[compiler] Cleanup for @enablePreserveExistingMemoizationGuarantees

I tried turning on `@enablePreserveExistingMemoizationGuarantees` by default and cleaned up a couple small things:

* We emit freeze calls for StartMemoize deps but these had ValueReason.Other so the message wasn't great. We now treat these like other hook arguments.
* PruneNonEscapingScopes was being too aggressive in this mode and memoizing even loads of globals. Switching to MemoizationLevel.Conditional ensures we build a graph that connects through to primitive-returning function calls, but doesn't unnecessarily force memoization otherwise.

.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

@@ -547,13 +547,10 @@ module.exports = {
   },
 
   globals: {
-    $Call: 'readonly',
-    $ElementType: 'readonly',
     $Flow$ModuleRef: 'readonly',
     $FlowFixMe: 'readonly',
     $Keys: 'readonly',
     $NonMaybeType: 'readonly',
-    $PropertyType: 'readonly',
     $ReadOnly: 'readonly',
     $ReadOnlyArray: 'readonly',
     $ArrayBufferView: 'readonly',
@@ -568,6 +565,7 @@ module.exports = {
     BigInt: 'readonly',
     BigInt64Array: 'readonly',
     BigUint64Array: 'readonly',
+    CacheType: 'readonly',
     Class: 'readonly',
     ClientRect: 'readonly',
     CopyInspectedElementPath: 'readonly',
@@ -582,13 +580,15 @@ module.exports = {
     IteratorResult: 'readonly',
     JSONValue: 'readonly',
     JSResourceReference: 'readonly',
+    mixin$Animatable: 'readonly',
     MouseEventHandler: 'readonly',
     NavigateEvent: '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',
@@ -620,7 +620,6 @@ module.exports = {
     PropertyIndexedKeyframes: 'readonly',
     KeyframeAnimationOptions: 'readonly',
     GetAnimationsOptions: 'readonly',
-    Animatable: 'readonly',
     ScrollTimeline: 'readonly',
     EventListenerOptionsOrUseCapture: 'readonly',
     FocusOptions: 'readonly',

.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/runtime_build_and_test.yml

@@ -316,7 +316,7 @@ 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
@@ -811,9 +811,18 @@ 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

.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

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

@@ -0,0 +1,101 @@
+/**
+ * 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 {useState} from 'react';
+import {Resizable} from 're-resizable';
+import {useStore, useStoreDispatch} from '../StoreContext';
+import {monacoOptions} from './monacoOptions';
+import {
+  generateOverridePragmaFromConfig,
+  updateSourceWithOverridePragma,
+} from '../../lib/configUtils';
+
+loader.config({monaco});
+
+export default function ConfigEditor(): JSX.Element {
+  const [, setMonaco] = useState<Monaco | null>(null);
+  const store = useStore();
+  const dispatchStore = useStoreDispatch();
+
+  const handleChange: (value: string | undefined) => void = async value => {
+    if (value === undefined) return;
+
+    try {
+      const newPragma = await generateOverridePragmaFromConfig(value);
+      const updatedSource = updateSourceWithOverridePragma(
+        store.source,
+        newPragma,
+      );
+
+      // Update the store with both the new config and updated source
+      dispatchStore({
+        type: 'updateFile',
+        payload: {
+          source: updatedSource,
+          config: value,
+        },
+      });
+    } catch (_) {
+      dispatchStore({
+        type: 'updateFile',
+        payload: {
+          source: store.source,
+          config: value,
+        },
+      });
+    }
+  };
+
+  const handleMount: (
+    _: editor.IStandaloneCodeEditor,
+    monaco: Monaco,
+  ) => void = (_, monaco) => {
+    setMonaco(monaco);
+
+    const uri = monaco.Uri.parse(`file:///config.js`);
+    const model = monaco.editor.getModel(uri);
+    if (model) {
+      model.updateOptions({tabSize: 2});
+    }
+  };
+
+  return (
+    <div className="relative flex flex-col flex-none border-r border-gray-200">
+      <h2 className="p-4 duration-150 ease-in border-b cursor-default border-grey-200 font-light text-secondary">
+        Config Overrides
+      </h2>
+      <Resizable
+        minWidth={300}
+        maxWidth={600}
+        defaultSize={{width: 350, height: 'auto'}}
+        enable={{right: true}}
+        className="!h-[calc(100vh_-_3.5rem_-_4rem)]">
+        <MonacoEditor
+          path={'config.js'}
+          language={'javascript'}
+          value={store.config}
+          onMount={handleMount}
+          onChange={handleChange}
+          options={{
+            ...monacoOptions,
+            lineNumbers: 'off',
+            folding: false,
+            renderLineHighlight: 'none',
+            scrollBeyondLastLine: false,
+            hideCursorInOverviewRuler: true,
+            overviewRulerBorder: false,
+            overviewRulerLanes: 0,
+            fontSize: 12,
+          }}
+        />
+      </Resizable>
+    </div>
+  );
+}

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

@@ -37,6 +37,7 @@ import {
   type Store,
 } from '../../lib/stores';
 import {useStore, useStoreDispatch} from '../StoreContext';
+import ConfigEditor from './ConfigEditor';
 import Input from './Input';
 import {
   CompilerOutput,
@@ -46,6 +47,8 @@ import {
 } from './Output';
 import {transformFromAstSync} from '@babel/core';
 import {LoggerEvent} from 'babel-plugin-react-compiler/dist/Entrypoint';
+import {useSearchParams} from 'next/navigation';
+import {parseAndFormatConfig} from '../../lib/configUtils';
 
 function parseInput(
   input: string,
@@ -291,7 +294,13 @@ export default function Editor(): JSX.Element {
     [deferredStore.source],
   );
 
+  // TODO: Remove this once the config editor is more stable
+  const searchParams = useSearchParams();
+  const search = searchParams.get('showConfig');
+  const shouldShowConfig = search === 'true';
+
   useMountEffect(() => {
+    // Initialize store
     let mountStore: Store;
     try {
       mountStore = initStoreFromUrlOrLocalStorage();
@@ -307,9 +316,17 @@ export default function Editor(): JSX.Element {
       });
       mountStore = defaultStore;
     }
-    dispatchStore({
-      type: 'setStore',
-      payload: {store: mountStore},
+
+    parseAndFormatConfig(mountStore.source).then(config => {
+      dispatchStore({
+        type: 'setStore',
+        payload: {
+          store: {
+            ...mountStore,
+            config,
+          },
+        },
+      });
     });
   });
 
@@ -328,6 +345,7 @@ export default function Editor(): JSX.Element {
   return (
     <>
       <div className="relative flex basis top-14">
+        {shouldShowConfig && <ConfigEditor />}
         <div className={clsx('relative sm:basis-1/4')}>
           <Input language={language} errors={errors} />
         </div>

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

@@ -17,6 +17,7 @@ import {useStore, useStoreDispatch} from '../StoreContext';
 import {monacoOptions} from './monacoOptions';
 // @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';
+import {parseAndFormatConfig} from '../../lib/configUtils.ts';
 
 loader.config({monaco});
 
@@ -79,13 +80,17 @@ 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;
 
+    // Parse and format the config
+    const config = await parseAndFormatConfig(value);
+
     dispatchStore({
       type: 'updateFile',
       payload: {
         source: value,
+        config,
       },
     });
   };

compiler/apps/playground/components/StoreContext.tsx

@@ -56,6 +56,7 @@ type ReducerAction =
       type: 'updateFile';
       payload: {
         source: string;
+        config?: string;
       };
     };
 
