is completely contained within the other)
+
+## Input Invariants
+- Reactive scope variables have been inferred (`InferReactiveScopeVariables` pass has run)
+- Scopes have been aligned to block scopes (`AlignReactiveScopesToBlockScopesHIR` pass has run)
+- Each `Place` may have an associated `ReactiveScope` with a `range` (start/end instruction IDs)
+- Scopes may still have overlapping ranges or contain instructions that mutate outer scopes
+
+## Output Guarantees
+- **No overlapping scopes**: All reactive scopes either are disjoint or properly nested
+- **Consistent mutation boundaries**: Instructions only mutate their "active" scope (the innermost containing scope)
+- **Merged scope ranges**: Merged scopes have their ranges extended to cover the union of all constituent scopes
+- **Updated references**: All `Place` references have their `identifier.scope` updated to point to the merged scope
+
+## Algorithm
+
+### Phase 1: Collect Scope Information (`collectScopeInfo`)
+- Iterates through all instructions and terminals in the function
+- Records for each `Place`:
+  - The scope it belongs to (`placeScopes` map)
+  - When scopes start and end (`scopeStarts` and `scopeEnds` arrays, sorted in descending order by ID)
+- Only records scopes with non-empty ranges (`range.start !== range.end`)
+
+### Phase 2: Detect Overlapping Scopes (`getOverlappingReactiveScopes`)
+Uses a stack-based traversal to track "active" scopes at each instruction:
+
+1. **For each instruction/terminal**:
+   - **Handle scope endings**: Pop completed scopes from the active stack. If a scope ends while other scopes that started later are still active (detected by finding the scope is not at the top of the stack), those scopes overlap and must be merged via `DisjointSet.union()`.
+
+   - **Handle scope starts**: Push new scopes onto the active stack (sorted by end time descending so earlier-ending scopes are at the top). Merge any scopes that have identical start/end ranges.
+
+   - **Handle mutations**: For each operand/lvalue, if it:
+     - Has an associated scope
+     - Is mutable at the current instruction
+     - The scope is active but not at the top of the stack (i.e., an outer scope)
+
+     Then merge all scopes from the mutated outer scope to the top of the stack.
+
+2. **Special case**: Primitive operands in `FunctionExpression` and `ObjectMethod` are skipped.
+
+### Phase 3: Merge Scopes and Rewrite References
+1. For each scope in the disjoint set, compute the merged range as the union (min start, max end)
+2. Update all `Place.identifier.scope` references to point to the merged "group" scope
+
+## Key Data Structures
+
+### ScopeInfo
+```typescript
+type ScopeInfo = {
+  scopeStarts: Array<{id: InstructionId; scopes: Set<ReactiveScope>}>;
+  scopeEnds: Array<{id: InstructionId; scopes: Set<ReactiveScope>}>;
+  placeScopes: Map<Place, ReactiveScope>;
+};
+```
+
+### TraversalState
+```typescript
+type TraversalState = {
+  joined: DisjointSet<ReactiveScope>;  // Union-find for merged scopes
+  activeScopes: Array<ReactiveScope>;   // Stack of currently active scopes
+};
+```
+
+### DisjointSet<ReactiveScope>
+A union-find data structure that tracks which scopes should be merged into the same group.
+
+## Edge Cases
+
+### Identical Scope Ranges
+When multiple scopes have the exact same start and end, they are automatically merged since they would produce the same reactive block.
+
+### Empty Scopes
+Scopes where `range.start === range.end` are skipped entirely.
+
+### Primitive Captures in Functions
+When a `FunctionExpression` or `ObjectMethod` captures a primitive operand, it's excluded from scope merging analysis.
+
+### JSX Single-Instruction Scopes
+The comment in the code notes this isn't perfect - mutating scopes may get merged with JSX single-instruction scopes.
+
+### Non-Mutating Captures
+The pass records both mutating and non-mutating scopes to handle cases where still-mutating values are aliased by inner scopes.
+
+## TODOs
+From the comments in the source file, the design constraints arise from the current compiler output design:
+- **Instruction ordering is preserved**: If reordering were allowed, disjoint ranges could be produced by reordering mutating instructions
+- **One if-else block per scope**: The current design doesn't allow composing a reactive scope from disconnected instruction ranges
+
+## Example
+
+### Fixture: `overlapping-scopes-interleaved.js`
+
+**Input Code:**
+```javascript
+function foo(a, b) {
+  let x = [];
+  let y = [];
+  x.push(a);
+  y.push(b);
+}
+```
+
+**Before MergeOverlappingReactiveScopesHIR:**
+```
+[1] $20_@0[1:9] = Array []        // x belongs to scope @0, range [1:9]
+[2] x$21_@0[1:9] = StoreLocal...
+[3] $23_@1[3:13] = Array []       // y belongs to scope @1, range [3:13]
+[4] y$24_@1[3:13] = StoreLocal...
+```
+Scopes @0 [1:9] and @1 [3:13] overlap: @0 starts at 1, @1 starts at 3, @0 ends at 9, @1 ends at 13. This is invalid.
+
+**After MergeOverlappingReactiveScopesHIR:**
+```
+[1] $20_@0[1:13] = Array []       // Merged scope @0, range [1:13]
+[2] x$21_@0[1:13] = StoreLocal...
+[3] $23_@0[1:13] = Array []       // Now also scope @0
+[4] y$24_@0[1:13] = StoreLocal...
+```
+
+Both `x` and `y` now belong to the same merged scope @0 with range [1:13], producing a single `if-else` memo block in the output.

compiler/packages/babel-plugin-react-compiler/docs/passes/17-buildReactiveScopeTerminalsHIR.md

@@ -0,0 +1,161 @@
+# buildReactiveScopeTerminalsHIR
+
+## File
+`src/HIR/BuildReactiveScopeTerminalsHIR.ts`
+
+## Purpose
+This pass transforms the HIR by inserting `ReactiveScopeTerminal` nodes to explicitly demarcate the boundaries of reactive scopes within the control flow graph. It converts the implicit scope ranges (stored on identifiers as `identifier.scope.range`) into explicit control flow structure by:
+
+1. Inserting a `scope` terminal at the **start** of each reactive scope
+2. Inserting a `goto` terminal at the **end** of each reactive scope
+3. Creating fallthrough blocks to properly connect the scopes to the rest of the CFG
+
+This transformation makes scope boundaries first-class elements in the CFG, which is essential for later passes that generate the memoization code (the `if ($[n] !== dep)` checks).
+
+## Input Invariants
+- **Properly nested scopes and blocks**: The pass assumes `assertValidBlockNesting` has passed, meaning all program blocks and reactive scopes form a proper tree hierarchy
+- **Aligned scope ranges**: Reactive scope ranges have been correctly aligned and merged by previous passes
+- **Valid instruction IDs**: All instructions have sequential IDs that define the scope boundaries
+- **Scopes attached to identifiers**: Reactive scopes are found by traversing all `Place` operands and collecting unique non-empty scopes
+
+## Output Guarantees
+- **Explicit scope terminals**: Each reactive scope is represented in the CFG as a `ReactiveScopeTerminal` with:
+  - `block` - The BlockId containing the scope's instructions
+  - `fallthrough` - The BlockId that executes after the scope
+- **Proper block structure**: Original blocks are split at scope boundaries
+- **Restored HIR invariants**: The pass restores RPO ordering, predecessor sets, instruction IDs, and scope/identifier ranges
+- **Updated phi nodes**: Phi operands are repointed when their source blocks are split
+
+## Algorithm
+
+### Step 1: Collect Scope Rewrites
+```
+for each reactive scope (in range pre-order):
+  push StartScope rewrite at scope.range.start
+  push EndScope rewrite at scope.range.end
+```
+The `recursivelyTraverseItems` helper traverses scopes in pre-order (outer scopes before inner scopes).
+
+### Step 2: Apply Rewrites by Splitting Blocks
+```
+reverse queuedRewrites (to pop in ascending instruction order)
+for each block:
+  for each instruction (or terminal):
+    while there are rewrites <= current instruction ID:
+      split block at current index
+      insert scope terminal (for start) or goto terminal (for end)
+  emit final block segment with original terminal
+```
+
+### Step 3: Repoint Phi Nodes
+When a block is split, its final segment gets a new BlockId. Phi operands that referenced the original block are updated to reference the new final block.
+
+### Step 4: Restore HIR Invariants
+- Recompute RPO (reverse post-order) block traversal
+- Recalculate predecessor sets
+- Renumber instruction IDs
+- Fix scope and identifier ranges to match new instruction IDs
+
+## Key Data Structures
+
+### TerminalRewriteInfo
+```typescript
+type TerminalRewriteInfo =
+  | {
+      kind: 'StartScope';
+      blockId: BlockId;        // New block for scope content
+      fallthroughId: BlockId;  // Block after scope ends
+      instrId: InstructionId;  // Where to insert
+      scope: ReactiveScope;    // The scope being created
+    }
+  | {
+      kind: 'EndScope';
+      instrId: InstructionId;  // Where to insert
+      fallthroughId: BlockId;  // Same as corresponding StartScope
+    };
+```
+
+### RewriteContext
+```typescript
+type RewriteContext = {
+  source: BasicBlock;        // Original block being split
+  instrSliceIdx: number;     // Current slice start index
+  nextPreds: Set<BlockId>;   // Predecessors for next emitted block
+  nextBlockId: BlockId;      // BlockId for next emitted block
+  rewrites: Array<BasicBlock>; // Accumulated split blocks
+};
+```
+
+### ScopeTraversalContext
+```typescript
+type ScopeTraversalContext = {
+  fallthroughs: Map<ScopeId, BlockId>; // Cache: scope -> its fallthrough block
+  rewrites: Array<TerminalRewriteInfo>;
+  env: Environment;
+};
+```
+
+## Edge Cases
+
+### Multiple Rewrites at Same Instruction ID
+The while loop in Step 2 handles multiple scope start/ends at the same instruction ID.
+
+### Nested Scopes
+The pre-order traversal ensures outer scopes are processed before inner scopes, creating proper nesting in the CFG.
+
+### Empty Blocks After Split
+When a scope boundary falls at the start of a block, the split may create a block with no instructions (only a terminal).
+
+### Control Flow Within Scopes
+The pass preserves existing control flow (if/else, loops) within scopes; it only adds scope entry/exit points.
+
+### Early Returns
+When a return occurs within a scope, the scope terminal still has a fallthrough block, but that block may contain `Unreachable` terminal.
+
+## TODOs
+Line 283-284:
+```typescript
+// TODO make consistent instruction IDs instead of reusing
+```
+
+## Example
+
+### Fixture: `reactive-scopes-if.js`
+
+**Before BuildReactiveScopeTerminalsHIR:**
+```
+bb0 (block):
+  [1] $29_@0[1:22] = Array []           // x with scope @0 range [1:22]
+  [2] StoreLocal x$30_@0 = $29_@0
+  [3] $32 = LoadLocal a$26
+  [4] If ($32) then:bb2 else:bb3 fallthrough=bb1
+bb2:
+  [5] $33_@1[5:11] = Array []           // y with scope @1 range [5:11]
+  ...
+```
+
+**After BuildReactiveScopeTerminalsHIR:**
+```
+bb0 (block):
+  [1] Scope @0 [1:28] block=bb9 fallthrough=bb10   // <-- scope terminal inserted
+bb9:
+  [2] $29_@0 = Array []
+  [3] StoreLocal x$30_@0 = $29_@0
+  [4] $32 = LoadLocal a$26
+  [5] If ($32) then:bb2 else:bb3 fallthrough=bb1
+bb2:
+  [6] Scope @1 [6:14] block=bb11 fallthrough=bb12  // <-- nested scope terminal
+bb11:
+  [7] $33_@1 = Array []
+  ...
+  [13] Goto bb12                                    // <-- scope end goto
+bb12:
+  ...
+bb1:
+  [27] Goto bb10                                    // <-- scope @0 end goto
+bb10:
+  [28] $50 = LoadLocal x$30_@0
+  [29] Return $50
+```
+
+The key transformation is that scope boundaries become explicit control flow: a `Scope` terminal enters the scope content block, and a `Goto` terminal exits to the fallthrough block. This structure is later used to generate the memoization checks.

compiler/packages/babel-plugin-react-compiler/docs/passes/18-flattenReactiveLoopsHIR.md

@@ -0,0 +1,158 @@
+# flattenReactiveLoopsHIR
+
+## File
+`src/ReactiveScopes/FlattenReactiveLoopsHIR.ts`
+
+## Purpose
+This pass **prunes reactive scopes that are nested inside loops** (for, for-in, for-of, while, do-while). The compiler does not yet support memoization within loops because:
+
+1. Loop iterations would require reconciliation across runs (similar to how `key` is used in JSX for lists)
+2. There is no way to identify values across iterations
+3. The current approach is to memoize *around* the loop rather than *within* it
+
+When a reactive scope is found inside a loop body, the pass converts its terminal from `scope` to `pruned-scope`. A `pruned-scope` terminal is later treated specially during codegen - its instructions are emitted inline without any memoization guards.
+
+## Input Invariants
+- The HIR has been through `buildReactiveScopeTerminalsHIR`, which creates `scope` terminal nodes for reactive scopes
+- The HIR is in valid block form with proper terminal kinds
+- The block ordering respects control flow (blocks are iterated in order, with loop fallthroughs appearing after loop bodies)
+
+## Output Guarantees
+- All `scope` terminals that appear inside any loop body are converted to `pruned-scope` terminals
+- Scopes outside of loops remain unchanged as `scope` terminals
+- The structure of blocks is preserved; only the terminal kind is mutated
+- The `pruned-scope` terminal retains all the same fields as `scope` (block, fallthrough, scope, id, loc)
+
+## Algorithm
+
+The algorithm uses a **linear scan with a stack-based loop tracking** approach:
+
+```
+1. Initialize an empty array `activeLoops` to track which loop(s) we are currently inside
+2. For each block in the function body (in order):
+   a. Remove the current block ID from activeLoops (if present)
+      - This happens when we reach a loop's fallthrough block, exiting the loop
+   b. Examine the block's terminal:
+      - If it's a loop terminal (do-while, for, for-in, for-of, while):
+        Push the loop's fallthrough block ID onto activeLoops
+      - If it's a scope terminal AND activeLoops is non-empty:
+        Convert the terminal to pruned-scope (keeping all other fields)
+      - All other terminal kinds are ignored
+```
+
+Key insight: The algorithm tracks when we "enter" a loop by pushing the fallthrough ID when encountering a loop terminal, and "exits" the loop when that fallthrough block is visited.
+
+## Key Data Structures
+
+### activeLoops: Array<BlockId>
+A stack of block IDs representing loop fallthroughs. When non-empty, we are inside one or more nested loops.
+
+### PrunedScopeTerminal
+```typescript
+export type PrunedScopeTerminal = {
+  kind: 'pruned-scope';
+  fallthrough: BlockId;
+  block: BlockId;
+  scope: ReactiveScope;
+  id: InstructionId;
+  loc: SourceLocation;
+};
+```
+
+### retainWhere
+Utility from utils.ts - an in-place array filter that removes elements not matching the predicate.
+
+## Edge Cases
+
+### Nested Loops
+The algorithm handles nested loops correctly because `activeLoops` is an array that can contain multiple fallthrough IDs. A scope deep inside multiple nested loops will still be pruned.
+
+### Scope Spanning the Loop
+If a scope terminal appears before the loop terminal but its body contains the loop, it is NOT pruned because the scope terminal itself is not inside the loop.
+
+### Multiple Loops in Sequence
+When exiting one loop (reaching its fallthrough) and entering another, `activeLoops` correctly clears the first loop before potentially adding the second.
+
+### Control Flow That Exits Loops (break/return)
+The algorithm relies on block ordering and fallthrough IDs. Early exits via break/return don't affect the tracking since we track by fallthrough block ID.
+
+## TODOs
+No explicit TODOs in this file. However, the docstring mentions future improvements:
+> "Eventually we may integrate more deeply into the runtime so that we can do a single level of reconciliation"
+
+This suggests a potential future feature to support memoization within loops via runtime integration.
+
+## Example
+
+### Fixture: `repro-memoize-for-of-collection-when-loop-body-returns.js`
+
+**Input:**
+```javascript
+function useHook(nodeID, condition) {
+  const graph = useContext(GraphContext);
+  const node = nodeID != null ? graph[nodeID] : null;
+
+  for (const key of Object.keys(node?.fields ?? {})) {
+    if (condition) {
+      return new Class(node.fields?.[field]);  // <-- Scope @4 is here
+    }
+  }
+  return new Class();  // <-- Scope @5 is here (outside loop)
+}
+```
+
+**Before FlattenReactiveLoopsHIR:**
+```
+[45] Scope scope @3 [45:72] ... block=bb35 fallthrough=bb36
+bb35:
+  [46] ForOf init=bb6 test=bb7 loop=bb8 fallthrough=bb5
+  ...
+  [66] Scope scope @4 [66:69] ... block=bb37 fallthrough=bb38  <-- Inside loop
+  ...
+  [73] Scope scope @5 [73:76] ... block=bb39 fallthrough=bb40  <-- Outside loop
+```
+
+**After FlattenReactiveLoopsHIR:**
+```
+[45] Scope scope @3 [45:72] ... block=bb35 fallthrough=bb36    <-- Unchanged
+...
+[66] <pruned> Scope scope @4 [66:69] ... block=bb37 fallthrough=bb38  <-- PRUNED!
+...
+[73] Scope scope @5 [73:76] ... block=bb39 fallthrough=bb40    <-- Unchanged
+```
+
+**Final Codegen Result:**
+```javascript
+function useHook(nodeID, condition) {
+  const $ = _c(7);
+  // ... memoized Object.keys call (scope @2)
+
+  let t1;
+  if ($[2] !== condition || $[3] !== node || $[4] !== t0) {
+    // Scope @3 wraps the loop
+    t1 = Symbol.for("react.early_return_sentinel");
+    bb0: for (const key of t0) {
+      if (condition) {
+        t1 = new Class(node.fields?.[field]);  // Scope @4 was PRUNED - no memoization
+        break bb0;
+      }
+    }
+    $[2] = condition;
+    $[3] = node;
+    $[4] = t0;
+    $[5] = t1;
+  } else {
+    t1 = $[5];
+  }
+  // ...
+
+  // Scope @5 - memoized (sentinel check)
+  if ($[6] === Symbol.for("react.memo_cache_sentinel")) {
+    t2 = new Class();
+    $[6] = t2;
+  }
+  return t2;
+}
+```
+
+The `new Class(...)` inside the loop has no memoization guards because scope @4 was pruned. The `new Class()` outside the loop retains its memoization via scope @5.