@@ -66,10 +67,11 @@ function storeReducer(store: Store, action: ReducerAction): Store {
       return newStore;
     }
     case 'updateFile': {
-      const {source} = action.payload;
+      const {source, config} = action.payload;
       const newStore = {
         ...store,
         source,
+        config,
       };
       return newStore;
     }

compiler/apps/playground/lib/configUtils.ts

@@ -0,0 +1,87 @@
+/**
+ * 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 parserBabel from 'prettier/plugins/babel';
+import prettierPluginEstree from 'prettier/plugins/estree';
+import * as prettier from 'prettier/standalone';
+import {parseConfigPragmaAsString} from '../../../packages/babel-plugin-react-compiler/src/Utils/TestUtils';
+
+/**
+ * Parse config from pragma and format it with prettier
+ */
+export async function parseAndFormatConfig(source: string): Promise<string> {
+  const pragma = source.substring(0, source.indexOf('\n'));
+  let configString = parseConfigPragmaAsString(pragma);
+  if (configString !== '') {
+    configString = `(${configString})`;
+  }
+
+  try {
+    const formatted = await prettier.format(configString, {
+      semi: true,
+      parser: 'babel-ts',
+      plugins: [parserBabel, prettierPluginEstree],
+    });
+    return formatted;
+  } catch (error) {
+    console.error('Error formatting config:', error);
+    return ''; // Return empty string if not valid for now
+  }
+}
+
+function extractCurlyBracesContent(input: string): string {
+  const startIndex = input.indexOf('{');
+  const endIndex = input.lastIndexOf('}');
+  if (startIndex === -1 || endIndex === -1 || endIndex <= startIndex) {
+    throw new Error('No outer curly braces found in input');
+  }
+  return input.slice(startIndex, endIndex + 1);
+}
+
+function cleanContent(content: string): string {
+  return content
+    .replace(/[\r\n]+/g, ' ')
+    .replace(/\s+/g, ' ')
+    .trim();
+}
+
+/**
+ * Generate a the override pragma comment from a formatted config object string
+ */
+export async function generateOverridePragmaFromConfig(
+  formattedConfigString: string,
+): Promise<string> {
+  const content = extractCurlyBracesContent(formattedConfigString);
+  const cleanConfig = cleanContent(content);
+
+  // Format the config to ensure it's valid
+  await prettier.format(`(${cleanConfig})`, {
+    semi: false,
+    parser: 'babel-ts',
+    plugins: [parserBabel, prettierPluginEstree],
+  });
+
+  return `// @OVERRIDE:${cleanConfig}`;
+}
+
+/**
+ * Update the override pragma comment in source code.
+ */
+export function updateSourceWithOverridePragma(
+  source: string,
+  newPragma: string,
+): string {
+  const firstLineEnd = source.indexOf('\n');
+  const firstLine = source.substring(0, firstLineEnd);
+
+  const pragmaRegex = /^\/\/\s*@/;
+  if (firstLineEnd !== -1 && pragmaRegex.test(firstLine.trim())) {
+    return newPragma + source.substring(firstLineEnd);
+  } else {
+    return newPragma + '\n' + source;
+  }
+}

compiler/apps/playground/lib/defaultStore.ts

@@ -15,8 +15,10 @@ export default function MyApp() {
 
 export const defaultStore: Store = {
   source: index,
+  config: '',
 };
 
 export const emptyStore: Store = {
   source: '',
+  config: '',
 };

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

@@ -17,6 +17,7 @@ import {defaultStore} from '../defaultStore';
  */
 export interface Store {
   source: string;
+  config?: string;
 }
 export function encodeStore(store: Store): string {
   return compressToEncodedURIComponent(JSON.stringify(store));
@@ -65,5 +66,14 @@ export function initStoreFromUrlOrLocalStorage(): Store {
   const raw = decodeStore(encodedSource);
 
   invariant(isValidStore(raw), 'Invalid Store');
+
+  // Add config property if missing for backwards compatibility
+  if (!('config' in raw)) {
+    return {
+      ...raw,
+      config: '',
+    };
+  }
+
   return raw;
 }

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

@@ -1,5 +1,6 @@
 /// <reference types="next" />
 /// <reference types="next/image-types/global" />
+/// <reference path="./.next/types/routes.d.ts" />
 
 // NOTE: This file should not be edited
 // see https://nextjs.org/docs/app/api-reference/config/typescript for more information.

compiler/apps/playground/package.json

@@ -34,26 +34,30 @@
     "invariant": "^2.2.4",
     "lz-string": "^1.5.0",
     "monaco-editor": "^0.52.0",
-    "next": "^15.2.0-canary.64",
+    "next": "15.5.2",
     "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": "19.1.1",
+    "react-dom": "19.1.1"
   },
   "devDependencies": {
     "@types/node": "18.11.9",
-    "@types/react": "^19.0.0",
-    "@types/react-dom": "^19.0.0",
+    "@types/react": "19.1.12",
+    "@types/react-dom": "19.1.9",
     "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": "19.1.12",
+    "@types/react-dom": "19.1.9"
   }
 }

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/src/CompilerError.ts

@@ -7,9 +7,10 @@
 
 import * as t from '@babel/types';
 import {codeFrameColumns} from '@babel/code-frame';
-import type {SourceLocation} from './HIR';
+import {type SourceLocation} from './HIR';
 import {Err, Ok, Result} from './Utils/Result';
 import {assertExhaustive} from './Utils/utils';
+import invariant from 'invariant';
 
 export enum ErrorSeverity {
   /**
@@ -35,6 +36,14 @@ export enum ErrorSeverity {
    * memoization.
    */
   CannotPreserveMemoization = 'CannotPreserveMemoization',
+  /**
+   * An API that is known to be incompatible with the compiler. Generally as a result of
+   * the library using "interior mutability", ie having a value whose referential identity
+   * stays the same but which provides access to values that can change. For example a
+   * function that doesn't change but returns different results, or an object that doesn't
+   * change identity but whose properties change.
+   */
+  IncompatibleLibrary = 'IncompatibleLibrary',
   /**
    * Unhandled syntax that we don't support yet.
    */
@@ -457,7 +466,8 @@ export class CompilerError extends Error {
         case ErrorSeverity.InvalidJS:
         case ErrorSeverity.InvalidReact:
         case ErrorSeverity.InvalidConfig:
-        case ErrorSeverity.UnsupportedJS: {
+        case ErrorSeverity.UnsupportedJS:
+        case ErrorSeverity.IncompatibleLibrary: {
           return true;
         }
         case ErrorSeverity.CannotPreserveMemoization:
@@ -505,8 +515,9 @@ function printErrorSummary(severity: ErrorSeverity, message: string): string {
       severityCategory = 'Error';
       break;
     }
+    case ErrorSeverity.IncompatibleLibrary:
     case ErrorSeverity.CannotPreserveMemoization: {
-      severityCategory = 'Memoization';
+      severityCategory = 'Compilation Skipped';
       break;
     }
     case ErrorSeverity.Invariant: {
@@ -540,9 +551,15 @@ export enum ErrorCategory {
   // Checking for valid usage of manual memoization
   UseMemo = 'UseMemo',
 
+  // Checking for higher order functions acting as factories for components/hooks
+  Factories = 'Factories',
+
   // Checks that manual memoization is preserved
   PreserveManualMemo = 'PreserveManualMemo',
 
+  // Checks for known incompatible libraries
+  IncompatibleLibrary = 'IncompatibleLibrary',
+
   // Checking for no mutations of props, hook arguments, hook return values
   Immutability = 'Immutability',
 
@@ -628,7 +645,18 @@ export type LintRule = {
   recommended: boolean;
 };
 