compiler/packages/babel-plugin-react-compiler/docs/passes/19-flattenScopesWithHooksOrUseHIR.md

@@ -0,0 +1,143 @@
+# flattenScopesWithHooksOrUseHIR
+
+## File
+`src/ReactiveScopes/FlattenScopesWithHooksOrUseHIR.ts`
+
+## Purpose
+This pass removes (flattens) reactive scopes that transitively contain hook calls or `use()` operator calls. The key insight is that:
+
+1. **Hooks cannot be called conditionally** - wrapping them in a memoized scope would make them conditionally called based on whether the cache is valid
+2. **The `use()` operator** - while it can be called conditionally in source code, React requires it to be called consistently if the component needs the returned value. Memoizing a scope containing `use()` would also make it conditionally called.
+
+By running reactive scope inference first (agnostic of hooks), the compiler knows which values "construct together" in the same scope. The pass then removes ALL memoization for scopes containing hook/use calls to ensure they are always executed unconditionally.
+
+## Input Invariants
+- HIR must have reactive scope terminals already built (pass runs after `BuildReactiveScopeTerminalsHIR`)
+- Blocks are visited in order (the pass iterates through `fn.body.blocks`)
+- Scope terminals have a `block` (body of the scope) and `fallthrough` (block after the scope)
+- Type inference has run so that `getHookKind()` and `isUseOperator()` can identify hooks and use() calls
+
+## Output Guarantees
+- All scopes that transitively contained a hook or `use()` call are either:
+  - Converted to `LabelTerminal` - if the scope body is trivial (just the hook call and a goto)
+  - Converted to `PrunedScopeTerminal` - if the scope body contains other instructions besides the hook call
+- The `PrunedScopeTerminal` still tracks the original scope information for downstream passes but will not generate memoization code
+- The control flow structure is preserved (same blocks, same fallthroughs)
+
+## Algorithm
+
+### Phase 1: Identify Scopes Containing Hook/Use Calls
+1. Maintain a stack `activeScopes` of currently "open" reactive scopes
+2. Iterate through all blocks in order
+3. When entering a block:
+   - Remove any scopes from `activeScopes` whose fallthrough equals the current block (those scopes have ended)
+4. For each instruction in the block:
+   - If it's a `CallExpression` or `MethodCall` and the callee is a hook or use operator:
+     - Add all currently active scopes to the `prune` list
+     - Clear `activeScopes` (these scopes are now marked for pruning)
+5. If the block's terminal is a `scope`:
+   - Push it onto `activeScopes`
+
+### Phase 2: Prune Identified Scopes
+For each block ID in `prune`:
+1. Get the scope terminal
+2. Check if the scope body is trivial (single instruction + goto to fallthrough):
+   - If trivial: Convert to `LabelTerminal` (will be removed by `PruneUnusedLabels`)
+   - If non-trivial: Convert to `PrunedScopeTerminal` (preserves scope info but skips memoization)
+
+## Key Data Structures
+
+```typescript
+// Stack tracking currently open scopes
+activeScopes: Array<{block: BlockId; fallthrough: BlockId}>
+
+// List of block IDs whose scope terminals should be pruned
+prune: Array<BlockId>
+
+// Terminal types used
+LabelTerminal: {kind: 'label', block, fallthrough, id, loc}
+PrunedScopeTerminal: {kind: 'pruned-scope', block, fallthrough, scope, id, loc}
+ReactiveScopeTerminal: {kind: 'scope', block, fallthrough, scope, id, loc}
+```
+
+## Edge Cases
+
+### Nested Scopes
+When a hook is found in an inner scope, ALL enclosing scopes are also pruned (the hook call would become conditional if any outer scope were memoized).
+
+### Method Call Hooks
+Handles both `CallExpression` (e.g., `useHook(...)`) and `MethodCall` (e.g., `obj.useHook(...)`).
+
+### Trivial Hook-Only Scopes
+If a scope exists just for a hook call (single instruction + goto), it's converted to a `LabelTerminal` which is a simpler structure that gets cleaned up by later passes.
+
+### Multiple Hooks in Sequence
+Once the first hook is encountered, all active scopes are pruned and cleared, so subsequent hooks in outer scopes still work correctly.
+
+## TODOs
+None explicitly marked in the source file.
+
+## Example
+
+### Fixture: `nested-scopes-hook-call.js`
+
+**Input:**
+```javascript
+function component(props) {
+  let x = [];
+  let y = [];
+  y.push(useHook(props.foo));
+  x.push(y);
+  return x;
+}
+```
+
+**Before FlattenScopesWithHooksOrUseHIR:**
+```
+bb0:
+  [1] Scope @0 [1:22] block=bb6 fallthrough=bb7    // Outer scope for x
+bb6:
+  [2] $22 = Array []         // x = []
+  [3] StoreLocal x = $22
+  [4] Scope @1 [4:17] block=bb8 fallthrough=bb9   // Inner scope for y
+bb8:
+  [5] $25 = Array []         // y = []
+  [6] StoreLocal y = $25
+  ...
+  [10] $33 = Call useHook(...)  // <-- Hook call here!
+  [11] MethodCall y.push($33)
+```
+
+**After FlattenScopesWithHooksOrUseHIR:**
+```
+bb0:
+  [1] <pruned> Scope @0 [1:22] block=bb6 fallthrough=bb7   // PRUNED
+bb6:
+  [2] $22 = Array []
+  [3] StoreLocal x = $22
+  [4] <pruned> Scope @1 [4:17] block=bb8 fallthrough=bb9  // PRUNED
+bb8:
+  [5] $25 = Array []
+  [6] StoreLocal y = $25
+  ...
+  [12] Label block=bb10 fallthrough=bb11   // Hook call converted to label
+bb10:
+  [13] $33 = Call useHook(...)
+  [14] Goto bb11
+...
+```
+
+**Final Output (no memoization):**
+```javascript
+function component(props) {
+  const x = [];
+  const y = [];
+  y.push(useHook(props.foo));
+  x.push(y);
+  return x;
+}
+```
+
+Notice that:
+1. Both scope @0 and scope @1 are marked as `<pruned>` because the hook call is inside scope @1, which is inside scope @0
+2. The final output has no memoization wrappers - just the raw code

compiler/packages/babel-plugin-react-compiler/docs/passes/20-propagateScopeDependenciesHIR.md

@@ -0,0 +1,158 @@
+# propagateScopeDependenciesHIR
+
+## File
+`src/HIR/PropagateScopeDependenciesHIR.ts`
+
+## Purpose
+The `propagateScopeDependenciesHIR` pass is responsible for computing and assigning the **dependencies** for each reactive scope in the compiled function. Dependencies are the external values that a scope reads, which determine when the scope needs to re-execute. This is a critical step for memoization correctness - the compiler must track exactly which values a scope depends on so it can generate proper cache invalidation checks.
+
+The pass also populates:
+- `scope.dependencies` - The set of `ReactiveScopeDependency` objects the scope reads
+- `scope.declarations` - Values declared within the scope that are used outside it
+
+## Input Invariants
+- Reactive scopes must be established (pass runs after `BuildReactiveScopeTerminalsHIR`)
+- The function must be in SSA form
+- `InferMutationAliasingRanges` must have run to establish when values are being mutated
+- `InferReactivePlaces` marks which identifiers are reactive
+- Scope ranges have been aligned and normalized by earlier passes
+
+## Output Guarantees
+After this pass completes:
+
+1. Each `ReactiveScope.dependencies` contains the minimal set of dependencies that:
+   - Were declared before the scope started
+   - Are read within the scope
+   - Are not ref values (which are always mutable)
+   - Are not object methods (which get codegen'd back into object literals)
+
+2. Each `ReactiveScope.declarations` contains identifiers that:
+   - Are assigned within the scope
+   - Are used outside the scope (need to be exposed as scope outputs)
+
+3. Property load chains are resolved to their root identifiers with paths (e.g., `props.user.name` becomes `{identifier: props, path: ["user", "name"]}`)
+
+4. Optional chains are handled correctly, distinguishing between `a?.b` and `a.b` access types
+
+## Algorithm
+
+### Phase 1: Build Sidemaps
+
+1. **findTemporariesUsedOutsideDeclaringScope**: Identifies temporaries that are used outside the scope where they were declared (cannot be hoisted/reordered safely)
+
+2. **collectTemporariesSidemap**: Creates a mapping from temporary IdentifierIds to their source `ReactiveScopeDependency`. For example:
+   ```
+   $0 = LoadLocal 'a'
+   $1 = PropertyLoad $0.'b'
+   ```
+   Maps `$1.id` to `{identifier: a, path: [{property: 'b', optional: false}]}`
+
+3. **collectOptionalChainSidemap**: Traverses optional chain blocks to map temporaries within optional chains to their full optional dependency path
+
+4. **collectHoistablePropertyLoads**: Uses CFG analysis to determine which property loads can be safely hoisted
+
+### Phase 2: Collect Dependencies
+
+The `collectDependencies` function traverses the HIR, maintaining a stack of active scopes:
+
+1. **Scope Entry/Exit**: When entering a scope terminal, push a new dependency array. When exiting, propagate collected dependencies to parent scopes if valid.
+
+2. **Instruction Processing**: For each instruction:
+   - Declare the lvalue with its instruction id and current scope
+   - Visit operands to record them as potential dependencies
+   - Handle special cases like `StoreLocal` (tracks reassignments), `Destructure`, `PropertyLoad`, etc.
+
+3. **Dependency Validation** (`#checkValidDependency`):
+   - Skip ref values (`isRefValueType`)
+   - Skip object methods (`isObjectMethodType`)
+   - Only include if declared before scope start
+
+### Phase 3: Derive Minimal Dependencies
+
+For each scope, use `ReactiveScopeDependencyTreeHIR` to:
+1. Build a tree from hoistable property loads
+2. Add all collected dependencies to the tree
+3. Truncate dependencies at their maximal safe-to-evaluate subpath
+4. Derive the minimal set (removing redundant nested dependencies)
+
+## Key Data Structures
+
+### ReactiveScopeDependency
+```typescript
+type ReactiveScopeDependency = {
+  identifier: Identifier;        // Root identifier
+  reactive: boolean;             // Whether the value is reactive
+  path: DependencyPathEntry[];   // Chain of property accesses
+}
+```
+
+### DependencyPathEntry
+```typescript
+type DependencyPathEntry = {
+  property: PropertyLiteral;  // Property name
+  optional: boolean;          // Is this `?.` access?
+}
+```
+
+### DependencyCollectionContext
+Maintains:
+- `#declarations`: Map of DeclarationId to {id, scope} recording where each value was declared
+- `#reassignments`: Map of Identifier to latest assignment info
+- `#scopes`: Stack of currently active ReactiveScopes
+- `#dependencies`: Stack of dependency arrays (one per active scope)
+- `#temporaries`: Sidemap for resolving property loads
+
+### ReactiveScopeDependencyTreeHIR
+A tree structure for efficient dependency deduplication that stores hoistable objects, tracks access types, and computes minimal dependencies.
+
+## Edge Cases
+
+### Values Used Outside Declaring Scope
+If a temporary is used outside its declaring scope, it cannot be tracked in the sidemap because reordering the read would be invalid.
+
+### Ref.current Access
+Accessing `ref.current` is treated specially - the dependency is truncated to just `ref`.
+
+### Optional Chains
+Optional chains like `a?.b?.c` produce different dependency paths than `a.b.c`. The pass distinguishes them and may merge optional loads into unconditional ones when control flow proves the object is non-null.
+
+### Inner Functions
+Dependencies from inner functions are collected recursively but with special handling for context variables.
+
+### Phi Nodes
+When a value comes from multiple control flow paths, optional chain dependencies from phi operands are also visited.
+
+## TODOs
+1. Line 374-375: `// TODO(mofeiZ): understand optional chaining` - More documentation needed for optional chain handling
+
+## Example
+
+### Fixture: `reactive-control-dependency-if.js`
+
+**Input:**
+```javascript
+function Component(props) {
+  let x;
+  if (props.cond) {
+    x = 1;
+  } else {
+    x = 2;
+  }
+  return [x];
+}
+```
+
+**Before PropagateScopeDependenciesHIR:**
+```
+Scope scope @0 [12:15] dependencies=[] declarations=[] reassignments=[] block=bb9
+```
+
+**After PropagateScopeDependenciesHIR:**
+```
+Scope scope @0 [12:15] dependencies=[x$24:TPrimitive] declarations=[$26_@0] reassignments=[] block=bb9
+```
+
+The pass identified that:
+- The scope at `[x]` depends on `x$24` (the phi node result from the if/else branches)
+- Even though `x` is assigned to constants (1 or 2), its value depends on the reactive control flow condition `props.cond`
+- The scope declares `$26_@0` (the array output)

compiler/packages/babel-plugin-react-compiler/docs/passes/21-buildReactiveFunction.md