+const RULE_NAME_PATTERN = /^[a-z]+(-[a-z]+)*$/;
+
 export function getRuleForCategory(category: ErrorCategory): LintRule {
+  const rule = getRuleForCategoryImpl(category);
+  invariant(
+    RULE_NAME_PATTERN.test(rule.name),
+    `Invalid rule name, got '${rule.name}' but rules must match ${RULE_NAME_PATTERN.toString()}`,
+  );
+  return rule;
+}
+
+function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
   switch (category) {
     case ErrorCategory.AutomaticEffectDependencies: {
       return {
@@ -636,7 +664,7 @@ export function getRuleForCategory(category: ErrorCategory): LintRule {
         name: 'automatic-effect-dependencies',
         description:
           'Verifies that automatic effect dependencies are compiled if opted-in',
-        recommended: true,
+        recommended: false,
       };
     }
     case ErrorCategory.CapitalizedCalls: {
@@ -652,7 +680,7 @@ export function getRuleForCategory(category: ErrorCategory): LintRule {
       return {
         category,
         name: 'config',
-        description: 'Validates the configuration',
+        description: 'Validates the compiler configuration options',
         recommended: true,
       };
     }
@@ -678,7 +706,7 @@ export function getRuleForCategory(category: ErrorCategory): LintRule {
         category,
         name: 'set-state-in-effect',
         description:
-          'Validates against calling setState synchronously in an effect',
+          'Validates against calling setState synchronously in an effect, which can lead to re-renders that degrade performance',
         recommended: true,
       };
     }
@@ -687,7 +715,17 @@ export function getRuleForCategory(category: ErrorCategory): LintRule {
         category,
         name: 'error-boundaries',
         description:
-          'Validates usage of error boundaries instead of try/catch for errors in JSX',
+          'Validates usage of error boundaries instead of try/catch for errors in child components',
+        recommended: true,
+      };
+    }
+    case ErrorCategory.Factories: {
+      return {
+        category,
+        name: 'component-hook-factories',
+        description:
+          'Validates against higher order functions defining nested components or hooks. ' +
+          'Components and hooks should be defined at the module level',
         recommended: true,
       };
     }
@@ -711,7 +749,8 @@ export function getRuleForCategory(category: ErrorCategory): LintRule {
       return {
         category,
         name: 'gating',
-        description: 'Validates configuration of gating mode',
+        description:
+          'Validates configuration of [gating mode](https://react.dev/reference/react-compiler/gating)',
         recommended: true,
       };
     }
@@ -720,7 +759,8 @@ export function getRuleForCategory(category: ErrorCategory): LintRule {
         category,
         name: 'globals',
         description:
-          'Validates against assignment/mutation of globals during render',
+          'Validates against assignment/mutation of globals during render, part of ensuring that ' +
+          '[side effects must render outside of render](https://react.dev/reference/rules/components-and-hooks-must-be-pure#side-effects-must-run-outside-of-render)',
         recommended: true,
       };
     }
@@ -742,7 +782,7 @@ export function getRuleForCategory(category: ErrorCategory): LintRule {
         category,
         name: 'immutability',
         description:
-          'Validates that immutable values (props, state, etc) are not mutated',
+          'Validates against mutating props, state, and other values that [are immutable](https://react.dev/reference/rules/components-and-hooks-must-be-pure#props-and-state-are-immutable)',
         recommended: true,
       };
     }
@@ -759,7 +799,9 @@ export function getRuleForCategory(category: ErrorCategory): LintRule {
         category,
         name: 'preserve-manual-memoization',
         description:
-          'Validates that existing manual memoized is preserved by the compiler',
+          'Validates that existing manual memoized is preserved by the compiler. ' +
+          'React Compiler will only compile components and hooks if its inference ' +
+          '[matches or exceeds the existing manual memoization](https://react.dev/learn/react-compiler/introduction#what-should-i-do-about-usememo-usecallback-and-reactmemo)',
         recommended: true,
       };
     }
@@ -768,7 +810,7 @@ export function getRuleForCategory(category: ErrorCategory): LintRule {
         category,
         name: 'purity',
         description:
-          'Validates that the component/hook is pure, and does not call known-impure functions',
+          'Validates that [components/hooks are pure](https://react.dev/reference/rules/components-and-hooks-must-be-pure) by checking that they do not call known-impure functions',
         recommended: true,
       };
     }
@@ -777,7 +819,7 @@ export function getRuleForCategory(category: ErrorCategory): LintRule {
         category,
         name: 'refs',
         description:
-          'Validates correct usage of refs, not reading/writing during render',
+          'Validates correct usage of refs, not reading/writing during render. See the "pitfalls" section in [`useRef()` usage](https://react.dev/reference/react/useRef#usage)',
         recommended: true,
       };
     }
@@ -785,7 +827,8 @@ export function getRuleForCategory(category: ErrorCategory): LintRule {
       return {
         category,
         name: 'set-state-in-render',
-        description: 'Validates against setting state during render',
+        description:
+          'Validates against setting state during render, which can trigger additional renders and potential infinite render loops',
         recommended: true,
       };
     }
@@ -794,7 +837,7 @@ export function getRuleForCategory(category: ErrorCategory): LintRule {
         category,
         name: 'static-components',
         description:
-          'Validates that components are static, not recreated every render',
+          'Validates that components are static, not recreated every render. Components that are recreated dynamically can reset state and trigger excessive re-rendering',
         recommended: true,
       };
     }
@@ -826,7 +869,8 @@ export function getRuleForCategory(category: ErrorCategory): LintRule {
       return {
         category,
         name: 'unsupported-syntax',
-        description: 'Validates against syntax that we do not plan to support',
+        description:
+          'Validates against syntax that we do not plan to support in React Compiler',
         recommended: true,
       };
     }
@@ -834,7 +878,17 @@ export function getRuleForCategory(category: ErrorCategory): LintRule {
       return {
         category,
         name: 'use-memo',
-        description: 'Validates usage of the useMemo() hook',
+        description:
+          'Validates usage of the useMemo() hook against common mistakes. See [`useMemo()` docs](https://react.dev/reference/react/useMemo) for more information.',
+        recommended: true,
+      };
+    }
+    case ErrorCategory.IncompatibleLibrary: {
+      return {
+        category,
+        name: 'incompatible-library',
+        description:
+          'Validates against usage of libraries which are incompatible with memoization (manual or automatic)',
         recommended: true,
       };
     }

compiler/packages/babel-plugin-react-compiler/src/Entrypoint/Program.ts

@@ -494,7 +494,20 @@ function findFunctionsToCompile(
 ): Array<CompileSource> {
   const queue: Array<CompileSource> = [];
   const traverseFunction = (fn: BabelFn, pass: CompilerPass): void => {
+    // In 'all' mode, compile only top level functions
+    if (
+      pass.opts.compilationMode === 'all' &&
+      fn.scope.getProgramParent() !== fn.scope.parent
+    ) {
+      return;
+    }
+
     const fnType = getReactFunctionType(fn, pass);
+
+    if (pass.opts.environment.validateNoDynamicallyCreatedComponentsOrHooks) {
+      validateNoDynamicallyCreatedComponentsOrHooks(fn, pass, programContext);
+    }
+
     if (fnType === null || programContext.alreadyCompiled.has(fn.node)) {
       return;
     }
@@ -839,6 +852,73 @@ function shouldSkipCompilation(
   return false;
 }
 
+/**
+ * Validates that Components/Hooks are always defined at module level. This prevents scope reference
+ * errors that occur when the compiler attempts to optimize the nested component/hook while its
+ * parent function remains uncompiled.
+ */
+function validateNoDynamicallyCreatedComponentsOrHooks(
+  fn: BabelFn,
+  pass: CompilerPass,
+  programContext: ProgramContext,
+): void {
+  const parentNameExpr = getFunctionName(fn);
+  const parentName =
+    parentNameExpr !== null && parentNameExpr.isIdentifier()
+      ? parentNameExpr.node.name
+      : '<anonymous>';
+
+  const validateNestedFunction = (
+    nestedFn: NodePath<
+      t.FunctionDeclaration | t.FunctionExpression | t.ArrowFunctionExpression
+    >,
+  ): void => {
+    if (
+      nestedFn.node === fn.node ||
+      programContext.alreadyCompiled.has(nestedFn.node)
+    ) {
+      return;
+    }
+
+    if (nestedFn.scope.getProgramParent() !== nestedFn.scope.parent) {
+      const nestedFnType = getReactFunctionType(nestedFn as BabelFn, pass);
+      const nestedFnNameExpr = getFunctionName(nestedFn as BabelFn);
+      const nestedName =
+        nestedFnNameExpr !== null && nestedFnNameExpr.isIdentifier()
+          ? nestedFnNameExpr.node.name
+          : '<anonymous>';
+      if (nestedFnType === 'Component' || nestedFnType === 'Hook') {
+        CompilerError.throwDiagnostic({
+          category: ErrorCategory.Factories,
+          severity: ErrorSeverity.InvalidReact,
+          reason: `Components and hooks cannot be created dynamically`,
+          description: `The function \`${nestedName}\` appears to be a React ${nestedFnType.toLowerCase()}, but it's defined inside \`${parentName}\`. Components and Hooks should always be declared at module scope`,
+          details: [
+            {
+              kind: 'error',
+              message: 'this function dynamically created a component/hook',
+              loc: parentNameExpr?.node.loc ?? fn.node.loc ?? null,
+            },
+            {
+              kind: 'error',
+              message: 'the component is created here',
+              loc: nestedFnNameExpr?.node.loc ?? nestedFn.node.loc ?? null,
+            },
+          ],
+        });
+      }
+    }
+
+    nestedFn.skip();
+  };
+
+  fn.traverse({
+    FunctionDeclaration: validateNestedFunction,
+    FunctionExpression: validateNestedFunction,
+    ArrowFunctionExpression: validateNestedFunction,
+  });
+}
+
 function getReactFunctionType(
   fn: BabelFn,
   pass: CompilerPass,
@@ -877,11 +957,6 @@ function getReactFunctionType(
       return componentSyntaxType;
     }
     case 'all': {
-      // Compile only top level functions
-      if (fn.scope.getProgramParent() !== fn.scope.parent) {
-        return null;
-      }
-
       return getComponentOrHookLike(fn, hookPattern) ?? 'Other';
     }
     default: {

compiler/packages/babel-plugin-react-compiler/src/HIR/DefaultModuleTypeProvider.ts