@@ -0,0 +1,180 @@
+# buildReactiveFunction
+
+## File
+`src/ReactiveScopes/BuildReactiveFunction.ts`
+
+## Purpose
+The `buildReactiveFunction` pass converts the compiler's HIR (High-level Intermediate Representation) from a **Control Flow Graph (CFG)** representation to a **tree-based ReactiveFunction** representation that is closer to an AST. This is a critical transformation in the React Compiler pipeline that:
+
+1. **Restores control flow constructs** - Reconstructs `if`, `while`, `for`, `switch`, and other control flow statements from the CFG's basic blocks and terminals
+2. **Eliminates phi nodes** - Replaces SSA phi nodes with compound value expressions (ternaries, logical expressions, sequence expressions)
+3. **Handles labeled break/continue** - Tracks control flow targets to emit explicit labeled `break` and `continue` statements when needed
+4. **Preserves reactive scope information** - Scope terminals are converted to `ReactiveScopeBlock` nodes in the tree
+
+## Input Invariants
+- HIR is in SSA form (variables have been renamed with unique identifiers)
+- Basic blocks are connected (valid predecessor/successor relationships)
+- Each block ends with a valid terminal
+- Phi nodes exist at merge points for values from different control flow paths
+- Reactive scopes have been constructed (`scope` terminals exist)
+- Scope dependencies are computed (`PropagateScopeDependenciesHIR` has run)
+
+## Output Guarantees
+- **Tree structure** - The output is a `ReactiveFunction` with a `body: ReactiveBlock` containing a tree of `ReactiveStatement` nodes
+- **No CFG structure** - Basic blocks are eliminated; control flow is represented through nested reactive terminals
+- **No phi nodes** - Value merges are represented as `ConditionalExpression`, `LogicalExpression`, or `SequenceExpression` values
+- **Labels emitted for all control flow** - Every terminal that can be a break/continue target has a label; unnecessary labels are removed by subsequent `PruneUnusedLabels` pass
+- **Each block emitted exactly once** - A block cannot be generated twice
+- **Scope blocks preserved** - `scope` terminals become `ReactiveScopeBlock` nodes
+
+## Algorithm
+
+### Core Classes
+
+1. **`Driver`** - Traverses blocks and emits ReactiveBlock arrays
+2. **`Context`** - Tracks state:
+   - `emitted: Set<BlockId>` - Which blocks have been generated
+   - `#scheduled: Set<BlockId>` - Blocks that will be emitted by parent constructs
+   - `#controlFlowStack: Array<ControlFlowTarget>` - Stack of active break/continue targets
+   - `scopeFallthroughs: Set<BlockId>` - Fallthroughs for scope blocks
+
+### Traversal Strategy
+
+1. Start at the entry block and call `traverseBlock(entryBlock)`
+2. For each block:
+   - Emit all instructions as `ReactiveInstructionStatement`
+   - Process the terminal based on its kind
+
+### Terminal Processing
+
+**Simple Terminals:**
+- `return`, `throw` - Emit directly as `ReactiveTerminal`
+- `unreachable` - No-op
+
+**Control Flow Terminals:**
+- `if` - Schedule fallthrough, recursively traverse consequent/alternate, emit `ReactiveIfTerminal`
+- `while`, `do-while`, `for`, `for-of`, `for-in` - Use `scheduleLoop()` which tracks continue targets
+- `switch` - Process cases in reverse order
+- `label` - Schedule fallthrough, traverse inner block
+
+**Value Terminals (expressions that produce values):**
+- `ternary`, `logical`, `optional`, `sequence` - Produce `ReactiveValue` compound expressions
+
+**Break/Continue:**
+- `goto` with `GotoVariant.Break` - Determine if break is implicit, unlabeled, or labeled
+- `goto` with `GotoVariant.Continue` - Determine continue type
+
+**Scope Terminals:**
+- `scope`, `pruned-scope` - Schedule fallthrough, traverse inner block, emit as `ReactiveScopeBlock`
+
+## Key Data Structures
+
+### ReactiveFunction
+```typescript
+type ReactiveFunction = {
+  loc: SourceLocation;
+  id: ValidIdentifierName | null;
+  params: Array<Place | SpreadPattern>;
+  generator: boolean;
+  async: boolean;
+  body: ReactiveBlock;
+  env: Environment;
+  directives: Array<string>;
+};
+```
+
+### ReactiveBlock
+```typescript
+type ReactiveBlock = Array<ReactiveStatement>;
+```
+
+### ReactiveStatement
+```typescript
+type ReactiveStatement =
+  | ReactiveInstructionStatement   // {kind: 'instruction', instruction}
+  | ReactiveTerminalStatement      // {kind: 'terminal', terminal, label}
+  | ReactiveScopeBlock             // {kind: 'scope', scope, instructions}
+  | PrunedReactiveScopeBlock;      // {kind: 'pruned-scope', ...}
+```
+
+### ReactiveValue (for compound expressions)
+```typescript
+type ReactiveValue =
+  | InstructionValue               // Regular instruction values
+  | ReactiveLogicalValue           // a && b, a || b, a ?? b
+  | ReactiveSequenceValue          // (a, b, c)
+  | ReactiveTernaryValue           // a ? b : c
+  | ReactiveOptionalCallValue;     // a?.b()
+```
+
+### ControlFlowTarget
+```typescript
+type ControlFlowTarget =
+  | {type: 'if'; block: BlockId; id: number}
+  | {type: 'switch'; block: BlockId; id: number}
+  | {type: 'case'; block: BlockId; id: number}
+  | {type: 'loop'; block: BlockId; continueBlock: BlockId; ...};
+```
+
+## Edge Cases
+
+### Nested Control Flow
+The scheduling mechanism handles arbitrarily nested control flow by pushing/popping from the control flow stack.
+
+### Value Blocks with Complex Expressions
+`SequenceExpression` handles cases where value blocks contain multiple instructions.
+
+### Scope Fallthroughs
+Breaks to scope fallthroughs are treated as implicit (no explicit break needed).
+
+### Catch Handlers
+Scheduled specially via `scheduleCatchHandler()` to prevent re-emission.
+
+### Unreachable Blocks
+The `reachable()` check prevents emitting unreachable blocks.
+
+## TODOs
+The code contains several `CompilerError.throwTodo()` calls for unsupported patterns:
+1. Optional chaining test blocks must end in `branch`
+2. Logical expression test blocks must end in `branch`
+3. Support for value blocks within try/catch statements
+4. Support for labeled statements combined with value blocks
+
+## Example
+
+### Fixture: `ternary-expression.js`
+
+**Input:**
+```javascript
+function ternary(props) {
+  const a = props.a && props.b ? props.c || props.d : (props.e ?? props.f);
+  const b = props.a ? (props.b && props.c ? props.d : props.e) : props.f;
+  return a ? b : null;
+}
+```
+
+**HIR (CFG with many basic blocks):**
+The HIR contains 33 basic blocks with `Ternary`, `Logical`, `Branch`, and `Goto` terminals, plus phi nodes at merge points.
+
+**ReactiveFunction Output (Tree):**
+```
+function ternary(props$62{reactive}) {
+  [1] $84 = Ternary
+    Sequence
+        [2] $66 = Logical
+          Sequence [...]
+          && Sequence [...]
+    ?
+      Sequence [...]  // props.c || props.d
+    :
+      Sequence [...]  // props.e ?? props.f
+  [40] StoreLocal a$99 = $98
+  ...
+  [82] return $145
+}
+```
+
+The transformation eliminates:
+- 33 basic blocks reduced to a single tree
+- Phi nodes replaced with nested `Ternary` and `Logical` value expressions
+- CFG edges replaced with tree nesting

compiler/packages/babel-plugin-react-compiler/docs/passes/22-pruneUnusedLabels.md

@@ -0,0 +1,145 @@
+# pruneUnusedLabels
+
+## File
+`src/ReactiveScopes/PruneUnusedLabels.ts`
+
+## Purpose
+The `pruneUnusedLabels` pass optimizes control flow by:
+
+1. **Flattening labeled terminals** where the label is not reachable via a `break` or `continue` statement
+2. **Marking labels as implicit** for terminals where the label exists but is never targeted
+
+This pass removes unnecessary labeled blocks that were introduced during compilation but serve no control flow purpose in the final output. JavaScript labeled statements are only needed when there is a corresponding `break label` or `continue label` that targets them.
+
+## Input Invariants
+- The input is a `ReactiveFunction` (after conversion from HIR)
+- All `break` and `continue` terminals have:
+  - A `target` (BlockId) indicating which label they jump to
+  - A `targetKind` that is one of: `'implicit'`, `'labeled'`, or `'unlabeled'`
+- Each `ReactiveTerminalStatement` has an optional `label` field containing `id` and `implicit`
+- The pass runs after `assertWellFormedBreakTargets` which validates break/continue targets
+
+## Output Guarantees
+- Labeled terminals where the label is unreachable are flattened into their parent block
+- When flattening, trailing unlabeled `break` statements (that would just fall through) are removed
+- Labels that exist but are never targeted have their `implicit` flag set to `true`
+- Control flow semantics are preserved - only structurally unnecessary labels are removed
+
+## Algorithm
+
+The pass uses a two-phase approach with a single traversal:
+
+**Phase 1: Collect reachable labels**
+```typescript
+if ((terminal.kind === 'break' || terminal.kind === 'continue') &&
+    terminal.targetKind === 'labeled') {
+  state.add(terminal.target);  // Mark this label as reachable
+}
+```
+
+**Phase 2: Transform terminals**
+```typescript
+const isReachableLabel = stmt.label !== null && state.has(stmt.label.id);
+
+if (stmt.terminal.kind === 'label' && !isReachableLabel) {
+  // Flatten: extract block contents, removing trailing unlabeled break
+  const block = [...stmt.terminal.block];
+  const last = block.at(-1);
+  if (last?.kind === 'terminal' && last.terminal.kind === 'break' &&
+      last.terminal.target === null) {
+    block.pop();  // Remove trailing break
+  }
+  return {kind: 'replace-many', value: block};
+} else {
+  if (!isReachableLabel && stmt.label != null) {
+    stmt.label.implicit = true;  // Mark as implicit
+  }
+  return {kind: 'keep'};
+}
+```
+
+## Edge Cases
+
+### Trailing Break Removal
+When flattening a labeled block, if the last statement is an unlabeled break (`target === null`), it is removed since it would just fall through anyway.
+
+### Implicit vs Labeled Breaks
+Only breaks with `targetKind === 'labeled'` count toward label reachability. Implicit breaks (fallthrough) and unlabeled breaks don't make a label "used".
+
+### Continue Statements
+Both `break` and `continue` with labeled targets mark the label as reachable.
+
+### Non-Label Terminals with Labels
+Other terminal types (like `if`, `while`, `for`) can also have labels. If unreachable, these labels are marked implicit but the terminal is not flattened.
+
+## TODOs
+None in the source file.
+
+## Example
+
+### Fixture: `unconditional-break-label.js`
+
+**Input:**
+```javascript
+function foo(a) {
+  let x = 0;
+  bar: {
+    x = 1;
+    break bar;
+  }
+  return a + x;
+}
+```
+
+**Output (after full compilation):**
+```javascript
+function foo(a) {
+  return a + 1;
+}
+```
+
+The labeled block `bar: { ... }` is removed because after the pass runs, constant propagation and dead code elimination further simplify the code.
+
+### Fixture: `conditional-break-labeled.js`
+
+**Input:**
+```javascript
+function Component(props) {
+  const a = [];
+  a.push(props.a);
+  label: {
+    if (props.b) {
+      break label;
+    }
+    a.push(props.c);
+  }
+  a.push(props.d);
+  return a;
+}
+```
+
+**Output:**
+```javascript
+function Component(props) {
+  const $ = _c(5);
+  let a;
+  if ($[0] !== props.a || $[1] !== props.b ||
+      $[2] !== props.c || $[3] !== props.d) {
+    a = [];
+    a.push(props.a);
+    bb0: {
+      if (props.b) {
+        break bb0;
+      }
+      a.push(props.c);
+    }
+    a.push(props.d);
+    // ... cache updates
+  } else {
+    a = $[4];
+  }
+  return a;
+}
+```
+
+The labeled block `bb0: { ... }` is preserved because the `break bb0` inside the conditional targets this label.

compiler/packages/babel-plugin-react-compiler/docs/passes/23-pruneNonEscapingScopes.md

@@ -0,0 +1,130 @@
+# pruneNonEscapingScopes
+
+## File
+`src/ReactiveScopes/PruneNonEscapingScopes.ts`
+
+## Purpose
+This pass prunes (removes) reactive scopes whose outputs do not "escape" the component and therefore do not need to be memoized. A value "escapes" in two ways:
+
+1. **Returned from the function** - The value is directly returned or transitively aliased by a return value
+2. **Passed to a hook** - Any value passed as an argument to a hook may be stored by React internally (e.g., the closure passed to `useEffect`)
+
+The key insight is that values which never escape the component boundary can be safely recreated on each render without affecting the behavior of consumers.
+
+## Input Invariants
+- The input is a `ReactiveFunction` after scope blocks have been identified
+- Reactive scopes have been assigned to instructions
+- The pass runs after `BuildReactiveFunction` and `PruneUnusedLabels`, before `PruneNonReactiveDependencies`
+
+## Output Guarantees
+- **Scopes with non-escaping outputs are removed** - Their instructions are inlined back into the parent scope/function body
+- **Scopes with escaping outputs are retained** - Values that escape via return or hook arguments remain memoized
+- **Transitive dependencies of escaping scopes are preserved** - If an escaping scope depends on a non-escaping value, that value's scope is also retained to prevent unnecessary invalidation
+- **`FinishMemoize` instructions are marked `pruned=true`** - When a scope is pruned, the associated memoization instructions are flagged
+
+## Algorithm
+
+### Phase 1: Build the Dependency Graph
+Using `CollectDependenciesVisitor`, build:
+- **Identifier nodes** - Each node tracks memoization level, dependencies, scopes, and whether ultimately memoized
+- **Scope nodes** - Each scope tracks its dependencies
+- **Escaping values** - Identifiers that escape via return or hook arguments
+
+### Phase 2: Classify Memoization Levels
+Each instruction value is classified:
+- `Memoized`: Arrays, objects, function calls, `new` expressions - always potentially aliasing
+- `Conditional`: Conditional/logical expressions, property loads - memoized only if dependencies are memoized
+- `Unmemoized`: JSX elements (when `memoizeJsxElements` is false), DeclareLocal
+- `Never`: Primitives, LoadGlobal, binary/unary expressions - can be cheaply compared
+
+### Phase 3: Compute Memoized Identifiers
+`computeMemoizedIdentifiers()` performs a graph traversal starting from escaping values:
+- For each escaping value, recursively visit its dependencies
+- Mark values and their scopes based on memoization level
+- When marking a scope, force-memoize all its dependencies
+
+### Phase 4: Prune Scopes
+`PruneScopesTransform` visits each scope block:
+- If any scope output is in the memoized set, keep the scope
+- If no outputs are memoized, replace the scope block with its inlined instructions
+
+## Edge Cases
+
+### Interleaved Mutations
+```javascript
+const a = [props.a];  // independently memoizable, non-escaping
+const b = [];
+const c = {};
+c.a = a;              // c captures a, but c doesn't escape
+b.push(props.b);      // b escapes via return
+return b;
+```
+Here `a` does not directly escape, but it is a dependency of the scope containing `b`. The algorithm correctly identifies that `a`'s scope must be preserved.
+
+### Hook Arguments Escape
+Values passed to hooks are treated as escaping because hooks may store references internally.
+
+### JSX Special Handling
+JSX elements are marked as `Unmemoized` by default because React.memo() can handle dynamic memoization.
+
+### noAlias Functions
+If a function signature indicates `noAlias === true`, its arguments are not treated as escaping.
+
+### Reassignments
+When a scope reassigns a variable, the scope is added as a dependency of that variable.
+
+## TODOs
+None explicitly in the source file.
+
+## Example
+
+### Fixture: `escape-analysis-non-escaping-interleaved-allocating-dependency.js`
+
+**Input:**
+```javascript
+function Component(props) {
+  const a = [props.a];
+
+  const b = [];
+  const c = {};
+  c.a = a;
+  b.push(props.b);
+
+  return b;
+}
+```
+
+**Output:**
+```javascript
+function Component(props) {
+  const $ = _c(5);
+  let t0;
+  if ($[0] !== props.a) {
+    t0 = [props.a];
+    $[0] = props.a;
+    $[1] = t0;
+  } else {
+    t0 = $[1];
+  }
+  const a = t0;  // a is memoized even though it doesn't escape directly
+
+  let b;
+  if ($[2] !== a || $[3] !== props.b) {
+    b = [];
+    const c = {};      // c is NOT memoized - it doesn't escape
+    c.a = a;
+    b.push(props.b);
+    $[2] = a;
+    $[3] = props.b;
+    $[4] = b;
+  } else {
+    b = $[4];
+  }
+  return b;
+}
+```
+
+Key observations:
+- `a` is memoized because it's a dependency of the scope containing `b`
+- `c` is not separately memoized because it doesn't escape
+- `b` is memoized because it's returned

compiler/packages/babel-plugin-react-compiler/docs/passes/24-pruneNonReactiveDependencies.md

@@ -0,0 +1,138 @@
+# pruneNonReactiveDependencies
+
+## File
+`src/ReactiveScopes/PruneNonReactiveDependencies.ts`
+
+## Purpose
+This pass removes dependencies from reactive scopes that are guaranteed to be **non-reactive** (i.e., their values cannot change between renders). This optimization reduces unnecessary memoization invalidations by ensuring scopes only depend on values that can actually change.
+
+The pass complements `PropagateScopeDependencies`, which infers dependencies without considering reactivity. This subsequent pruning step filters out dependencies that are semantically constant.
+
+## Input Invariants
+- The function has been converted to a ReactiveFunction structure
+- `InferReactivePlaces` has annotated places with `{reactive: true}` where values can change
+- Each `ReactiveScopeBlock` has a `scope.dependencies` set populated by `PropagateScopeDependenciesHIR`
+- Type inference has run, so identifiers have type information for `isStableType` checks
+
+## Output Guarantees
+- **Non-reactive dependencies removed**: All dependencies in `scope.dependencies` are reactive after this pass
+- **Scope outputs marked reactive if needed**: If a scope has any reactive dependencies remaining, all its outputs are marked reactive
+- **Stable types remain non-reactive through property loads**: When loading properties from stable types (like `useReducer` dispatch functions), the result is not added to the reactive set
+
+## Algorithm
+
+### Phase 1: Collect Reactive Identifiers
+The `collectReactiveIdentifiers` helper builds the initial set of reactive identifiers by:
+1. Visiting all places in the ReactiveFunction
+2. Adding any place marked `{reactive: true}` to the set
+3. For pruned scopes, adding declarations that are not primitives and not stable ref types
+
+### Phase 2: Propagate Reactivity and Prune Dependencies
+The main `Visitor` class traverses the ReactiveFunction and:
+
+1. **For Instructions** - Propagates reactivity through data flow:
+   - `LoadLocal`: If source is reactive, mark the lvalue as reactive
+   - `StoreLocal`: If source value is reactive, mark both the local variable and lvalue as reactive
+   - `Destructure`: If source is reactive, mark all pattern operands as reactive (except stable types)
+   - `PropertyLoad`: If object is reactive AND result is not a stable type, mark result as reactive
+   - `ComputedLoad`: If object OR property is reactive, mark result as reactive
+
+2. **For Scopes** - Prunes non-reactive dependencies and propagates outputs:
+   - Delete each dependency from `scope.dependencies` if its identifier is not in the reactive set
+   - If any dependencies remain after pruning, mark all scope outputs as reactive
+
+### Key Insight: Stable Types
+The pass leverages `isStableType` to prevent reactivity from flowing through certain React-provided stable values:
+
+```typescript
+function isStableType(id: Identifier): boolean {
+  return (
+    isSetStateType(id) ||       // useState setter
+    isSetActionStateType(id) || // useActionState setter
+    isDispatcherType(id) ||     // useReducer dispatcher
+    isUseRefType(id) ||         // useRef result
+    isStartTransitionType(id) ||// useTransition startTransition
+    isSetOptimisticType(id)     // useOptimistic setter
+  );
+}
+```
+
+## Edge Cases
+
+### Unmemoized Values Spanning Hook Calls
+A value created before a hook call and mutated after cannot be memoized. However, if it's non-reactive, it still should not appear as a dependency of downstream scopes.
+
+### Stable Types from Reactive Containers
+When `useReducer` returns `[state, dispatch]`, `state` is reactive but `dispatch` is stable. The pass correctly handles this.
+
+### Pruned Scopes with Reactive Content
+The `CollectReactiveIdentifiers` pass also examines pruned scopes and adds their non-primitive, non-stable-ref declarations to the reactive set.
+
+### Transitive Reactivity Through Scopes
+When a scope retains at least one reactive dependency, ALL its outputs become reactive.
+
+## TODOs
+None in the source file.
+
+## Example
+
+### Fixture: `unmemoized-nonreactive-dependency-is-pruned-as-dependency.js`
+
+**Input:**
+```javascript
+function Component(props) {
+  const x = [];
+  useNoAlias();
+  mutate(x);
+
+  return <div>{x}</div>;
+}
+```
+
+**Before PruneNonReactiveDependencies:**
+```
+scope @2 dependencies=[x$15_@0:TObject<BuiltInArray>] declarations=[$23_@2]
+```
+
+**After PruneNonReactiveDependencies:**
+```
+scope @2 dependencies=[] declarations=[$23_@2]
+```
+
+The dependency on `x` is removed because `x` is created locally and therefore non-reactive.
+
+### Fixture: `useReducer-returned-dispatcher-is-non-reactive.js`
+
+**Input:**
+```javascript
+function f() {
+  const [state, dispatch] = useReducer();
+
+  const onClick = () => {
+    dispatch();
+  };
+
+  return <div onClick={onClick} />;
+}
+```
+
+**Generated Code:**
+```javascript
+function f() {
+  const $ = _c(1);
+  const [, dispatch] = useReducer();
+  let t0;
+  if ($[0] === Symbol.for("react.memo_cache_sentinel")) {
+    const onClick = () => {
+      dispatch();
+    };
+    t0 = <div onClick={onClick} />;
+    $[0] = t0;
+  } else {
+    t0 = $[0];
+  }
+  return t0;
+}
+```
+
+The `onClick` function only captures `dispatch`, which is a stable type. Therefore, `onClick` is non-reactive, and the JSX element can be memoized with zero dependencies.

compiler/packages/babel-plugin-react-compiler/docs/passes/25-pruneUnusedScopes.md

@@ -0,0 +1,111 @@
+# pruneUnusedScopes
+
+## File
+`src/ReactiveScopes/PruneUnusedScopes.ts`
+
+## Purpose
+This pass converts reactive scopes that have no meaningful outputs into "pruned scopes". A pruned scope is no longer memoized - its instructions are executed unconditionally on every render. This optimization removes unnecessary memoization overhead for scopes that don't produce values that need to be cached.
+
+## Input Invariants
+- The input is a `ReactiveFunction` that has already been transformed into reactive scope form
+- Scopes have been created and have `declarations`, `reassignments`, and potentially `earlyReturnValue` populated
+- The pass is called after:
+  - `pruneUnusedLabels` - cleans up unnecessary labels
+  - `pruneNonEscapingScopes` - removes scopes whose outputs don't escape
+  - `pruneNonReactiveDependencies` - removes non-reactive dependencies from scopes
+- Scopes may already be marked as pruned by earlier passes
+
+## Output Guarantees
+Scopes that meet ALL of the following criteria are converted to `pruned-scope`:
+- No return statement within the scope
+- No reassignments (`scope.reassignments.size === 0`)
+- Either no declarations (`scope.declarations.size === 0`), OR all declarations "bubbled up" from inner scopes
+
+Pruned scopes:
+- Keep their original scope metadata (for debugging/tracking)
+- Keep their instructions intact
+- Will be executed unconditionally during codegen (no memoization check)
+
+## Algorithm
+
+The pass uses the visitor pattern with `ReactiveFunctionTransform`:
+
+1. **State Tracking**: A `State` object tracks whether a return statement was encountered:
+   ```typescript
+   type State = {
+     hasReturnStatement: boolean;
+   };
+   ```
+
+2. **Terminal Visitor** (`visitTerminal`): Checks if any terminal is a `return` statement
+
+3. **Scope Transform** (`transformScope`): For each scope:
+   - Creates a fresh state for this scope
+   - Recursively visits the scope's contents
+   - Checks pruning criteria:
+     - `!scopeState.hasReturnStatement` - no early return
+     - `scope.reassignments.size === 0` - no reassignments
+     - `scope.declarations.size === 0` OR `!hasOwnDeclaration(scopeBlock)` - no outputs
+
+4. **hasOwnDeclaration Helper**: Determines if a scope has "own" declarations vs declarations propagated from nested scopes
+
+## Edge Cases
+
+### Return Statements
+Scopes containing return statements are preserved because early returns need memoization to avoid re-executing the return check on every render.
+
+### Bubbled-Up Declarations
+When nested scopes are flattened or merged, their declarations may be propagated to parent scopes. The `hasOwnDeclaration` check ensures that parent scopes with only inherited declarations can still be pruned.
+
+### Reassignments
+Scopes with reassignments are kept because the reassignment represents a side effect that needs to be tracked for memoization.
+
+### Already-Pruned Scopes
+The pass operates on `ReactiveScopeBlock` (kind: 'scope'), not `PrunedReactiveScopeBlock`. Scopes already pruned by earlier passes are not revisited.
+
+### Interaction with Subsequent Passes
+The `MergeReactiveScopesThatInvalidateTogether` pass explicitly handles pruned scopes - it does not merge across them.
+
+## TODOs
+None in the source file.
+
+## Example
+
+### Fixture: `prune-scopes-whose-deps-invalidate-array.js`
+
+**Input:**
+```javascript
+function Component(props) {
+  const x = [];
+  useHook();
+  x.push(props.value);
+  const y = [x];
+  return [y];
+}
+```
+
+What happens:
+- The scope for `x` cannot be memoized because `useHook()` is called inside it
+- `FlattenScopesWithHooksOrUseHIR` marks scope @0 as `pruned-scope`
+- `PruneUnusedScopes` doesn't change it further since it's already pruned
+
+**Output (no memoization for x):**
+```javascript
+function Component(props) {
+  const x = [];
+  useHook();
+  x.push(props.value);
+  const y = [x];
+  return [y];
+}
+```
+
+### Key Insight
+
+The `pruneUnusedScopes` pass is part of a multi-pass pruning strategy:
+1. `FlattenScopesWithHooksOrUseHIR` - Prunes scopes that contain hook/use calls
+2. `pruneNonEscapingScopes` - Prunes scopes whose outputs don't escape
+3. `pruneNonReactiveDependencies` - Removes non-reactive dependencies
+4. **`pruneUnusedScopes`** - Prunes scopes with no remaining outputs
+
+This pass acts as a cleanup for scopes that became "empty" after previous pruning passes removed their outputs.

compiler/packages/babel-plugin-react-compiler/docs/passes/26-mergeReactiveScopesThatInvalidateTogether.md

@@ -0,0 +1,213 @@
+# mergeReactiveScopesThatInvalidateTogether
+
+## File
+`src/ReactiveScopes/MergeReactiveScopesThatInvalidateTogether.ts`
+
+## Purpose
+This pass is an optimization that reduces memoization overhead in the compiled output by merging reactive scopes that will always invalidate together. The pass operates on the ReactiveFunction representation and works in two main scenarios:
+
+1. **Consecutive Scopes**: When two scopes appear sequentially in the same reactive block with identical dependencies (or where the output of the first scope is the sole input to the second), they are merged into a single scope. This reduces the number of memo cache slots used and eliminates redundant dependency comparisons.
+
+2. **Nested Scopes**: When an inner scope has the same dependencies as its parent scope, the inner scope is flattened into the parent. Since PropagateScopeDependencies propagates dependencies upward, nested scopes can only have equal or fewer dependencies than their parents, never more. When they're equal, the inner scope always invalidates with the parent, making it safe and beneficial to flatten.
+
+## Input Invariants
+- The ReactiveFunction has already undergone scope dependency propagation (via `PropagateScopeDependencies`)
+- The function has been pruned of unused scopes (via `pruneNonReactiveDependencies` and `pruneUnusedScopes`)
+- Scopes have valid `dependencies`, `declarations`, `range`, and `reassignments` fields
+- The ReactiveFunction is in a valid structural state with properly formed blocks and instructions
+
+## Output Guarantees
+- **Fewer scopes**: Consecutive and nested scopes with identical dependencies are merged
+- **Valid scope ranges**: Merged scopes have their `range.end` updated to cover all merged instructions
+- **Updated declarations**: Scope declarations are updated to remove any that are no longer used after the merged scope
+- **Merged scope tracking**: The `scope.merged` set tracks which scope IDs were merged into each surviving scope
+- **Preserved semantics**: Only safe-to-memoize intermediate instructions are absorbed into merged scopes
+
+## Algorithm
+
+The pass operates in multiple phases:
+
+### Phase 1: Find Last Usage
+A visitor (`FindLastUsageVisitor`) collects the last usage instruction ID for each declaration:
+
+```typescript
+class FindLastUsageVisitor extends ReactiveFunctionVisitor<void> {
+  lastUsage: Map<DeclarationId, InstructionId> = new Map();
+
+  override visitPlace(id: InstructionId, place: Place, _state: void): void {
+    const previousUsage = this.lastUsage.get(place.identifier.declarationId);
+    const lastUsage =
+      previousUsage !== undefined
+        ? makeInstructionId(Math.max(previousUsage, id))
+        : id;
+    this.lastUsage.set(place.identifier.declarationId, lastUsage);
+  }
+}
+```
+
+### Phase 2: Transform (Nested Scope Flattening)
+The `transformScope` method flattens nested scopes with identical dependencies:
+
+```typescript
+override transformScope(
+  scopeBlock: ReactiveScopeBlock,
+  state: ReactiveScopeDependencies | null,
+): Transformed<ReactiveStatement> {
+  this.visitScope(scopeBlock, scopeBlock.scope.dependencies);
+  if (
+    state !== null &&
+    areEqualDependencies(state, scopeBlock.scope.dependencies)
+  ) {
+    return {kind: 'replace-many', value: scopeBlock.instructions};
+  } else {
+    return {kind: 'keep'};
+  }
+}
+```
+
+### Phase 3: Visit Block (Consecutive Scope Merging)
+Within `visitBlock`, the pass:
+1. First traverses nested blocks recursively
+2. Iterates through instructions, tracking merge candidates
+3. Determines if consecutive scopes can merge based on:
+   - Identical dependencies, OR
+   - Output of first scope is input to second scope (with always-invalidating types)
+4. Collects intermediate lvalues and ensures they're only used by the next scope
+5. Merges eligible scopes by combining instructions and updating range/declarations
+
+### Key Merging Conditions (`canMergeScopes`):
+```typescript
+function canMergeScopes(
+  current: ReactiveScopeBlock,
+  next: ReactiveScopeBlock,
+  temporaries: Map<DeclarationId, DeclarationId>,
+): boolean {
+  // Don't merge scopes with reassignments
+  if (current.scope.reassignments.size !== 0 || next.scope.reassignments.size !== 0) {
+    return false;
+  }
+  // Merge scopes whose dependencies are identical
+  if (areEqualDependencies(current.scope.dependencies, next.scope.dependencies)) {
+    return true;
+  }
+  // Merge scopes where outputs of previous are inputs of next
+  // (with always-invalidating type check)
+  // ...
+}
+```
+
+### Always-Invalidating Types:
+```typescript
+export function isAlwaysInvalidatingType(type: Type): boolean {
+  switch (type.kind) {
+    case 'Object': {
+      switch (type.shapeId) {
+        case BuiltInArrayId:
+        case BuiltInObjectId:
+        case BuiltInFunctionId:
+        case BuiltInJsxId: {
+          return true;
+        }
+      }
+      break;
+    }
+    case 'Function': {
+      return true;
+    }
+  }
+  return false;
+}
+```
+
+## Edge Cases
+
+### Terminals
+The pass does not merge across terminals (control flow boundaries).
+
+### Pruned Scopes
+Merging stops at pruned scopes.
+
+### Reassignments
+Scopes containing reassignments cannot be merged (side-effect ordering concerns).
+
+### Intermediate Reassignments
+Non-const StoreLocal instructions between scopes prevent merging.
+
+### Safe Intermediate Instructions
+Only certain instruction types are allowed between merged scopes: `BinaryExpression`, `ComputedLoad`, `JSXText`, `LoadGlobal`, `LoadLocal`, `Primitive`, `PropertyLoad`, `TemplateLiteral`, `UnaryExpression`, and const `StoreLocal`.
+
+### Lvalue Usage
+Intermediate values must be last-used at or before the next scope to allow merging.
+
+### Non-Invalidating Outputs
+If a scope's output may not change when inputs change (e.g., `foo(x) { return x < 10 }` returns same boolean for different x values), that scope cannot be a merge candidate for subsequent scopes.
+
+## TODOs
+```typescript
+/*
+ * TODO LeaveSSA: use IdentifierId for more precise tracking
+ * Using DeclarationId is necessary for compatible output but produces suboptimal results
+ * in cases where a scope defines a variable, but that version is never read and always
+ * overwritten later.
+ * see reassignment-separate-scopes.js for example
+ */
+lastUsage: Map<DeclarationId, InstructionId> = new Map();
+```
+
+## Example
+
+### Fixture: `merge-consecutive-scopes-deps-subset-of-decls.js`
+
+**Input:**
+```javascript
+import {useState} from 'react';
+
+function Component() {
+  const [count, setCount] = useState(0);
+  return (
+    <div>
+      <button onClick={() => setCount(count - 1)}>Decrement</button>
+      <button onClick={() => setCount(count + 1)}>Increment</button>
+    </div>
+  );
+}
+```
+
+**After MergeReactiveScopesThatInvalidateTogether** (from `yarn snap -p merge-consecutive-scopes-deps-subset-of-decls.js -d`):
+```
+scope @1 [7:24] dependencies=[count$32:TPrimitive] declarations=[$51_@5] reassignments=[] {
+  [8] $35_@1 = Function @context[setCount$33, count$32]  // decrement callback
+  [10] $41 = JSXText "Decrement"
+  [12] $42_@2 = JSX <button onClick={$35_@1}>{$41}</button>
+  [15] $43_@3 = Function @context[setCount$33, count$32]  // increment callback
+  [17] $49 = JSXText "Increment"
+  [19] $50_@4 = JSX <button onClick={$43_@3}>{$49}</button>
+  [22] $51_@5 = JSX <div>{$42_@2}{$50_@4}</div>
+}
+```
+
+All scopes are merged because they share `count` as a dependency. Without merging, this would have separate scopes for each callback and button element.
+
+**Generated Code:**
+```javascript
+function Component() {
+  const $ = _c(2);
+  const [count, setCount] = useState(0);
+  let t0;
+  if ($[0] !== count) {
+    t0 = (
+      <div>
+        <button onClick={() => setCount(count - 1)}>Decrement</button>
+        <button onClick={() => setCount(count + 1)}>Increment</button>
+      </div>
+    );
+    $[0] = count;
+    $[1] = t0;
+  } else {
+    t0 = $[1];
+  }
+  return t0;
+}
+```
+
+The merged version uses only 2 cache slots instead of potentially 6-8.