@@ -0,0 +1,91 @@
+/**
+ * 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 {Effect, ValueKind} from '..';
+import {TypeConfig} from './TypeSchema';
+
+/**
+ * Libraries developed before we officially documented the [Rules of React](https://react.dev/reference/rules)
+ * implement APIs which cannot be memoized safely, either via manual or automatic memoization.
+ *
+ * Any non-hook API that is designed to be called during render (not events/effects) should be safe to memoize:
+ *
+ * ```js
+ * function Component() {
+ *   const {someFunction} = useLibrary();
+ *   // it should always be safe to memoize functions like this
+ *  const result = useMemo(() => someFunction(), [someFunction]);
+ * }
+ * ```
+ *
+ * However, some APIs implement "interior mutability" — mutating values rather than copying into a new value
+ * and setting state with the new value. Such functions (`someFunction()` in the example) could return different
+ * values even though the function itself is the same object. This breaks memoization, since React relies on
+ * the outer object (or function) changing if part of its value has changed.
+ *
+ * Given that we didn't have the Rules of React precisely documented prior to the introduction of React compiler,
+ * it's understandable that some libraries accidentally shipped APIs that break this rule. However, developers
+ * can easily run into pitfalls with these APIs. They may manually memoize them, which can break their app. Or
+ * they may try using React Compiler, and think that the compiler has broken their code.
+ *
+ * To help ensure that developers can successfully use the compiler with existing code, this file teaches the
+ * compiler about specific APIs that are known to be incompatible with memoization. We've tried to be as precise
+ * as possible.
+ *
+ * The React team is open to collaborating with library authors to help develop compatible versions of these APIs,
+ * and we have already reached out to the teams who own any API listed here to ensure they are aware of the issue.
+ */
+export function defaultModuleTypeProvider(
+  moduleName: string,
+): TypeConfig | null {
+  switch (moduleName) {
+    case 'react-hook-form': {
+      return {
+        kind: 'object',
+        properties: {
+          useForm: {
+            kind: 'hook',
+            returnType: {
+              kind: 'object',
+              properties: {
+                // Only the `watch()` function returned by react-hook-form's `useForm()` API is incompatible
+                watch: {
+                  kind: 'function',
+                  positionalParams: [],
+                  restParam: Effect.Read,
+                  calleeEffect: Effect.Read,
+                  returnType: {kind: 'type', name: 'Any'},
+                  returnValueKind: ValueKind.Mutable,
+                  knownIncompatible: `React Hook Form's \`useForm()\` API returns a \`watch()\` function which cannot be memoized safely.`,
+                },
+              },
+            },
+          },
+        },
+      };
+    }
+    case '@tanstack/react-table': {
+      return {
+        kind: 'object',
+        properties: {
+          /*
+           * Many of the properties of `useReactTable()`'s return value are incompatible, so we mark the entire hook
+           * as incompatible
+           */
+          useReactTable: {
+            kind: 'hook',
+            positionalParams: [],
+            restParam: Effect.Read,
+            returnType: {kind: 'type', name: 'Any'},
+            knownIncompatible: `TanStack Table's \`useReactTable()\` API returns functions that cannot be memoized safely`,
+          },
+        },
+      };
+    }
+  }
+  return null;
+}

compiler/packages/babel-plugin-react-compiler/src/HIR/Environment.ts

@@ -50,6 +50,7 @@ import {
 import {Scope as BabelScope, NodePath} from '@babel/traverse';
 import {TypeSchema} from './TypeSchema';
 import {FlowTypeEnv} from '../Flood/Types';
+import {defaultModuleTypeProvider} from './DefaultModuleTypeProvider';
 
 export const ReactElementSymbolSchema = z.object({
   elementSymbol: z.union([
@@ -650,6 +651,13 @@ export const EnvironmentConfigSchema = z.object({
    *   useMemo(() => { ... }, [...]);
    */
   validateNoVoidUseMemo: z.boolean().default(false),
+
+  /**
+   * Validates that Components/Hooks are always defined at module level. This prevents scope
+   * reference errors that occur when the compiler attempts to optimize the nested component/hook
+   * while its parent function remains uncompiled.
+   */
+  validateNoDynamicallyCreatedComponentsOrHooks: z.boolean().default(false),
 });
 
 export type EnvironmentConfig = z.infer<typeof EnvironmentConfigSchema>;
@@ -853,10 +861,16 @@ export class Environment {
   #resolveModuleType(moduleName: string, loc: SourceLocation): Global | null {
     let moduleType = this.#moduleTypes.get(moduleName);
     if (moduleType === undefined) {
-      if (this.config.moduleTypeProvider == null) {
+      /*
+       * NOTE: Zod doesn't work when specifying a function as a default, so we have to
+       * fallback to the default value here
+       */
+      const moduleTypeProvider =
+        this.config.moduleTypeProvider ?? defaultModuleTypeProvider;
+      if (moduleTypeProvider == null) {
         return null;
       }
-      const unparsedModuleConfig = this.config.moduleTypeProvider(moduleName);
+      const unparsedModuleConfig = moduleTypeProvider(moduleName);
       if (unparsedModuleConfig != null) {
         const parsedModuleConfig = TypeSchema.safeParse(unparsedModuleConfig);
         if (!parsedModuleConfig.success) {

compiler/packages/babel-plugin-react-compiler/src/HIR/Globals.ts

@@ -1001,6 +1001,7 @@ export function installTypeConfig(
         mutableOnlyIfOperandsAreMutable:
           typeConfig.mutableOnlyIfOperandsAreMutable === true,
         aliasing: typeConfig.aliasing,
+        knownIncompatible: typeConfig.knownIncompatible ?? null,
       });
     }
     case 'hook': {
@@ -1019,6 +1020,7 @@ export function installTypeConfig(
         returnValueKind: typeConfig.returnValueKind ?? ValueKind.Frozen,
         noAlias: typeConfig.noAlias === true,
         aliasing: typeConfig.aliasing,
+        knownIncompatible: typeConfig.knownIncompatible ?? null,
       });
     }
     case 'object': {

compiler/packages/babel-plugin-react-compiler/src/HIR/HIRBuilder.ts

@@ -323,6 +323,22 @@ export default class HIRBuilder {
         ],
       });
     }
+    if (node.name === 'this') {
+      CompilerError.throwDiagnostic({
+        severity: ErrorSeverity.UnsupportedJS,
+        category: ErrorCategory.UnsupportedSyntax,
+        reason: '`this` is not supported syntax',
+        description:
+          'React Compiler does not support compiling functions that use `this`',
+        details: [
+          {
+            kind: 'error',
+            message: '`this` was used here',
+            loc: node.loc ?? GeneratedSource,
+          },
+        ],
+      });
+    }
     const originalName = node.name;
     let name = originalName;
     let index = 0;

compiler/packages/babel-plugin-react-compiler/src/HIR/ObjectShape.ts

@@ -332,6 +332,7 @@ export type FunctionSignature = {
   mutableOnlyIfOperandsAreMutable?: boolean;
 
   impure?: boolean;
+  knownIncompatible?: string | null | undefined;
 
   canonicalName?: string;
 