compiler/packages/babel-plugin-react-compiler/docs/passes/27-pruneAlwaysInvalidatingScopes.md

@@ -0,0 +1,143 @@
+# pruneAlwaysInvalidatingScopes
+
+## File
+`src/ReactiveScopes/PruneAlwaysInvalidatingScopes.ts`
+
+## Purpose
+This pass identifies and prunes reactive scopes whose dependencies will *always* invalidate on every render, making memoization pointless. Specifically, it tracks values that are guaranteed to be new allocations (arrays, objects, JSX, new expressions) and checks if those values are used outside of any memoization scope. When a downstream scope depends on such an unmemoized always-invalidating value, the scope is pruned because it would re-execute on every render anyway.
+
+The optimization avoids wasted comparisons in the generated code. Without this pass, the compiler would emit dependency checks for scopes that will never cache-hit, adding runtime overhead with no benefit. By converting these scopes to `pruned-scope` nodes, the codegen emits the instructions inline without memoization guards.
+
+## Input Invariants
+- The pass expects a `ReactiveFunction` with scopes already formed
+- Scopes should have their `dependencies` populated with the identifiers they depend on
+- The pass runs after `MergeReactiveScopesThatInvalidateTogether`
+- Hook calls have already caused scope flattening via `FlattenScopesWithHooksOrUseHIR`
+
+## Output Guarantees
+- Scopes that depend on unmemoized always-invalidating values are converted to `pruned-scope` nodes
+- The `unmemoizedValues` set correctly propagates through `StoreLocal`/`LoadLocal` instructions
+- All declarations and reassignments within pruned scopes that are themselves always-invalidating are added to `unmemoizedValues`, enabling cascading pruning of downstream scopes
+
+## Algorithm
+
+The pass uses a `ReactiveFunctionTransform` visitor with two key methods:
+
+### 1. `transformInstruction` - Tracks always-invalidating values:
+
+```typescript
+switch (value.kind) {
+  case 'ArrayExpression':
+  case 'ObjectExpression':
+  case 'JsxExpression':
+  case 'JsxFragment':
+  case 'NewExpression': {
+    if (lvalue !== null) {
+      this.alwaysInvalidatingValues.add(lvalue.identifier);
+      if (!withinScope) {
+        this.unmemoizedValues.add(lvalue.identifier);  // Key: only if outside a scope
+      }
+    }
+    break;
+  }
+  // Also propagates through StoreLocal and LoadLocal
+}
+```
+
+### 2. `transformScope` - Prunes scopes with unmemoized dependencies:
+
+```typescript
+for (const dep of scopeBlock.scope.dependencies) {
+  if (this.unmemoizedValues.has(dep.identifier)) {
+    // Propagate unmemoized status to scope outputs
+    for (const [_, decl] of scopeBlock.scope.declarations) {
+      if (this.alwaysInvalidatingValues.has(decl.identifier)) {
+        this.unmemoizedValues.add(decl.identifier);
+      }
+    }
+    return {
+      kind: 'replace',
+      value: {
+        kind: 'pruned-scope',
+        scope: scopeBlock.scope,
+        instructions: scopeBlock.instructions,
+      },
+    };
+  }
+}
+```
+
+## Edge Cases
+
+### Function Calls Not Considered Always-Invalidating
+The pass optimistically assumes function calls may return primitives, so `makeArray()` doesn't trigger pruning even though it might return a new array.
+
+### Conditional Allocations
+Code like `x = cond ? [] : 42` doesn't trigger pruning because the value might be a primitive.
+
+### Propagation Through Locals
+The pass correctly tracks values through `StoreLocal` and `LoadLocal` to handle variable reassignments and loads.
+
+### Cascading Pruning
+When a scope is pruned, its always-invalidating outputs become unmemoized, potentially causing downstream scopes to be pruned as well.
+
+## TODOs
+None in the source file.
+
+## Example
+
+### Fixture: `prune-scopes-whose-deps-invalidate-array.js`
+
+**Input:**
+```javascript
+function Component(props) {
+  const x = [];
+  useHook();
+  x.push(props.value);
+  const y = [x];
+  return [y];
+}
+```
+
+**After PruneAlwaysInvalidatingScopes** (from `yarn snap -p prune-scopes-whose-deps-invalidate-array.js -d`):
+```
+<pruned> scope @0 [1:14] dependencies=[] declarations=[x$21_@0] reassignments=[] {
+  [2] $20_@0 = Array []
+  [3] StoreLocal Const x$21_@0 = $20_@0
+  [4] $23 = LoadGlobal import { useHook }
+  [6] $24_@1 = Call $23()       // Hook flattens scope
+  [7] break bb9 (implicit)
+  [8] $25_@0 = LoadLocal x$21_@0
+  [9] $26 = PropertyLoad $25_@0.push
+  [10] $27 = LoadLocal props$19
+  [11] $28 = PropertyLoad $27.value
+  [12] $29 = MethodCall $25_@0.$26($28)
+}
+[14] $30 = LoadLocal x$21_@0
+<pruned> scope @2 [15:23] dependencies=[x$21_@0:TObject<BuiltInArray>] declarations=[$35_@3] {
+  [16] $31_@2 = Array [$30]
+  [18] StoreLocal Const y$32 = $31_@2
+  [19] $34 = LoadLocal y$32
+  [21] $35_@3 = Array [$34]
+}
+[23] return $35_@3
+```
+
+Key observations:
+- Scope @0 is pruned because the hook call (`useHook()`) flattens it (hook rules prevent memoization around hooks)
+- `x` is an `ArrayExpression` created in the pruned scope @0, making it unmemoized
+- Scope @2 depends on `x$21_@0` which is unmemoized and always-invalidating (it's an array)
+- Therefore, scope @2 is also pruned - cascading pruning
+
+**Generated Code:**
+```javascript
+function Component(props) {
+  const x = [];
+  useHook();
+  x.push(props.value);
+  const y = [x];
+  return [y];
+}
+```
+
+The output matches the input because all memoization was pruned - the code runs unconditionally on every render.

compiler/packages/babel-plugin-react-compiler/docs/passes/28-propagateEarlyReturns.md

@@ -0,0 +1,183 @@
+# propagateEarlyReturns
+
+## File
+`src/ReactiveScopes/PropagateEarlyReturns.ts`
+
+## Purpose
+The `propagateEarlyReturns` pass ensures that reactive scopes (memoization blocks) correctly honor the control flow behavior of the original code, particularly when a function returns early from within a reactive scope. Without this transformation, if a component returned early on the previous render and the inputs have not changed, the cached memoization block would be skipped entirely, but the early return would not occur, causing incorrect behavior.
+
+The pass solves this by transforming `return` statements inside reactive scopes into assignments to a temporary variable followed by a labeled `break`. After the reactive scope completes, generated code checks whether the early return sentinel value was replaced with an actual return value; if so, the function returns that value.
+
+## Input Invariants
+1. **ReactiveFunction structure**: The input must be a `ReactiveFunction` with scopes already inferred (reactive scope blocks are already established)
+2. **Scope earlyReturnValue not set**: The pass expects `scopeBlock.scope.earlyReturnValue === null` for scopes it processes
+3. **Return statements within reactive scopes**: The pass specifically targets `return` terminal statements that appear within a `withinReactiveScope` context
+
+## Output Guarantees
+1. **Labeled scope blocks**: Top-level reactive scopes containing early returns are wrapped in a labeled block (e.g., `bb14: { ... }`)
+2. **Sentinel initialization**: At the start of each such scope, a temporary variable is initialized to `Symbol.for("react.early_return_sentinel")`
+3. **Return-to-break transformation**: All `return` statements inside the scope are replaced with:
+   - An assignment of the return value to the early return temporary
+   - A `break` to the scope's label
+4. **Early return declaration**: The temporary variable is registered as a declaration of the scope so it gets memoized
+5. **Post-scope check**: During codegen, an if-statement is added after the scope to check if the temporary differs from the sentinel and return it if so
+
+## Algorithm
+
+The pass uses a visitor pattern with a `ReactiveFunctionTransform` that tracks two pieces of state:
+
+```typescript
+type State = {
+  withinReactiveScope: boolean;  // Are we inside a reactive scope?
+  earlyReturnValue: ReactiveScope['earlyReturnValue'];  // Bubble up early return info
+};
+```
+
+### Key Steps:
+
+1. **visitScope** - When entering a reactive scope:
+   - Create an inner state with `withinReactiveScope: true`
+   - Traverse the scope's contents
+   - If any early returns were found (`earlyReturnValue !== null`):
+     - If this is the **outermost** scope (parent's `withinReactiveScope` is false):
+       - Store the early return info on the scope
+       - Add the temporary as a scope declaration
+       - Prepend sentinel initialization instructions
+       - Wrap the original instructions in a labeled block
+     - Otherwise, propagate the early return info to the parent scope
+
+2. **transformTerminal** - When encountering a `return` inside a reactive scope:
+   - Create or reuse an early return value identifier
+   - Replace the return with:
+     ```typescript
+     [
+       {kind: 'instruction', /* StoreLocal: reassign earlyReturnValue = returnValue */},
+       {kind: 'terminal', /* break to earlyReturnValue.label */}
+     ]
+     ```
+
+### Sentinel Initialization Code (synthesized at scope start):
+```typescript
+// Load Symbol.for and call it with the sentinel string
+let t0 = Symbol.for("react.early_return_sentinel");
+```
+
+## Edge Cases
+
+### Nested Reactive Scopes
+When early returns occur in nested scopes, only the **outermost** scope gets the labeled block wrapper. Inner scopes bubble their early return information up via `parentState.earlyReturnValue`.
+
+### Multiple Early Returns in Same Scope
+All returns share the same temporary variable and label. The first return found creates the identifier, subsequent returns reuse it.
+
+### Partial Early Returns
+When only some control flow paths return early (e.g., one branch returns, the other falls through), the sentinel check after the scope allows normal execution to continue if no early return occurred.
+
+### Already Processed Scopes
+If `scopeBlock.scope.earlyReturnValue !== null` on entry, the pass exits early without modification.
+
+### Returns Outside Reactive Scopes
+The pass only transforms returns where `state.withinReactiveScope === true`. Returns outside scopes are left unchanged.
+
+## TODOs
+None in the source file.
+
+## Example
+
+### Fixture: `early-return-within-reactive-scope.js`
+
+**Input:**
+```javascript
+function Component(props) {
+  let x = [];
+  if (props.cond) {
+    x.push(props.a);
+    return x;
+  } else {
+    return makeArray(props.b);
+  }
+}
+```
+
+**After PropagateEarlyReturns** (from `yarn snap -p early-return-within-reactive-scope.js -d`):
+```
+scope @0 [...] earlyReturn={id: #t34$34, label: 14} {
+  [0] $36 = LoadGlobal(global) Symbol
+  [0] $37 = PropertyLoad $36.for
+  [0] $38 = "react.early_return_sentinel"
+  [0] $35 = MethodCall $36.$37($38)
+  [0] StoreLocal Let #t34$34{reactive} = $35    // Initialize sentinel
+  bb14: {
+    [2] $19_@0 = Array []
+    [3] StoreLocal Const x$20_@0 = $19_@0
+    [4] $22{reactive} = LoadLocal props$18
+    [5] $23{reactive} = PropertyLoad $22.cond
+    [6] if ($23) {
+      [7] $24_@0 = LoadLocal x$20_@0
+      [8] $25 = PropertyLoad $24_@0.push
+      [9] $26 = LoadLocal props$18
+      [10] $27 = PropertyLoad $26.a
+      [11] $28 = MethodCall $24_@0.$25($27)
+      [12] $29 = LoadLocal x$20_@0
+      [0] StoreLocal Reassign #t34$34 = $29     // was: return x
+      [0] break bb14 (labeled)
+    } else {
+      [14] $30 = LoadGlobal import { makeArray }
+      [15] $31 = LoadLocal props$18
+      [16] $32 = PropertyLoad $31.b
+      scope @1 [...] {
+        [18] $33_@1 = Call $30($32)
+      }
+      [0] StoreLocal Reassign #t34$34 = $33_@1  // was: return makeArray(props.b)
+      [0] break bb14 (labeled)
+    }
+  }
+}
+```
+
+Key observations:
+- Scope @0 now has `earlyReturn={id: #t34$34, label: 14}`
+- Sentinel initialization code is prepended to the scope
+- The scope body is wrapped in `bb14: { ... }`
+- Both `return x` and `return makeArray(props.b)` are transformed to `StoreLocal Reassign + break bb14`
+
+**Generated Code:**
+```javascript
+function Component(props) {
+  const $ = _c(6);
+  let t0;
+  if ($[0] !== props.a || $[1] !== props.b || $[2] !== props.cond) {
+    t0 = Symbol.for("react.early_return_sentinel");
+    bb0: {
+      const x = [];
+      if (props.cond) {
+        x.push(props.a);
+        t0 = x;
+        break bb0;
+      } else {
+        let t1;
+        if ($[4] !== props.b) {
+          t1 = makeArray(props.b);
+          $[4] = props.b;
+          $[5] = t1;
+        } else {
+          t1 = $[5];
+        }
+        t0 = t1;
+        break bb0;
+      }
+    }
+    $[0] = props.a;
+    $[1] = props.b;
+    $[2] = props.cond;
+    $[3] = t0;
+  } else {
+    t0 = $[3];
+  }
+  if (t0 !== Symbol.for("react.early_return_sentinel")) {
+    return t0;
+  }
+}
+```
+
+This transformation ensures that when inputs don't change, the cached return value is used and returned, preserving referential equality and correct early return behavior.

compiler/packages/babel-plugin-react-compiler/docs/passes/29-promoteUsedTemporaries.md

@@ -0,0 +1,203 @@
+# promoteUsedTemporaries
+
+## File
+`src/ReactiveScopes/PromoteUsedTemporaries.ts`
+
+## Purpose
+This pass promotes temporary variables (identifiers with no name) to named variables when they need to be referenced across scope boundaries or in code generation. Temporaries are intermediate values that the compiler creates during lowering; they are typically inlined at their use sites during codegen. However, some temporaries must be emitted as separate declarations - this pass identifies and names them.
+
+The pass ensures that:
+1. Scope dependencies and declarations have proper names for codegen
+2. Variables referenced across reactive scope boundaries are named
+3. JSX tag identifiers get special naming (`T0`, `T1`, etc.)
+4. Temporaries with interposing side-effects are promoted to preserve ordering
+
+## Input Invariants
+- The ReactiveFunction has undergone scope construction and dependency propagation
+- Identifiers may have `name === null` (temporaries) or be named
+- Scopes have `dependencies`, `declarations`, and `reassignments` populated
+- Pruned scopes are properly marked with `kind: 'pruned-scope'`
+
+## Output Guarantees
+- All scope dependencies have non-null names
+- All scope declarations have non-null names
+- JSX tag temporaries use uppercase naming (`T0`, `T1`, ...)
+- Regular temporaries use lowercase naming (`#t{id}`)
+- All instances of a promoted identifier share the same name (via DeclarationId tracking)
+- Temporaries with interposing mutating instructions are promoted to preserve source ordering
+
+## Algorithm
+
+The pass operates in four phases using visitor classes:
+
+### Phase 1: CollectPromotableTemporaries
+Collects information about which temporaries may need promotion:
+
+```typescript
+class CollectPromotableTemporaries {
+  // Tracks pruned scope declarations and whether they're used outside their scope
+  pruned: Map<DeclarationId, {activeScopes: Array<ScopeId>; usedOutsideScope: boolean}>
+
+  // Tracks identifiers used as JSX tags (need uppercase names)
+  tags: Set<DeclarationId>
+}
+```
+
+- When visiting a `JsxExpression`, adds the tag identifier to `tags`
+- When visiting a `PrunedScope`, records its declarations
+- Tracks when pruned declarations are used in different scopes
+
+### Phase 2: PromoteTemporaries
+Promotes temporaries that appear in positions requiring names:
+
+```typescript
+override visitScope(scopeBlock: ReactiveScopeBlock, state: State): void {
+  // Promote all dependencies without names
+  for (const dep of scopeBlock.scope.dependencies) {
+    if (identifier.name == null) {
+      promoteIdentifier(identifier, state);
+    }
+  }
+  // Promote all declarations without names
+  for (const [, declaration] of scopeBlock.scope.declarations) {
+    if (declaration.identifier.name == null) {
+      promoteIdentifier(declaration.identifier, state);
+    }
+  }
+}
+```
+
+Also promotes:
+- Function parameters without names
+- Pruned scope declarations used outside their scope
+
+### Phase 3: PromoteInterposedTemporaries
+Handles ordering-sensitive promotion:
+
+```typescript
+class PromoteInterposedTemporaries {
+  // Instructions that emit as statements can interpose between temp defs and uses
+  // If such an instruction occurs, mark pending temporaries as needing promotion
+
+  override visitInstruction(instruction: ReactiveInstruction, state: InterState): void {
+    // For instructions that become statements (calls, stores, etc.):
+    if (willBeStatement && !constStore) {
+      // Mark all pending temporaries as needing promotion
+      for (const [key, [ident, _]] of state.entries()) {
+        state.set(key, [ident, true]);  // Mark as needing promotion
+      }
+    }
+  }
+}
+```
+
+This preserves source ordering when side-effects occur between a temporary's definition and use.
+
+### Phase 4: PromoteAllInstancesOfPromotedTemporaries
+Ensures all instances of a promoted identifier share the same name:
+
+```typescript
+class PromoteAllInstancesOfPromotedTemporaries {
+  override visitPlace(_id: InstructionId, place: Place, state: State): void {
+    if (place.identifier.name === null &&
+        state.promoted.has(place.identifier.declarationId)) {
+      promoteIdentifier(place.identifier, state);
+    }
+  }
+}
+```
+
+### Naming Convention
+```typescript
+function promoteIdentifier(identifier: Identifier, state: State): void {
+  if (state.tags.has(identifier.declarationId)) {
+    promoteTemporaryJsxTag(identifier);  // Uses #T{id} for JSX tags
+  } else {
+    promoteTemporary(identifier);  // Uses #t{id} for regular temps
+  }
+  state.promoted.add(identifier.declarationId);
+}
+```
+
+## Edge Cases
+
+### JSX Tag Temporaries
+JSX tags require uppercase names to be valid JSX syntax. The pass tracks which temporaries are used as JSX tags and uses `T0`, `T1`, etc. instead of `t0`, `t1`.
+
+### Pruned Scope Declarations
+Declarations in pruned scopes are only promoted if they're actually used outside the pruned scope, avoiding unnecessary variable declarations.
+
+### Const vs Let Temporaries
+The pass tracks const identifiers specially - they don't need promotion for ordering purposes since they can't be mutated by interposing instructions.
+
+### Global Loads
+Values loaded from globals (and their property loads) are treated as const-like for promotion purposes.
+
+### Method Call Properties
+The property identifier in a method call is treated as const-like to avoid unnecessary promotion.
+
+## TODOs
+None in the source file.
+
+## Example
+
+### Fixture: `simple.js`
+
+**Input:**
+```javascript
+export default function foo(x, y) {
+  if (x) {
+    return foo(false, y);
+  }
+  return [y * 10];
+}
+```
+
+**Before PromoteUsedTemporaries:**
+```
+scope @0 [...] dependencies=[y$14] declarations=[$19_@0]
+scope @1 [...] dependencies=[$22] declarations=[$23_@1]
+```
+
+**After PromoteUsedTemporaries:**
+```
+scope @0 [...] dependencies=[y$14] declarations=[#t5$19_@0]
+scope @1 [...] dependencies=[#t9$22] declarations=[#t10$23_@1]
+```
+
+Key observations:
+- `$19_@0` is promoted to `#t5$19_@0` because it's a scope declaration
+- `$22` is promoted to `#t9$22` because it's a scope dependency
+- `$23_@1` is promoted to `#t10$23_@1` because it's a scope declaration
+- The `#t` prefix indicates this is a promoted temporary (later renamed by `renameVariables`)
+
+**Generated Code:**
+```javascript
+import { c as _c } from "react/compiler-runtime";
+export default function foo(x, y) {
+  const $ = _c(4);
+  if (x) {
+    let t0;
+    if ($[0] !== y) {
+      t0 = foo(false, y);
+      $[0] = y;
+      $[1] = t0;
+    } else {
+      t0 = $[1];
+    }
+    return t0;
+  }
+  const t0 = y * 10;
+  let t1;
+  if ($[2] !== t0) {
+    t1 = [t0];
+    $[2] = t0;
+    $[3] = t1;
+  } else {
+    t1 = $[3];
+  }
+  return t1;
+}
+```
+
+The promoted temporaries (`#t5`, `#t9`, `#t10`) become the named variables (`t0`, `t1`) in the output after `renameVariables` runs.

compiler/packages/babel-plugin-react-compiler/docs/passes/30-renameVariables.md

@@ -0,0 +1,200 @@
+# renameVariables
+
+## File
+`src/ReactiveScopes/RenameVariables.ts`
+
+## Purpose
+This pass ensures that every named variable in the function has a unique name that doesn't conflict with other variables in the same block scope or with global identifiers. After scope construction and temporary promotion, variables from different source scopes may end up in the same reactive block - this pass resolves any naming conflicts.
+
+The pass also converts the `#t{id}` promoted temporary names into clean output names like `t0`, `t1`, etc.
+
+## Input Invariants
+- The ReactiveFunction has been through `promoteUsedTemporaries`
+- Variables may have names that conflict with:
+  - Other variables in the same or ancestor block scope
+  - Global identifiers referenced by the function
+  - Promoted temporaries with `#t{id}` or `#T{id}` naming
+- The function parameters have names (either from source or promoted)
+
+## Output Guarantees
+- Every named variable has a unique name within its scope
+- No variable shadows a global identifier referenced by the function
+- Promoted temporaries are renamed to `t0`, `t1`, ... (for regular temps)
+- Promoted JSX temporaries are renamed to `T0`, `T1`, ... (for JSX tags)
+- Conflicting source names get disambiguated with `$` suffix (e.g., `foo$0`, `foo$1`)
+- Returns a `Set<string>` of all unique variable names in the function
+
+## Algorithm
+
+### Phase 1: Collect Referenced Globals
+Uses `collectReferencedGlobals(fn)` to build a set of all global identifiers referenced by the function. Variable names must not conflict with these.
+
+### Phase 2: Rename with Scope Stack
+The `Scopes` class maintains:
+
+```typescript
+class Scopes {
+  #seen: Map<DeclarationId, IdentifierName> = new Map();  // Canonical name for each declaration
+  #stack: Array<Map<string, DeclarationId>> = [new Map()];  // Block scope stack
+  #globals: Set<string>;  // Global names to avoid
+  names: Set<ValidIdentifierName> = new Set();  // All assigned names
+}
+```
+
+### Renaming Logic
+```typescript
+visit(identifier: Identifier): void {
+  // Skip unnamed identifiers
+  if (originalName === null) return;
+
+  // If we've already named this declaration, reuse that name
+  const mappedName = this.#seen.get(identifier.declarationId);
+  if (mappedName !== undefined) {
+    identifier.name = mappedName;
+    return;
+  }
+
+  // Find a unique name
+  let name = originalName.value;
+  let id = 0;
+
+  // Promoted temporaries start with t0/T0
+  if (isPromotedTemporary(originalName.value)) {
+    name = `t${id++}`;
+  } else if (isPromotedJsxTemporary(originalName.value)) {
+    name = `T${id++}`;
+  }
+
+  // Increment until we find a unique name
+  while (this.#lookup(name) !== null || this.#globals.has(name)) {
+    if (isPromotedTemporary(...)) {
+      name = `t${id++}`;
+    } else if (isPromotedJsxTemporary(...)) {
+      name = `T${id++}`;
+    } else {
+      name = `${originalName.value}$${id++}`;  // foo$0, foo$1, etc.
+    }
+  }
+
+  identifier.name = makeIdentifierName(name);
+  this.#seen.set(identifier.declarationId, identifier.name);
+}
+```
+
+### Scope Management
+```typescript
+enter(fn: () => void): void {
+  this.#stack.push(new Map());
+  fn();
+  this.#stack.pop();
+}
+
+#lookup(name: string): DeclarationId | null {
+  // Search from innermost to outermost scope
+  for (let i = this.#stack.length - 1; i >= 0; i--) {
+    const entry = this.#stack[i].get(name);
+    if (entry !== undefined) return entry;
+  }
+  return null;
+}
+```
+
+### Visitor Pattern
+```typescript
+class Visitor extends ReactiveFunctionVisitor<Scopes> {
+  override visitBlock(block: ReactiveBlock, state: Scopes): void {
+    state.enter(() => {
+      this.traverseBlock(block, state);
+    });
+  }
+
+  override visitScope(scope: ReactiveScopeBlock, state: Scopes): void {
+    // Visit scope declarations first
+    for (const [_, declaration] of scope.scope.declarations) {
+      state.visit(declaration.identifier);
+    }
+    this.traverseScope(scope, state);
+  }
+
+  override visitPlace(id: InstructionId, place: Place, state: Scopes): void {
+    state.visit(place.identifier);
+  }
+}
+```
+
+## Edge Cases
+
+### Shadowed Variables
+When the compiler merges scopes that had shadowing in the source:
+```javascript
+function foo() {
+  const x = 1;
+  {
+    const x = 2;  // Shadowed in source
+  }
+}
+```
+If both `x` declarations end up in the same compiled scope, they become `x` and `x$0`.
+
+### Global Name Conflicts
+If a local variable would conflict with a referenced global:
+```javascript
+function foo() {
+  const Math = 1;  // Conflicts with global Math if used
+}
+```
+The local gets renamed to `Math$0` if `Math` global is referenced.
+
+### Nested Functions
+The pass recursively processes nested function expressions, entering a new scope for each function body.
+
+### Pruned Scopes
+Pruned scopes don't create a new block scope in the output - the pass traverses their instructions without entering a new scope level.
+
+### DeclarationId Consistency
+The pass uses `DeclarationId` to track which identifiers refer to the same variable, ensuring all references get the same renamed name.
+
+## TODOs
+None in the source file.
+
+## Example
+
+### Fixture: `simple.js`
+
+**Before RenameVariables:**
+```
+scope @0 [...] declarations=[#t5$19_@0]
+scope @1 [...] dependencies=[#t9$22] declarations=[#t10$23_@1]
+```
+
+**After RenameVariables:**
+```
+scope @0 [...] declarations=[t0$19_@0]
+scope @1 [...] dependencies=[t0$22] declarations=[t1$23_@1]
+```
+
+Key observations:
+- `#t5$19_@0` becomes `t0$19_@0` (first temporary in scope)
+- `#t9$22` becomes `t0$22` (first temporary in a different block scope)
+- `#t10$23_@1` becomes `t1$23_@1` (second temporary in that block)
+- The `#t` prefix is removed and sequential numbering is applied
+
+**Generated Code:**
+```javascript
+export default function foo(x, y) {
+  const $ = _c(4);
+  if (x) {
+    let t0;  // Was #t5
+    if ($[0] !== y) {
+      t0 = foo(false, y);
+      // ...
+    }
+    return t0;
+  }
+  const t0 = y * 10;  // Was #t9, reuses t0 since different block scope
+  let t1;  // Was #t10
+  // ...
+}
+```
+
+The pass produces clean, readable output with minimal variable names while avoiding conflicts.

compiler/packages/babel-plugin-react-compiler/docs/passes/31-codegenReactiveFunction.md

@@ -0,0 +1,286 @@
+# codegenReactiveFunction
+
+## File
+`src/ReactiveScopes/CodegenReactiveFunction.ts`
+
+## Purpose
+This is the final pass that converts the ReactiveFunction representation back into a Babel AST. It generates the memoization code that makes React components and hooks efficient by:
+1. Creating the `useMemoCache` call to allocate cache slots
+2. Generating dependency comparisons to check if values have changed
+3. Emitting conditional blocks that skip computation when cached values are valid
+4. Storing computed values in the cache
+5. Loading cached values when dependencies haven't changed
+
+## Input Invariants
+- The ReactiveFunction has been through all prior passes
+- All identifiers that need names have been promoted and renamed
+- Reactive scopes have finalized `dependencies`, `declarations`, and `reassignments`
+- Early returns have been transformed with sentinel values (via `propagateEarlyReturns`)
+- Pruned scopes are marked with `kind: 'pruned-scope'`
+- Unique identifiers set is available to avoid naming conflicts
+
+## Output Guarantees
+- Returns a `CodegenFunction` with Babel AST `body`
+- All reactive scopes become if-else blocks checking dependencies
+- The `$` cache array is properly sized with `useMemoCache(n)`
+- Each dependency and output gets its own cache slot
+- Pruned scopes emit their instructions inline without memoization
+- Early returns use the sentinel pattern with post-scope checks
+- Statistics are collected: `memoSlotsUsed`, `memoBlocks`, `memoValues`, etc.
+
+## Algorithm
+
+### Entry Point: codegenFunction
+```typescript
+export function codegenFunction(fn: ReactiveFunction): Result<CodegenFunction, CompilerError> {
+  const cx = new Context(...);
+
+  // Optional: Fast Refresh source hash tracking
+  if (enableResetCacheOnSourceFileChanges) {
+    fastRefreshState = { cacheIndex: cx.nextCacheIndex, hash: sha256(source) };
+  }
+
+  const compiled = codegenReactiveFunction(cx, fn);
+
+  // Prepend useMemoCache call if any cache slots used
+  if (cacheCount !== 0) {
+    body.unshift(
+      t.variableDeclaration('const', [
+        t.variableDeclarator(
+          t.identifier('$'),
+          t.callExpression(t.identifier('useMemoCache'), [t.numericLiteral(cacheCount)])
+        )
+      ])
+    );
+  }
+
+  return compiled;
+}
+```
+
+### Context Class
+Tracks state during codegen:
+```typescript
+class Context {
+  #nextCacheIndex: number = 0;  // Allocates cache slots
+  #declarations: Set<DeclarationId> = new Set();  // Tracks declared variables
+  temp: Temporaries;  // Maps identifiers to their expressions
+  errors: CompilerError;
+
+  get nextCacheIndex(): number {
+    return this.#nextCacheIndex++;  // Returns and increments
+  }
+}
+```
+
+### codegenReactiveScope
+The core of memoization code generation:
+
+```typescript
+function codegenReactiveScope(cx: Context, statements: Array<t.Statement>,
+                              scope: ReactiveScope, block: ReactiveBlock): void {
+  const changeExpressions: Array<t.Expression> = [];
+  const cacheStoreStatements: Array<t.Statement> = [];
+  const cacheLoadStatements: Array<t.Statement> = [];
+
+  // 1. Generate dependency checks
+  for (const dep of scope.dependencies) {
+    const index = cx.nextCacheIndex;
+    changeExpressions.push(
+      t.binaryExpression('!==',
+        t.memberExpression(t.identifier('$'), t.numericLiteral(index), true),
+        codegenDependency(cx, dep)
+      )
+    );
+    cacheStoreStatements.push(
+      t.assignmentExpression('=', $[index], dep)
+    );
+  }
+
+  // 2. Generate output cache slots
+  for (const {identifier} of scope.declarations) {
+    const index = cx.nextCacheIndex;
+    // Declare variable if not already declared
+    if (!cx.hasDeclared(identifier)) {
+      statements.push(t.variableDeclaration('let', [t.variableDeclarator(name, null)]));
+    }
+    cacheLoads.push({name, index, value: name});
+  }
+
+  // 3. Build test condition
+  let testCondition = changeExpressions.reduce((acc, expr) =>
+    t.logicalExpression('||', acc, expr)
+  );
+
+  // 4. If no dependencies, use sentinel check
+  if (testCondition === null) {
+    testCondition = t.binaryExpression('===',
+      $[firstOutputIndex],
+      t.callExpression(Symbol.for, ['react.memo_cache_sentinel'])
+    );
+  }
+
+  // 5. Generate the memoization if-else
+  statements.push(
+    t.ifStatement(
+      testCondition,
+      computationBlock,  // Compute + store in cache
+      cacheLoadBlock     // Load from cache
+    )
+  );
+}
+```
+
+### Generated Structure
+For a scope with dependencies `[a, b]` and output `result`:
+
+```javascript
+let result;
+if ($[0] !== a || $[1] !== b) {
+  // Computation block
+  result = compute(a, b);
+
+  // Store dependencies
+  $[0] = a;
+  $[1] = b;
+
+  // Store output
+  $[2] = result;
+} else {
+  // Load from cache
+  result = $[2];
+}
+```
+
+### Early Return Handling
+When a scope has an early return (from `propagateEarlyReturns`):
+
+```typescript
+// Before scope: initialize sentinel
+t0 = Symbol.for("react.early_return_sentinel");
+
+// Scope generates labeled block
+bb0: {
+  // ... computation ...
+  if (cond) {
+    t0 = returnValue;
+    break bb0;
+  }
+}
+
+// After scope: check for early return
+if (t0 !== Symbol.for("react.early_return_sentinel")) {
+  return t0;
+}
+```
+
+### Pruned Scopes
+Pruned scopes emit their instructions inline without memoization:
+```typescript
+case 'pruned-scope': {
+  const scopeBlock = codegenBlockNoReset(cx, item.instructions);
+  statements.push(...scopeBlock.body);  // Inline, no memoization
+  break;
+}
+```
+
+## Edge Cases
+
+### Zero Dependencies
+Scopes with no dependencies use a sentinel value check instead:
+```javascript
+if ($[0] === Symbol.for("react.memo_cache_sentinel")) {
+  // First render only
+}
+```
+
+### Fast Refresh / HMR
+When `enableResetCacheOnSourceFileChanges` is enabled, the generated code includes a source hash check that resets the cache when the source changes:
+```javascript
+if ($[0] !== "source_hash_abc123") {
+  for (let $i = 0; $i < cacheCount; $i++) {
+    $[$i] = Symbol.for("react.memo_cache_sentinel");
+  }
+  $[0] = "source_hash_abc123";
+}
+```
+
+
+### Labeled Breaks
+Control flow with labeled breaks (for early returns or loop exits) uses `codegenLabel` to generate consistent label names:
+```typescript
+function codegenLabel(id: BlockId): string {
+  return `bb${id}`;  // e.g., "bb0", "bb1"
+}
+```
+
+### Nested Functions
+Function expressions and object methods are recursively processed with their own contexts.
+
+### FBT/Internationalization
+Special handling for FBT operands ensures they're memoized in the same scope for correct internationalization behavior.
+
+## Statistics Collected
+```typescript
+type CodegenFunction = {
+  memoSlotsUsed: number;     // Total cache slots allocated
+  memoBlocks: number;        // Number of reactive scopes
+  memoValues: number;        // Total memoized values
+  prunedMemoBlocks: number;  // Scopes that were pruned
+  prunedMemoValues: number;  // Values in pruned scopes
+  hasInferredEffect: boolean;
+};
+```
+
+## TODOs
+None in the source file.
+
+## Example
+
+### Fixture: `simple.js`
+
+**Input:**
+```javascript
+export default function foo(x, y) {
+  if (x) {
+    return foo(false, y);
+  }
+  return [y * 10];
+}
+```
+
+**Generated Code:**
+```javascript
+import { c as _c } from "react/compiler-runtime";
+export default function foo(x, y) {
+  const $ = _c(4);  // Allocate 4 cache slots
+  if (x) {
+    let t0;
+    if ($[0] !== y) {           // Check dependency
+      t0 = foo(false, y);       // Compute
+      $[0] = y;                 // Store dependency
+      $[1] = t0;                // Store output
+    } else {
+      t0 = $[1];                // Load from cache
+    }
+    return t0;
+  }
+  const t0 = y * 10;
+  let t1;
+  if ($[2] !== t0) {            // Check dependency
+    t1 = [t0];                  // Compute
+    $[2] = t0;                  // Store dependency
+    $[3] = t1;                  // Store output
+  } else {
+    t1 = $[3];                  // Load from cache
+  }
+  return t1;
+}
+```
+
+Key observations:
+- `_c(4)` allocates 4 cache slots total
+- First scope uses slots 0-1: slot 0 for `y` dependency, slot 1 for `t0` output
+- Second scope uses slots 2-3: slot 2 for `t0` (the computed `y * 10`), slot 3 for `t1` (the array)
+- Each scope has an if-else structure: compute/store vs load
+- The memoization ensures referential equality of the returned array when `y` hasn't changed