compiler/packages/babel-plugin-react-compiler/src/HIR/PrintHIR.ts

@@ -880,7 +880,8 @@ export function printType(type: Type): string {
   if (type.kind === 'Object' && type.shapeId != null) {
     return `:T${type.kind}<${type.shapeId}>`;
   } else if (type.kind === 'Function' && type.shapeId != null) {
-    return `:T${type.kind}<${type.shapeId}>`;
+    const returnType = printType(type.return);
+    return `:T${type.kind}<${type.shapeId}>()${returnType !== '' ? `:  ${returnType}` : ''}`;
   } else {
     return `:T${type.kind}`;
   }
@@ -983,7 +984,7 @@ export function printAliasingEffect(effect: AliasingEffect): string {
     case 'MutateConditionally':
     case 'MutateTransitive':
     case 'MutateTransitiveConditionally': {
-      return `${effect.kind} ${printPlaceForAliasEffect(effect.value)}`;
+      return `${effect.kind} ${printPlaceForAliasEffect(effect.value)}${effect.kind === 'Mutate' && effect.reason?.kind === 'AssignCurrentProperty' ? ' (assign `.current`)' : ''}`;
     }
     case 'MutateFrozen': {
       return `MutateFrozen ${printPlaceForAliasEffect(effect.place)} reason=${JSON.stringify(effect.error.reason)}`;

compiler/packages/babel-plugin-react-compiler/src/HIR/TypeSchema.ts

@@ -251,6 +251,7 @@ export type FunctionTypeConfig = {
   impure?: boolean | null | undefined;
   canonicalName?: string | null | undefined;
   aliasing?: AliasingSignatureConfig | null | undefined;
+  knownIncompatible?: string | null | undefined;
 };
 export const FunctionTypeSchema: z.ZodType<FunctionTypeConfig> = z.object({
   kind: z.literal('function'),
@@ -264,6 +265,7 @@ export const FunctionTypeSchema: z.ZodType<FunctionTypeConfig> = z.object({
   impure: z.boolean().nullable().optional(),
   canonicalName: z.string().nullable().optional(),
   aliasing: AliasingSignatureSchema.nullable().optional(),
+  knownIncompatible: z.string().nullable().optional(),
 });
 
 export type HookTypeConfig = {
@@ -274,6 +276,7 @@ export type HookTypeConfig = {
   returnValueKind?: ValueKind | null | undefined;
   noAlias?: boolean | null | undefined;
   aliasing?: AliasingSignatureConfig | null | undefined;
+  knownIncompatible?: string | null | undefined;
 };
 export const HookTypeSchema: z.ZodType<HookTypeConfig> = z.object({
   kind: z.literal('hook'),
@@ -283,6 +286,7 @@ export const HookTypeSchema: z.ZodType<HookTypeConfig> = z.object({
   returnValueKind: ValueKindSchema.nullable().optional(),
   noAlias: z.boolean().nullable().optional(),
   aliasing: AliasingSignatureSchema.nullable().optional(),
+  knownIncompatible: z.string().nullable().optional(),
 });
 
 export type BuiltInTypeConfig =

compiler/packages/babel-plugin-react-compiler/src/Inference/InferMutationAliasingEffects.ts

@@ -27,6 +27,7 @@ import {
   InstructionKind,
   InstructionValue,
   isArrayType,
+  isJsxType,
   isMapType,
   isPrimitiveType,
   isRefOrRefValue,
@@ -1871,6 +1872,23 @@ function computeSignatureForInstruction(
             });
           }
         }
+        for (const prop of value.props) {
+          if (
+            prop.kind === 'JsxAttribute' &&
+            prop.place.identifier.type.kind === 'Function' &&
+            (isJsxType(prop.place.identifier.type.return) ||
+              (prop.place.identifier.type.return.kind === 'Phi' &&
+                prop.place.identifier.type.return.operands.some(operand =>
+                  isJsxType(operand),
+                )))
+          ) {
+            // Any props which return jsx are assumed to be called during render
+            effects.push({
+              kind: 'Render',
+              place: prop.place,
+            });
+          }
+        }
       }
       break;
     }
@@ -2071,7 +2089,7 @@ function computeSignatureForInstruction(
           effects.push({
             kind: 'Freeze',
             value: operand,
-            reason: ValueReason.Other,
+            reason: ValueReason.HookCaptured,
           });
         }
       }
@@ -2152,6 +2170,27 @@ function computeEffectsForLegacySignature(
       }),
     });
   }
+  if (signature.knownIncompatible != null && state.env.isInferredMemoEnabled) {
+    const errors = new CompilerError();
+    errors.pushDiagnostic(
+      CompilerDiagnostic.create({
+        category: ErrorCategory.IncompatibleLibrary,
+        severity: ErrorSeverity.IncompatibleLibrary,
+        reason: 'Use of incompatible library',
+        description: [
+          'This API returns functions which cannot be memoized without leading to stale UI. ' +
+            'To prevent this, by default React Compiler will skip memoizing this component/hook. ' +
+            'However, you may see issues if values from this API are passed to other components/hooks that are ' +
+            'memoized.',
+        ].join(''),
+      }).withDetail({
+        kind: 'error',
+        loc: receiver.loc,
+        message: signature.knownIncompatible,
+      }),
+    );
+    throw errors;
+  }
   const stores: Array<Place> = [];
   const captures: Array<Place> = [];
   function visit(place: Place, effect: Effect): void {

compiler/packages/babel-plugin-react-compiler/src/Inference/InferMutationAliasingRanges.ts

@@ -27,7 +27,7 @@ import {
 } from '../HIR/visitors';
 import {assertExhaustive, getOrInsertWith} from '../Utils/utils';
 import {Err, Ok, Result} from '../Utils/Result';
-import {AliasingEffect} from './AliasingEffects';
+import {AliasingEffect, MutationReason} from './AliasingEffects';
 
 /**
  * This pass builds an abstract model of the heap and interprets the effects of the
@@ -101,6 +101,7 @@ export function inferMutationAliasingRanges(
     transitive: boolean;
     kind: MutationKind;
     place: Place;
+    reason: MutationReason | null;
   }> = [];
   const renders: Array<{index: number; place: Place}> = [];
 
@@ -176,6 +177,7 @@ export function inferMutationAliasingRanges(
               effect.kind === 'MutateTransitive'
                 ? MutationKind.Definite
                 : MutationKind.Conditional,
+            reason: null,
             place: effect.value,
           });
         } else if (
@@ -190,6 +192,7 @@ export function inferMutationAliasingRanges(
               effect.kind === 'Mutate'
                 ? MutationKind.Definite
                 : MutationKind.Conditional,
+            reason: effect.kind === 'Mutate' ? (effect.reason ?? null) : null,
             place: effect.value,
           });
         } else if (
@@ -241,6 +244,7 @@ export function inferMutationAliasingRanges(
       mutation.transitive,
       mutation.kind,
       mutation.place.loc,
+      mutation.reason,
       errors,
     );
   }
@@ -267,6 +271,7 @@ export function inferMutationAliasingRanges(
         functionEffects.push({
           kind: 'Mutate',
           value: {...place, loc: node.local.loc},
+          reason: node.mutationReason,
         });
       }
     }
@@ -507,6 +512,7 @@ export function inferMutationAliasingRanges(
       true,
       MutationKind.Conditional,
       into.loc,
+      null,
       ignoredErrors,
     );
     for (const from of tracked) {
@@ -580,6 +586,7 @@ type Node = {
   transitive: {kind: MutationKind; loc: SourceLocation} | null;
   local: {kind: MutationKind; loc: SourceLocation} | null;
   lastMutated: number;
+  mutationReason: MutationReason | null;
   value:
     | {kind: 'Object'}
     | {kind: 'Phi'}
@@ -599,6 +606,7 @@ class AliasingState {
       transitive: null,
       local: null,
       lastMutated: 0,
+      mutationReason: null,
       value,
     });
   }
@@ -697,6 +705,7 @@ class AliasingState {
     transitive: boolean,
     startKind: MutationKind,
     loc: SourceLocation,
+    reason: MutationReason | null,
     errors: CompilerError,
   ): void {
     const seen = new Map<Identifier, MutationKind>();
@@ -717,6 +726,7 @@ class AliasingState {
       if (node == null) {
         continue;
       }
+      node.mutationReason ??= reason;
       node.lastMutated = Math.max(node.lastMutated, index);
       if (end != null) {
         node.id.mutableRange.end = makeInstructionId(

compiler/packages/babel-plugin-react-compiler/src/ReactiveScopes/AlignReactiveScopesToBlockScopesHIR.ts