compiler/packages/babel-plugin-react-compiler/docs/passes/34-optimizePropsMethodCalls.md

@@ -0,0 +1,132 @@
+# optimizePropsMethodCalls
+
+## File
+`src/Optimization/OptimizePropsMethodCalls.ts`
+
+## Purpose
+This pass converts method calls on the props object to regular function calls. Method calls like `props.onClick()` are transformed to `const t0 = props.onClick; t0()`. This normalization enables better analysis and optimization by the compiler.
+
+The transformation is important because method calls have different semantics than regular calls - the receiver (`props`) would normally be passed as `this` to the method. For React props, methods are typically just callback functions where `this` binding doesn't matter, so converting them to regular calls is safe and enables better memoization.
+
+## Input Invariants
+- The function has been through type inference
+- Props parameters are typed as `TObject<BuiltInProps>`
+
+## Output Guarantees
+- All `MethodCall` instructions where the receiver has props type are converted to `CallExpression`
+- The method property becomes the callee of the call
+- Arguments are preserved exactly
+
+## Algorithm
+
+```typescript
+export function optimizePropsMethodCalls(fn: HIRFunction): void {
+  for (const [, block] of fn.body.blocks) {
+    for (let i = 0; i < block.instructions.length; i++) {
+      const instr = block.instructions[i]!;
+
+      if (
+        instr.value.kind === 'MethodCall' &&
+        isPropsType(instr.value.receiver.identifier)
+      ) {
+        // Transform: props.onClick(arg)
+        // To: const t0 = props.onClick; t0(arg)
+        instr.value = {
+          kind: 'CallExpression',
+          callee: instr.value.property,  // The method becomes the callee
+          args: instr.value.args,
+          loc: instr.value.loc,
+        };
+      }
+    }
+  }
+}
+
+function isPropsType(identifier: Identifier): boolean {
+  return (
+    identifier.type.kind === 'Object' &&
+    identifier.type.shapeId === BuiltInPropsId
+  );
+}
+```
+
+## Edge Cases
+
+### Non-Props Method Calls
+Method calls on non-props objects are left unchanged:
+```javascript
+// Unchanged - array.map is not on props
+array.map(x => x * 2)
+
+// Unchanged - obj is not props
+obj.method()
+```
+
+### Props Type Detection
+The pass uses type information to identify props:
+```javascript
+function Component(props) {
+  // props has type TObject<BuiltInProps>
+  props.onClick();  // Transformed
+}
+
+function Regular(obj) {
+  // obj has unknown type
+  obj.onClick();  // Not transformed
+}
+```
+
+### Nested Props Access
+Only direct method calls on props are transformed:
+```javascript
+props.onClick();       // Transformed
+props.nested.onClick(); // Not transformed (receiver is props.nested, not props)
+```
+
+### Arrow Function Callbacks
+Works with any method on props:
+```javascript
+props.onChange(value);   // Transformed
+props.onSubmit(data);    // Transformed
+props.validate(input);   // Transformed
+```
+
+## TODOs
+None in the source file.
+
+## Example
+
+### Fixture: Using props method
+
+**Input:**
+```javascript
+function Component(props) {
+  return <button onClick={() => props.onClick()} />;
+}
+```
+
+**Before OptimizePropsMethodCalls:**
+```
+[1] $5 = Function @context[props$1] ...
+    <<anonymous>>():
+      [1] $2 = LoadLocal props$1
+      [2] $3 = PropertyLoad $2.onClick
+      [3] $4 = MethodCall $2.$3()  // Method call on props
+      [4] Return Void
+```
+
+**After OptimizePropsMethodCalls:**
+```
+[1] $5 = Function @context[props$1] ...
+    <<anonymous>>():
+      [1] $2 = LoadLocal props$1
+      [2] $3 = PropertyLoad $2.onClick
+      [3] $4 = Call $3()  // Now a regular call
+      [4] Return Void
+```
+
+Key observations:
+- `MethodCall $2.$3()` becomes `Call $3()`
+- The property load (`$3 = PropertyLoad $2.onClick`) is preserved
+- The receiver (`$2`) is no longer part of the call
+- This enables the compiler to analyze `onClick` as a regular function

compiler/packages/babel-plugin-react-compiler/docs/passes/35-optimizeForSSR.md

@@ -0,0 +1,187 @@
+# optimizeForSSR
+
+## File
+`src/Optimization/OptimizeForSSR.ts`
+
+## Purpose
+This pass applies Server-Side Rendering (SSR) specific optimizations. During SSR, React renders components to HTML strings without mounting them in the DOM. This means:
+
+1. **Effects don't run** - `useEffect` and `useLayoutEffect` are no-ops
+2. **Event handlers aren't needed** - There's no DOM to attach handlers to
+3. **State is never updated** - Components render once with initial state
+4. **Refs aren't attached** - There's no DOM to ref
+
+The pass leverages these SSR characteristics to inline and simplify code, removing unnecessary runtime overhead.
+
+## Input Invariants
+- The function has been through type inference
+- Hook types are properly identified (useState, useReducer, useEffect, etc.)
+- Function types for callbacks are properly inferred
+
+## Output Guarantees
+- `useState(initialValue)` is inlined to just `[initialValue, noop]`
+- `useReducer(reducer, initialArg, init?)` is inlined to `[init ? init(initialArg) : initialArg, noop]`
+- `useEffect` and `useLayoutEffect` calls are removed entirely
+- Event handler functions (functions that call setState) are replaced with empty functions
+- Ref-typed values are removed from JSX props
+
+## Algorithm
+
+### Phase 1: Identify Inlinable State
+```typescript
+const inlinedState = new Map<IdentifierId, InstructionValue>();
+
+for (const instr of block.instructions) {
+  if (isUseStateCall(instr)) {
+    // Store the initial value for inlining
+    inlinedState.set(instr.lvalue.id, {
+      kind: 'ArrayExpression',
+      elements: [initialValue, noopFunction],
+    });
+  }
+
+  if (isUseReducerCall(instr)) {
+    // Compute initial state and store for inlining
+    const initialState = init ? callInit(initialArg) : initialArg;
+    inlinedState.set(instr.lvalue.id, {
+      kind: 'ArrayExpression',
+      elements: [initialState, noopFunction],
+    });
+  }
+}
+```
+
+### Phase 2: Inline State Hooks
+Replace useState/useReducer with their computed initial values:
+```typescript
+// Before:
+$0 = useState(0)
+[state, setState] = $0
+
+// After (inlined):
+$0 = [0, () => {}]
+[state, setState] = $0
+```
+
+### Phase 3: Remove Effects
+```typescript
+if (isUseEffectCall(instr) || isUseLayoutEffectCall(instr)) {
+  // Remove the instruction entirely
+  block.instructions.splice(i, 1);
+}
+```
+
+### Phase 4: Identify and Neuter Event Handlers
+```typescript
+// Functions that capture and call setState are event handlers
+if (capturesSetState(functionExpr)) {
+  // Replace with empty function
+  instr.value = {
+    kind: 'FunctionExpression',
+    params: originalParams,
+    body: emptyBody,
+  };
+}
+```
+
+### Phase 5: Remove Ref Props
+```typescript
+if (isJSX(instr) && hasRefProp(instr)) {
+  // Remove ref={...} from JSX props
+  removeRefProp(instr.value);
+}
+```
+
+## Edge Cases
+
+### useState with Function Initializer
+When `useState` receives a function initializer, it must be called:
+```javascript
+// useState(() => expensive())
+// SSR: Call the initializer to get the value
+const [state] = [expensiveComputation(), noop];
+```
+
+### useReducer with Init Function
+The optional `init` function is called with `initialArg`:
+```javascript
+// useReducer(reducer, arg, init)
+// SSR: [init(arg), noop]
+```
+
+### Nested State Setters
+Functions that transitively call setState are also event handlers:
+```javascript
+function outer() {
+  function inner() {
+    setState(x);  // inner is event handler
+  }
+  inner();  // outer is also event handler
+}
+```
+
+### Conditional Event Handlers
+Event handler detection is conservative - if a function might call setState, it's treated as an event handler.
+
+### Refs in Nested Objects
+Only direct `ref` props on JSX are removed:
+```javascript
+<div ref={myRef} />           // ref removed
+<div config={{ref: myRef}} /> // ref NOT removed (nested)
+```
+
+## TODOs
+None in the source file.
+
+## Example
+
+### Fixture: `ssr/optimize-ssr.js`
+
+**Input:**
+```javascript
+function Component() {
+  const [state, setState] = useState(0);
+  const ref = useRef(null);
+  const onChange = (e) => {
+    setState(e.target.value);
+  };
+  useEffect(() => {
+    log(ref.current.value);
+  });
+  return <input value={state} onChange={onChange} ref={ref} />;
+}
+```
+
+**After SSR Optimization:**
+```javascript
+function Component() {
+  const $ = _c(1);
+  // useState inlined to [initialValue, noop]
+  const [state] = [0, () => {}];
+
+  // useRef returns object with current: null
+  const ref = { current: null };
+
+  // Event handler replaced with noop (it calls setState)
+  const onChange = () => {};
+
+  // useEffect removed entirely (no-op on SSR)
+
+  // ref prop removed from JSX
+  let t0;
+  if ($[0] === Symbol.for("react.memo_cache_sentinel")) {
+    t0 = <input value={state} onChange={onChange} />;
+    $[0] = t0;
+  } else {
+    t0 = $[0];
+  }
+  return t0;
+}
+```
+
+Key observations:
+- `useState(0)` becomes `[0, () => {}]` - no hook call
+- `useEffect(...)` is removed entirely
+- `onChange` is replaced with empty function since it called `setState`
+- `ref={ref}` prop is removed from JSX
+- SSR output is simpler and has less runtime overhead