@@ -175,6 +175,41 @@ export function alignReactiveScopesToBlockScopesHIR(fn: HIRFunction): void {
       if (node != null) {
         valueBlockNodes.set(fallthrough, node);
       }
+    } else if (terminal.kind === 'goto') {
+      /**
+       * If we encounter a goto that is not to the natural fallthrough of the current
+       * block (not the topmost fallthrough on the stack), then this is a goto to a
+       * label. Any scopes that extend beyond the goto must be extended to include
+       * the labeled range, so that the break statement doesn't accidentally jump
+       * out of the scope. We do this by extending the start and end of the scope's
+       * range to the label and its fallthrough respectively.
+       */
+      const start = activeBlockFallthroughRanges.find(
+        range => range.fallthrough === terminal.block,
+      );
+      if (start != null && start !== activeBlockFallthroughRanges.at(-1)) {
+        const fallthroughBlock = fn.body.blocks.get(start.fallthrough)!;
+        const firstId =
+          fallthroughBlock.instructions[0]?.id ?? fallthroughBlock.terminal.id;
+        for (const scope of activeScopes) {
+          /**
+           * activeScopes is only filtered at block start points, so some of the
+           * scopes may not actually be active anymore, ie we've past their end
+           * instruction. Only extend ranges for scopes that are actually active.
+           *
+           * TODO: consider pruning activeScopes per instruction
+           */
+          if (scope.range.end <= terminal.id) {
+            continue;
+          }
+          scope.range.start = makeInstructionId(
+            Math.min(start.range.start, scope.range.start),
+          );
+          scope.range.end = makeInstructionId(
+            Math.max(firstId, scope.range.end),
+          );
+        }
+      }
     }
 
     /*

compiler/packages/babel-plugin-react-compiler/src/ReactiveScopes/PruneNonEscapingScopes.ts

@@ -411,7 +411,9 @@ class CollectDependenciesVisitor extends ReactiveFunctionVisitor<
     this.state = state;
     this.options = {
       memoizeJsxElements: !this.env.config.enableForest,
-      forceMemoizePrimitives: this.env.config.enableForest,
+      forceMemoizePrimitives:
+        this.env.config.enableForest ||
+        this.env.config.enablePreserveExistingMemoizationGuarantees,
     };
   }
 
@@ -534,9 +536,23 @@ class CollectDependenciesVisitor extends ReactiveFunctionVisitor<
       case 'JSXText':
       case 'BinaryExpression':
       case 'UnaryExpression': {
-        const level = options.forceMemoizePrimitives
-          ? MemoizationLevel.Memoized
-          : MemoizationLevel.Never;
+        if (options.forceMemoizePrimitives) {
+          /**
+           * Because these instructions produce primitives we usually don't consider
+           * them as escape points: they are known to copy, not return references.
+           * However if we're forcing memoization of primitives then we mark these
+           * instructions as needing memoization and walk their rvalues to ensure
+           * any scopes transitively reachable from the rvalues are considered for
+           * memoization. Note: we may still prune primitive-producing scopes if
+           * they don't ultimately escape at all.
+           */
+          const level = MemoizationLevel.Conditional;
+          return {
+            lvalues: lvalue !== null ? [{place: lvalue, level}] : [],
+            rvalues: [...eachReactiveValueOperand(value)],
+          };
+        }
+        const level = MemoizationLevel.Never;
         return {
           // All of these instructions return a primitive value and never need to be memoized
           lvalues: lvalue !== null ? [{place: lvalue, level}] : [],
@@ -685,9 +701,7 @@ class CollectDependenciesVisitor extends ReactiveFunctionVisitor<
       }
       case 'ComputedLoad':
       case 'PropertyLoad': {
-        const level = options.forceMemoizePrimitives
-          ? MemoizationLevel.Memoized
-          : MemoizationLevel.Conditional;
+        const level = MemoizationLevel.Conditional;
         return {
           // Indirection for the inner value, memoized if the value is
           lvalues: lvalue !== null ? [{place: lvalue, level}] : [],

compiler/packages/babel-plugin-react-compiler/src/TypeInference/InferTypes.ts

@@ -777,6 +777,15 @@ class Unifier {
       return {kind: 'Phi', operands: type.operands.map(o => this.get(o))};
     }
 
+    if (type.kind === 'Function') {
+      return {
+        kind: 'Function',
+        isConstructor: type.isConstructor,
+        shapeId: type.shapeId,
+        return: this.get(type.return),
+      };
+    }
+
     return type;
   }
 }

compiler/packages/babel-plugin-react-compiler/src/Utils/TestUtils.ts

@@ -182,6 +182,11 @@ export function parseConfigPragmaForTests(
     environment?: PartialEnvironmentConfig;
   },
 ): PluginOptions {
+  const overridePragma = parseConfigPragmaAsString(pragma);
+  if (overridePragma !== '') {
+    return parseConfigStringAsJS(overridePragma, defaults);
+  }
+
   const environment = parseConfigPragmaEnvironmentForTest(
     pragma,
     defaults.environment ?? {},
@@ -217,3 +222,90 @@ export function parseConfigPragmaForTests(
   }
   return parsePluginOptions(options);
 }
+
+export function parseConfigPragmaAsString(pragma: string): string {
+  // Check if it's in JS override format
+  for (const {key, value: val} of splitPragma(pragma)) {
+    if (key === 'OVERRIDE' && val != null) {
+      return val;
+    }
+  }
+  return '';
+}
+
+function parseConfigStringAsJS(
+  configString: string,
+  defaults: {
+    compilationMode: CompilationMode;
+    environment?: PartialEnvironmentConfig;
+  },
+): PluginOptions {
+  let parsedConfig: any;
+  try {
+    // Parse the JavaScript object literal
+    parsedConfig = new Function(`return ${configString}`)();
+  } catch (error) {
+    CompilerError.invariant(false, {
+      reason: 'Failed to parse config pragma as JavaScript object',
+      description: `Could not parse: ${configString}. Error: ${error}`,
+      loc: null,
+      suggestions: null,
+    });
+  }
+
+  console.log('OVERRIDE:', parsedConfig);
+
+  const environment = parseConfigPragmaEnvironmentForTest(
+    '',
+    defaults.environment ?? {},
+  );
+
+  const options: Record<keyof PluginOptions, unknown> = {
+    ...defaultOptions,
+    panicThreshold: 'all_errors',
+    compilationMode: defaults.compilationMode,
+    environment,
+  };
+
+  // Apply parsed config, merging environment if it exists
+  if (parsedConfig.environment) {
+    const mergedEnvironment = {
+      ...(options.environment as Record<string, unknown>),
+      ...parsedConfig.environment,
+    };
+
+    // Validate environment config
+    const validatedEnvironment =
+      EnvironmentConfigSchema.safeParse(mergedEnvironment);
+    if (!validatedEnvironment.success) {
+      CompilerError.invariant(false, {
+        reason: 'Invalid environment configuration in config pragma',
+        description: `${fromZodError(validatedEnvironment.error)}`,
+        loc: null,
+        suggestions: null,
+      });
+    }
+
+    options.environment = validatedEnvironment.data;
+  }
+
+  // Apply other config options
+  for (const [key, value] of Object.entries(parsedConfig)) {
+    if (key === 'environment') {
+      continue;
+    }
+
+    if (hasOwnProperty(defaultOptions, key)) {
+      if (key === 'target' && value === 'donotuse_meta_internal') {
+        options[key] = {
+          kind: value,
+          runtimeModule: 'react',
+        };
+      } else {
+        options[key] = value;
+      }
+    }
+  }
+
+  return parsePluginOptions(options);
+}

compiler/packages/babel-plugin-react-compiler/src/Validation/ValidatePreservedManualMemoization.ts