compiler/packages/babel-plugin-react-compiler/docs/passes/36-outlineJSX.md

@@ -0,0 +1,224 @@
+# outlineJSX
+
+## File
+`src/Optimization/OutlineJsx.ts`
+
+## Purpose
+This pass outlines nested JSX elements into separate component functions. When a callback function contains JSX, this pass can extract that JSX into a new component, which enables:
+
+1. **Better code splitting** - Outlined components can be lazily loaded
+2. **Memoization at component boundaries** - React's reconciliation can skip unchanged subtrees
+3. **Reduced closure captures** - Outlined components receive props explicitly
+
+The pass specifically targets JSX within callbacks (like `.map()` callbacks) rather than top-level component returns.
+
+## Input Invariants
+- The `enableJsxOutlining` feature flag must be enabled
+- The function must be a React component or hook
+- JSX must appear within a nested function expression (callback)
+
+## Output Guarantees
+- Nested functions containing only JSX returns are extracted as separate components
+- The original callback is replaced with a call to the outlined component
+- Captured variables become explicit props to the outlined component
+- The outlined component is registered with the environment for emission
+
+## Algorithm
+
+### Phase 1: Identify Outlinable JSX
+```typescript
+function outlineJsxImpl(fn: HIRFunction, outlinedFns: Array<HIRFunction>): void {
+  for (const [, block] of fn.body.blocks) {
+    for (const instr of block.instructions) {
+      if (instr.value.kind === 'FunctionExpression') {
+        const innerFn = instr.value.loweredFunc.func;
+
+        // Check if function only returns JSX
+        if (canOutline(innerFn)) {
+          const outlined = createOutlinedComponent(innerFn);
+          outlinedFns.push(outlined);
+          replaceWithComponentCall(instr, outlined);
+        }
+      }
+    }
+  }
+}
+```
+
+### Phase 2: Check Outlinability
+```typescript
+function canOutline(fn: HIRFunction): boolean {
+  // Must have exactly one block with only JSX-related instructions
+  // Must end with returning JSX
+  // Must not have complex control flow
+
+  return (
+    fn.body.blocks.size === 1 &&
+    returnsJSX(fn) &&
+    !hasComplexControlFlow(fn)
+  );
+}
+```
+
+### Phase 3: Create Outlined Component
+```typescript
+function createOutlinedComponent(fn: HIRFunction): HIRFunction {
+  // Convert captured context to props
+  const props = fn.context.map(capture => ({
+    name: capture.identifier.name,
+    type: capture.identifier.type,
+  }));
+
+  // Create new component function
+  return {
+    ...fn,
+    params: [{kind: 'Identifier', name: 'props', ...}],
+    context: [],  // No captures - all via props
+  };
+}
+```
+
+### Phase 4: Replace Original Callback
+```typescript
+function replaceWithComponentCall(instr: Instruction, outlined: HIRFunction): void {
+  // Original: items.map(item => <Stringify item={item} />)
+  // Becomes: items.map(item => <OutlinedComponent item={item} />)
+
+  instr.value = {
+    kind: 'JSX',
+    tag: {kind: 'LoadGlobal', name: outlined.id},
+    props: capturedVariablesToProps(instr.value.context),
+  };
+}
+```
+
+### Phase 5: Register Outlined Functions
+```typescript
+export function outlineJSX(fn: HIRFunction): void {
+  const outlinedFns: Array<HIRFunction> = [];
+  outlineJsxImpl(fn, outlinedFns);
+
+  for (const outlinedFn of outlinedFns) {
+    fn.env.outlineFunction(outlinedFn, 'Component');
+  }
+}
+```
+
+## Edge Cases
+
+### Context Captures
+Variables captured by the callback become props:
+```javascript
+// Before:
+items.map(item => <Card key={item.id} user={currentUser} item={item} />)
+
+// After (outlined):
+function OutlinedCard(props) {
+  return <Card key={props.item.id} user={props.currentUser} item={props.item} />;
+}
+items.map(item => <OutlinedCard currentUser={currentUser} item={item} />)
+```
+
+### Complex Control Flow
+Callbacks with conditionals or loops are not outlined:
+```javascript
+// Not outlined - has conditional
+items.map(item => item.show ? <Card item={item} /> : null)
+```
+
+### Multiple JSX Returns
+Only single-JSX-return callbacks are outlined:
+```javascript
+// Not outlined - multiple potential returns
+items.map(item => {
+  if (item.type === 'a') return <TypeA item={item} />;
+  return <TypeB item={item} />;
+})
+```
+
+### Top-Level JSX
+Only JSX in nested callbacks is outlined, not component return values:
+```javascript
+function Component() {
+  return <div />;  // Not outlined - this is the component's return
+}
+```
+
+### Recursive Outlining
+The pass recursively processes outlined components to outline their nested JSX.
+
+## TODOs
+None in the source file.
+
+## Example
+
+### Fixture: `outlined-helper.js`
+
+**Input:**
+```javascript
+// @enableFunctionOutlining
+function Component(props) {
+  return (
+    <div>
+      {props.items.map(item => (
+        <Stringify key={item.id} item={item.name} />
+      ))}
+    </div>
+  );
+}
+```
+
+**After OutlineJSX:**
+```
+// Outlined component:
+function _outlined_Component$1(props) {
+  return <Stringify key={props.item.id} item={props.item.name} />;
+}
+
+// Original component modified:
+function Component(props) {
+  return (
+    <div>
+      {props.items.map(item => (
+        <_outlined_Component$1 item={item} />
+      ))}
+    </div>
+  );
+}
+```
+
+**Generated Code:**
+```javascript
+function _outlined_Component$1(props) {
+  const $ = _c(2);
+  const item = props.item;
+  let t0;
+  if ($[0] !== item.id || $[1] !== item.name) {
+    t0 = <Stringify key={item.id} item={item.name} />;
+    $[0] = item.id;
+    $[1] = item.name;
+  } else {
+    t0 = $[1];
+  }
+  return t0;
+}
+
+function Component(props) {
+  const $ = _c(2);
+  let t0;
+  if ($[0] !== props.items) {
+    t0 = props.items.map((item) => <_outlined_Component$1 item={item} />);
+    $[0] = props.items;
+    $[1] = t0;
+  } else {
+    t0 = $[1];
+  }
+  return <div>{t0}</div>;
+}
+```
+
+Key observations:
+- The map callback JSX is extracted into `_outlined_Component$1`
+- The `item` variable becomes a prop instead of a closure capture
+- The outlined component gets its own memoization cache
+- This enables React to skip re-rendering unchanged list items

compiler/packages/babel-plugin-react-compiler/docs/passes/37-outlineFunctions.md

@@ -0,0 +1,169 @@
+# outlineFunctions
+
+## File
+`src/Optimization/OutlineFunctions.ts`
+
+## Purpose
+This pass outlines pure function expressions that have no captured context into top-level helper functions. By moving these functions outside the component, they become truly static and can be shared across renders without any memoization overhead.
+
+A function with no captured context is completely self-contained - it only uses its parameters and globals. Such functions don't need to be recreated on each render and can be hoisted to module scope.
+
+## Input Invariants
+- The `enableFunctionOutlining` feature flag must be enabled
+- Functions must have `context.length === 0` (no captured variables)
+- Functions must be anonymous (no `id` property)
+- Functions must not be FBT macro operands (tracked by `fbtOperands` parameter)
+
+## Output Guarantees
+- Pure function expressions are replaced with `LoadGlobal` of the outlined function
+- Outlined functions are registered with the environment for emission
+- The original instruction is transformed to load the global
+
+## Algorithm
+
+```typescript
+export function outlineFunctions(
+  fn: HIRFunction,
+  fbtOperands: Set<IdentifierId>,
+): void {
+  for (const [, block] of fn.body.blocks) {
+    for (let i = 0; i < block.instructions.length; i++) {
+      const instr = block.instructions[i]!;
+
+      if (
+        instr.value.kind === 'FunctionExpression' &&
+        instr.value.loweredFunc.func.context.length === 0 &&
+        instr.value.loweredFunc.func.id === null &&
+        !fbtOperands.has(instr.lvalue.identifier.id)
+      ) {
+        // Outline this function
+        const outlinedId = fn.env.outlineFunction(
+          instr.value.loweredFunc.func,
+          'helper',
+        );
+
+        // Replace with LoadGlobal
+        instr.value = {
+          kind: 'LoadGlobal',
+          binding: {
+            kind: 'ModuleLocal',
+            name: outlinedId,
+          },
+          loc: instr.value.loc,
+        };
+      }
+    }
+  }
+}
+```
+
+## Edge Cases
+
+### Functions with Context
+Functions that capture variables are not outlined:
+```javascript
+function Component(props) {
+  const x = props.value;
+  const fn = () => x * 2;  // Captures x, not outlined
+}
+```
+
+### Named Functions
+Functions with explicit names are not outlined:
+```javascript
+const foo = function namedFn() { ... };  // Has id, not outlined
+```
+
+### FBT Operands
+Functions used as FBT operands cannot be outlined due to translation requirements:
+```javascript
+<fbt>
+  Hello <fbt:param name="user">{() => getName()}</fbt:param>
+</fbt>
+// The function cannot be outlined - FBT needs it inline
+```
+
+### Arrow Functions vs Function Expressions
+Both arrow functions and function expressions are candidates:
+```javascript
+const a = () => 1;           // Outlined if no context
+const b = function() {};     // Outlined if no context
+```
+
+### Recursive Functions
+Self-referencing functions cannot be outlined (they would have themselves in context):
+```javascript
+const fib = (n) => n <= 1 ? n : fib(n-1) + fib(n-2);  // References self
+```
+
+## TODOs
+None in the source file.
+
+## Example
+
+### Fixture: `outlined-helper.js`
+
+**Input:**
+```javascript
+// @enableFunctionOutlining
+function Component(props) {
+  return (
+    <div>
+      {props.items.map(item => (
+        <Stringify key={item.id} item={item.name} />
+      ))}
+    </div>
+  );
+}
+```
+
+**Analysis:**
+The map callback `item => <Stringify .../>` has one captured variable: nothing from the component (only uses `item` parameter). However, it receives `item` as a parameter, not from context.
+
+If we have a truly pure helper:
+```javascript
+// @enableFunctionOutlining
+function Component(props) {
+  const double = (x) => x * 2;  // No context, pure
+  return <div>{double(props.value)}</div>;
+}
+```
+
+**After OutlineFunctions:**
+```
+// Outlined to module scope:
+function _outlined_double$1(x) {
+  return x * 2;
+}
+
+// In component:
+[1] $1 = LoadGlobal _outlined_double$1  // Instead of FunctionExpression
+[2] StoreLocal Const double = $1
+```
+
+**Generated Code:**
+```javascript
+function _outlined_double$1(x) {
+  return x * 2;
+}
+
+function Component(props) {
+  const $ = _c(2);
+  const double = _outlined_double$1;  // Just a reference, no recreation
+  let t0;
+  if ($[0] !== props.value) {
+    t0 = <div>{double(props.value)}</div>;
+    $[0] = props.value;
+    $[1] = t0;
+  } else {
+    t0 = $[1];
+  }
+  return t0;
+}
+```
+
+Key observations:
+- The pure function is hoisted to module scope
+- The component just references the outlined function
+- No memoization needed for the function itself
+- Reduces runtime overhead by avoiding function recreation

compiler/packages/babel-plugin-react-compiler/docs/passes/38-memoizeFbtAndMacroOperandsInSameScope.md

@@ -0,0 +1,231 @@
+# memoizeFbtAndMacroOperandsInSameScope
+
+## File
+`src/ReactiveScopes/MemoizeFbtAndMacroOperandsInSameScope.ts`
+
+## Purpose
+This pass ensures that FBT (Facebook Translation) expressions and their operands are memoized within the same reactive scope. FBT is Facebook's internationalization system that requires special handling to ensure translations work correctly.
+
+The key insight is that FBT operands must be computed and frozen together with the FBT call itself. If operands were memoized in separate scopes, the translation system could receive stale operand values when only some inputs change.
+
+## Input Invariants
+- The function has been through type inference
+- FBT calls (`fbt`, `fbt.c`, `fbt:param`, etc.) are properly identified
+- Custom macros are configured in `fn.env.config.customMacros`
+- Reactive scope variables have been inferred
+
+## Output Guarantees
+- All operands of FBT calls are assigned to the same reactive scope as the FBT call
+- The `fbtOperands` set is returned for use by other passes (e.g., `outlineFunctions`)
+- Operand scope assignments use either transitive or shallow inlining based on macro definition
+
+## Algorithm
+
+### Phase 1: Collect Macro Kinds
+```typescript
+const macroKinds = new Map<Macro, MacroDefinition>([
+  ...Array.from(FBT_TAGS.entries()),  // Built-in fbt tags
+  ...(fn.env.config.customMacros ?? []).map(([name, def]) => [name, def]),
+]);
+```
+
+### Phase 2: Populate Macro Tags
+```typescript
+function populateMacroTags(
+  fn: HIRFunction,
+  macroKinds: Map<Macro, MacroDefinition>,
+): Map<IdentifierId, MacroDefinition> {
+  const macroTags = new Map();
+
+  for (const instr of allInstructions(fn)) {
+    if (isLoadGlobal(instr) || isPropertyLoad(instr)) {
+      const name = getName(instr);
+      if (macroKinds.has(name)) {
+        macroTags.set(instr.lvalue.id, macroKinds.get(name));
+      }
+    }
+  }
+
+  return macroTags;
+}
+```
+
+### Phase 3: Merge Macro Arguments
+```typescript
+function mergeMacroArguments(
+  fn: HIRFunction,
+  macroTags: Map<IdentifierId, MacroDefinition>,
+  macroKinds: Map<Macro, MacroDefinition>,
+): Set<IdentifierId> {
+  const macroValues = new Set<IdentifierId>();
+
+  for (const instr of allInstructions(fn)) {
+    if (isCall(instr) || isMethodCall(instr) || isJSX(instr)) {
+      const callee = getCallee(instr);
+      const macroDef = macroTags.get(callee.id);
+
+      if (macroDef !== undefined) {
+        // Mark all operands to be in same scope
+        for (const operand of getOperands(instr)) {
+          macroValues.add(operand.id);
+
+          // Merge scope to match macro call scope
+          if (macroDef.inlineLevel === InlineLevel.Transitive) {
+            mergeScopesTransitively(operand, instr.lvalue);
+          } else {
+            mergeScopes(operand, instr.lvalue);
+          }
+        }
+      }
+    }
+  }
+
+  return macroValues;
+}
+```
+
+### InlineLevel Types
+```typescript
+enum InlineLevel {
+  Shallow,     // Only merge direct operands
+  Transitive,  // Merge operands and their dependencies
+}
+```
+
+## Edge Cases
+
+### Nested FBT Params
+FBT params can be nested, and all levels must be in the same scope:
+```javascript
+<fbt>
+  Hello <fbt:param name="user">
+    <fbt:param name="firstName">{user.firstName}</fbt:param>
+  </fbt:param>
+</fbt>
+```
+
+### FBT with Complex Expressions
+Complex expressions as operands have their entire dependency chain merged:
+```javascript
+<fbt>
+  Count: <fbt:param name="count">{items.length * multiplier}</fbt:param>
+</fbt>
+// Both items.length and multiplier expressions are merged into fbt scope
+```
+
+### Custom Macros
+User-defined macros can specify their inlining behavior:
+```typescript
+customMacros: [
+  ['myMacro', { inlineLevel: InlineLevel.Transitive }],
+]
+```
+
+### Method Calls on FBT
+`fbt.param()`, `fbt.plural()`, etc. are handled as method calls:
+```javascript
+fbt(
+  fbt.param('count', items.length),  // MethodCall on fbt
+  'description'
+)
+```
+
+### JSX vs Call Syntax
+Both JSX and call syntax for FBT are handled:
+```javascript
+// JSX syntax
+<fbt desc="greeting">Hello</fbt>
+
+// Call syntax
+fbt('Hello', 'greeting')
+```
+
+## Built-in FBT Tags
+The pass recognizes these FBT constructs:
+- `fbt` / `fbt.c` - Main translation functions
+- `fbt:param` - Parameter substitution
+- `fbt:plural` - Plural handling
+- `fbt:enum` - Enumeration values
+- `fbt:name` - Name parameters
+- `fbt:pronoun` - Pronoun handling
+- `fbs` - Simple string translation
+
+## TODOs
+None in the source file.
+
+## Example
+
+### Fixture: `fbt/fbt-call.js`
+
+**Input:**
+```javascript
+function Component(props) {
+  const text = fbt(
+    `${fbt.param('count', props.count)} items`,
+    'Number of items'
+  );
+  return <div>{text}</div>;
+}
+```
+
+**Before MemoizeFbtAndMacroOperandsInSameScope:**
+```
+[1] $18 = LoadGlobal import fbt from 'fbt'
+[2] $19 = LoadGlobal import fbt from 'fbt'
+[3] $20_@0[3:8] = PropertyLoad $19.param
+[4] $21 = "(key) count"
+[5] $22 = LoadLocal props$17
+[6] $23 = PropertyLoad $22.count
+[7] $24_@0[3:8] = MethodCall $19.$20_@0($21, $23)  // fbt.param call
+[8] $25 = `${$24_@0} items`
+[9] $26 = "(description) Number of items"
+[10] $27_@1 = Call $18($25, $26)  // fbt call
+```
+
+**After MemoizeFbtAndMacroOperandsInSameScope:**
+```
+[1] $18_@1[1:11] = LoadGlobal import fbt from 'fbt'  // Merged to @1
+[2] $19 = LoadGlobal import fbt from 'fbt'
+[3] $20_@0[3:8] = PropertyLoad $19.param
+[4] $21 = "(key) count"
+[5] $22 = LoadLocal props$17
+[6] $23 = PropertyLoad $22.count
+[7] $24_@1[1:11] = MethodCall $19.$20_@0($21, $23)  // Merged to @1
+[8] $25_@1[1:11] = `${$24_@1} items`                // Merged to @1
+[9] $26_@1[1:11] = "(description) Number of items" // Merged to @1
+[10] $27_@1[1:11] = Call $18_@1($25_@1, $26_@1)     // Main fbt scope @1
+```
+
+**Generated Code:**
+```javascript
+function Component(props) {
+  const $ = _c(3);
+  let t0;
+  if ($[0] !== props.count) {
+    // All fbt operands computed in same memoization block
+    t0 = fbt(
+      `${fbt.param("count", props.count)} items`,
+      "Number of items"
+    );
+    $[0] = props.count;
+    $[1] = t0;
+  } else {
+    t0 = $[1];
+  }
+  const text = t0;
+  let t1;
+  if ($[2] === Symbol.for("react.memo_cache_sentinel")) {
+    t1 = <div>{text}</div>;
+    $[2] = t1;
+  } else {
+    t1 = $[2];
+  }
+  return t1;
+}
+```
+
+Key observations:
+- All FBT-related operations are in the same memoization scope `@1`
+- `fbt.param`, template literal, and `fbt` call are memoized together
+- This ensures the translation system receives consistent operand values
+- The entire translation is recomputed when any operand (`props.count`) changes