@@ -284,8 +284,7 @@ function validateInferredDep(
     CompilerDiagnostic.create({
       category: ErrorCategory.PreserveManualMemo,
       severity: ErrorSeverity.CannotPreserveMemoization,
-      reason:
-        'Compilation skipped because existing memoization could not be preserved',
+      reason: 'Existing memoization could not be preserved',
       description: [
         'React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. ',
         'The inferred dependencies did not match the manually specified dependencies, which could cause the value to change more or less frequently than expected. ',
@@ -539,8 +538,7 @@ class Visitor extends ReactiveFunctionVisitor<VisitorState> {
             CompilerDiagnostic.create({
               category: ErrorCategory.PreserveManualMemo,
               severity: ErrorSeverity.CannotPreserveMemoization,
-              reason:
-                'Compilation skipped because existing memoization could not be preserved',
+              reason: 'Existing memoization could not be preserved',
               description: [
                 'React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. ',
                 'This dependency may be mutated later, which could cause the value to change unexpectedly.',
@@ -588,8 +586,7 @@ class Visitor extends ReactiveFunctionVisitor<VisitorState> {
                 CompilerDiagnostic.create({
                   category: ErrorCategory.PreserveManualMemo,
                   severity: ErrorSeverity.CannotPreserveMemoization,
-                  reason:
-                    'Compilation skipped because existing memoization could not be preserved',
+                  reason: 'Existing memoization could not be preserved',
                   description: [
                     'React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. This value was memoized in source but not in compilation output. ',
                     DEBUG

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/align-scopes-reactive-scope-overlaps-if.expect.md

@@ -46,14 +46,16 @@ function useFoo(t0) {
     t1 = $[0];
   }
   let items = t1;
-  bb0: if ($[1] !== cond) {
-    if (cond) {
-      items = [];
-    } else {
-      break bb0;
-    }
+  if ($[1] !== cond) {
+    bb0: {
+      if (cond) {
+        items = [];
+      } else {
+        break bb0;
+      }
 
-    items.push(2);
+      items.push(2);
+    }
     $[1] = cond;
     $[2] = items;
   } else {

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/dont-memoize-primitive-function-call-non-escaping-useMemo.expect.md

@@ -0,0 +1,77 @@
+
+## Input
+
+```javascript
+// @compilationMode:"infer" @enablePreserveExistingMemoizationGuarantees @validatePreserveExistingMemoizationGuarantees
+import {useMemo} from 'react';
+import {makeObject_Primitives, ValidateMemoization} from 'shared-runtime';
+
+function Component(props) {
+  const result = useMemo(
+    () => makeObject(props.value).value + 1,
+    [props.value]
+  );
+  console.log(result);
+  return 'ok';
+}
+
+function makeObject(value) {
+  console.log(value);
+  return {value};
+}
+
+export const TODO_FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{value: 42}],
+  sequentialRenders: [
+    {value: 42},
+    {value: 42},
+    {value: 3.14},
+    {value: 3.14},
+    {value: 42},
+    {value: 3.14},
+    {value: 42},
+    {value: 3.14},
+  ],
+};
+
+```
+
+## Code
+
+```javascript
+// @compilationMode:"infer" @enablePreserveExistingMemoizationGuarantees @validatePreserveExistingMemoizationGuarantees
+import { useMemo } from "react";
+import { makeObject_Primitives, ValidateMemoization } from "shared-runtime";
+
+function Component(props) {
+  const result = makeObject(props.value).value + 1;
+
+  console.log(result);
+  return "ok";
+}
+
+function makeObject(value) {
+  console.log(value);
+  return { value };
+}
+
+export const TODO_FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ value: 42 }],
+  sequentialRenders: [
+    { value: 42 },
+    { value: 42 },
+    { value: 3.14 },
+    { value: 3.14 },
+    { value: 42 },
+    { value: 3.14 },
+    { value: 42 },
+    { value: 3.14 },
+  ],
+};
+
+```
+      
+### Eval output
+(kind: exception) Fixture not implemented

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/dont-memoize-primitive-function-call-non-escaping-useMemo.js

@@ -0,0 +1,32 @@
+// @compilationMode:"infer" @enablePreserveExistingMemoizationGuarantees @validatePreserveExistingMemoizationGuarantees
+import {useMemo} from 'react';
+import {makeObject_Primitives, ValidateMemoization} from 'shared-runtime';
+
+function Component(props) {
+  const result = useMemo(
+    () => makeObject(props.value).value + 1,
+    [props.value]
+  );
+  console.log(result);
+  return 'ok';
+}
+
+function makeObject(value) {
+  console.log(value);
+  return {value};
+}
+
+export const TODO_FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{value: 42}],
+  sequentialRenders: [
+    {value: 42},
+    {value: 42},
+    {value: 3.14},
+    {value: 3.14},
+    {value: 42},
+    {value: 3.14},
+    {value: 42},
+    {value: 3.14},
+  ],
+};

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/dont-memoize-primitive-function-call-non-escaping.expect.md

@@ -0,0 +1,81 @@
+
+## Input
+
+```javascript
+// @compilationMode:"infer" @enablePreserveExistingMemoizationGuarantees @validatePreserveExistingMemoizationGuarantees
+import {useMemo} from 'react';
+import {makeObject_Primitives, ValidateMemoization} from 'shared-runtime';
+
+function Component(props) {
+  const result = makeObject(props.value).value + 1;
+  console.log(result);
+  return 'ok';
+}
+
+function makeObject(value) {
+  console.log(value);
+  return {value};
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{value: 42}],
+  sequentialRenders: [
+    {value: 42},
+    {value: 42},
+    {value: 3.14},
+    {value: 3.14},
+    {value: 42},
+    {value: 3.14},
+    {value: 42},
+    {value: 3.14},
+  ],
+};
+
+```
+
+## Code
+
+```javascript
+// @compilationMode:"infer" @enablePreserveExistingMemoizationGuarantees @validatePreserveExistingMemoizationGuarantees
+import { useMemo } from "react";
+import { makeObject_Primitives, ValidateMemoization } from "shared-runtime";
+
+function Component(props) {
+  const result = makeObject(props.value).value + 1;
+  console.log(result);
+  return "ok";
+}
+
+function makeObject(value) {
+  console.log(value);
+  return { value };
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ value: 42 }],
+  sequentialRenders: [
+    { value: 42 },
+    { value: 42 },
+    { value: 3.14 },
+    { value: 3.14 },
+    { value: 42 },
+    { value: 3.14 },
+    { value: 42 },
+    { value: 3.14 },
+  ],
+};
+
+```
+      
+### Eval output
+(kind: ok) "ok"
+"ok"
+"ok"
+"ok"
+"ok"
+"ok"
+"ok"
+"ok"
+logs: [42,43,42,43,3.14,4.140000000000001,3.14,4.140000000000001,42,43,3.14,4.140000000000001,42,43,3.14,4.140000000000001]

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/dont-memoize-primitive-function-call-non-escaping.js

@@ -0,0 +1,29 @@
+// @compilationMode:"infer" @enablePreserveExistingMemoizationGuarantees @validatePreserveExistingMemoizationGuarantees
+import {useMemo} from 'react';
+import {makeObject_Primitives, ValidateMemoization} from 'shared-runtime';
+
+function Component(props) {
+  const result = makeObject(props.value).value + 1;
+  console.log(result);
+  return 'ok';
+}
+
+function makeObject(value) {
+  console.log(value);
+  return {value};
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{value: 42}],
+  sequentialRenders: [
+    {value: 42},
+    {value: 42},
+    {value: 3.14},
+    {value: 3.14},
+    {value: 42},
+    {value: 3.14},
+    {value: 42},
+    {value: 3.14},
+  ],
+};

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/ecma/error.reserved-words.expect.md

@@ -24,9 +24,18 @@ function useThing(fn) {
 ```
 Found 1 error:
 
-Error: Expected a non-reserved identifier name
+Error: `this` is not supported syntax
 
-`this` is a reserved word in JavaScript and cannot be used as an identifier name.
+React Compiler does not support compiling functions that use `this`
+
+error.reserved-words.ts:8:28
+   6 |
+   7 |   if (ref.current === null) {
+>  8 |     ref.current = function (this: unknown, ...args) {
+     |                             ^^^^^^^^^^^^^ `this` was used here
+   9 |       return fnRef.current.call(this, ...args);
+  10 |     };
+  11 |   }
 ```
           
       

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.assign-ref-in-effect-hint.expect.md

@@ -0,0 +1,37 @@
+
+## Input
+
+```javascript
+// Fixture to test that we show a hint to name as `ref` or `-Ref` when attempting
+// to assign .current inside an effect
+function Component({foo}) {
+  useEffect(() => {
+    foo.current = true;
+  }, [foo]);
+}
+
+```
+
+
+## Error
+
+```
+Found 1 error:
+
+Error: This value cannot be modified
+
+Modifying component props or hook arguments is not allowed. Consider using a local variable instead.
+
+error.assign-ref-in-effect-hint.ts:5:4
+  3 | function Component({foo}) {
+  4 |   useEffect(() => {
+> 5 |     foo.current = true;
+    |     ^^^ `foo` cannot be modified
+  6 |   }, [foo]);
+  7 | }
+  8 |
+
+Hint: If this value is a Ref (value returned by `useRef()`), rename the variable to end in "Ref".
+```
+          
+      

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.assign-ref-in-effect-hint.js

@@ -0,0 +1,7 @@
+// Fixture to test that we show a hint to name as `ref` or `-Ref` when attempting
+// to assign .current inside an effect
+function Component({foo}) {
+  useEffect(() => {
+    foo.current = true;
+  }, [foo]);
+}

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.hoist-optional-member-expression-with-conditional-optional.expect.md

@@ -26,7 +26,7 @@ function Component(props) {
 ```
 Found 1 error:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. The inferred dependencies did not match the manually specified dependencies, which could cause the value to change more or less frequently than expected. The inferred dependency was `props.items`, but the source dependencies were [props?.items, props.cond]. Inferred different dependency than source.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.hoist-optional-member-expression-with-conditional.expect.md

@@ -26,7 +26,7 @@ function Component(props) {
 ```
 Found 1 error:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. The inferred dependencies did not match the manually specified dependencies, which could cause the value to change more or less frequently than expected. The inferred dependency was `props.items`, but the source dependencies were [props?.items, props.cond]. Inferred different dependency than source.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.invalid-known-incompatible-function.expect.md

@@ -0,0 +1,34 @@
+
+## Input
+
+```javascript
+import {knownIncompatible} from 'ReactCompilerKnownIncompatibleTest';
+
+function Component() {
+  const data = knownIncompatible();
+  return <div>Error</div>;
+}
+
+```
+
+
+## Error
+
+```
+Found 1 error:
+
+Compilation Skipped: Use of incompatible library
+
+This API returns functions which cannot be memoized without leading to stale UI. To prevent this, by default React Compiler will skip memoizing this component/hook. However, you may see issues if values from this API are passed to other components/hooks that are memoized.
+
+error.invalid-known-incompatible-function.ts:4:15
+  2 |
+  3 | function Component() {
+> 4 |   const data = knownIncompatible();
+    |                ^^^^^^^^^^^^^^^^^ useKnownIncompatible is known to be incompatible
+  5 |   return <div>Error</div>;
+  6 | }
+  7 |
+```
+          
+      

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.invalid-known-incompatible-function.js

@@ -0,0 +1,6 @@
+import {knownIncompatible} from 'ReactCompilerKnownIncompatibleTest';
+
+function Component() {
+  const data = knownIncompatible();
+  return <div>Error</div>;
+}

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.invalid-known-incompatible-hook-return-property.expect.md

@@ -0,0 +1,33 @@
+
+## Input
+
+```javascript
+import {useKnownIncompatibleIndirect} from 'ReactCompilerKnownIncompatibleTest';
+
+function Component() {
+  const {incompatible} = useKnownIncompatibleIndirect();
+  return <div>{incompatible()}</div>;
+}
+
+```
+
+
+## Error
+
+```
+Found 1 error:
+
+Compilation Skipped: Use of incompatible library
+
+This API returns functions which cannot be memoized without leading to stale UI. To prevent this, by default React Compiler will skip memoizing this component/hook. However, you may see issues if values from this API are passed to other components/hooks that are memoized.
+
+error.invalid-known-incompatible-hook-return-property.ts:5:15
+  3 | function Component() {
+  4 |   const {incompatible} = useKnownIncompatibleIndirect();
+> 5 |   return <div>{incompatible()}</div>;
+    |                ^^^^^^^^^^^^ useKnownIncompatibleIndirect returns an incompatible() function that is known incompatible
+  6 | }
+  7 |
+```
+          
+      

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.invalid-known-incompatible-hook-return-property.js

@@ -0,0 +1,6 @@
+import {useKnownIncompatibleIndirect} from 'ReactCompilerKnownIncompatibleTest';
+
+function Component() {
+  const {incompatible} = useKnownIncompatibleIndirect();
+  return <div>{incompatible()}</div>;
+}

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.invalid-known-incompatible-hook.expect.md

@@ -0,0 +1,34 @@
+
+## Input
+
+```javascript
+import {useKnownIncompatible} from 'ReactCompilerKnownIncompatibleTest';
+
+function Component() {
+  const data = useKnownIncompatible();
+  return <div>Error</div>;
+}
+
+```
+
+
+## Error
+
+```
+Found 1 error:
+
+Compilation Skipped: Use of incompatible library
+
+This API returns functions which cannot be memoized without leading to stale UI. To prevent this, by default React Compiler will skip memoizing this component/hook. However, you may see issues if values from this API are passed to other components/hooks that are memoized.
+
+error.invalid-known-incompatible-hook.ts:4:15
+  2 |
+  3 | function Component() {
+> 4 |   const data = useKnownIncompatible();
+    |                ^^^^^^^^^^^^^^^^^^^^ useKnownIncompatible is known to be incompatible
+  5 |   return <div>Error</div>;
+  6 | }
+  7 |
+```
+          
+      

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.invalid-known-incompatible-hook.js

@@ -0,0 +1,6 @@
+import {useKnownIncompatible} from 'ReactCompilerKnownIncompatibleTest';
+
+function Component() {
+  const data = useKnownIncompatible();
+  return <div>Error</div>;
+}

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.invalid-mutate-context-in-callback.expect.md

@@ -38,6 +38,8 @@ error.invalid-mutate-context-in-callback.ts:12:4
   13 |   };
   14 |   return <div onClick={onClick} />;
   15 | }
+
+Hint: If this value is a Ref (value returned by `useRef()`), rename the variable to end in "Ref".
 ```
           
       

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.invalid-mutate-global-in-render-helper-phi-return-prop.expect.md

@@ -0,0 +1,44 @@
+
+## Input
+
+```javascript
+function Component() {
+  const renderItem = item => {
+    // Multiple returns so that the return type is a Phi (union)
+    if (item == null) {
+      return null;
+    }
+    // Normally we assume that it's safe to mutate globals in a function passed
+    // as a prop, because the prop could be used as an event handler or effect.
+    // But if the function returns JSX we can assume it's a render helper, ie
+    // called during render, and thus it's unsafe to mutate globals or call
+    // other impure code.
+    global.property = true;
+    return <Item item={item} value={rand} />;
+  };
+  return <ItemList renderItem={renderItem} />;
+}
+
+```
+
+
+## Error
+
+```
+Found 1 error:
+
+Error: This value cannot be modified
+
+Modifying a variable defined outside a component or hook is not allowed. Consider using an effect.
+
+error.invalid-mutate-global-in-render-helper-phi-return-prop.ts:12:4
+  10 |     // called during render, and thus it's unsafe to mutate globals or call
+  11 |     // other impure code.
+> 12 |     global.property = true;
+     |     ^^^^^^ value cannot be modified
+  13 |     return <Item item={item} value={rand} />;
+  14 |   };
+  15 |   return <ItemList renderItem={renderItem} />;
+```
+          
+      

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.invalid-mutate-global-in-render-helper-phi-return-prop.js

@@ -0,0 +1,16 @@
+function Component() {
+  const renderItem = item => {
+    // Multiple returns so that the return type is a Phi (union)
+    if (item == null) {
+      return null;
+    }
+    // Normally we assume that it's safe to mutate globals in a function passed
+    // as a prop, because the prop could be used as an event handler or effect.
+    // But if the function returns JSX we can assume it's a render helper, ie
+    // called during render, and thus it's unsafe to mutate globals or call
+    // other impure code.
+    global.property = true;
+    return <Item item={item} value={rand} />;
+  };
+  return <ItemList renderItem={renderItem} />;
+}

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.invalid-mutate-global-in-render-helper-prop.expect.md

@@ -0,0 +1,40 @@
+
+## Input
+
+```javascript
+function Component() {
+  const renderItem = item => {
+    // Normally we assume that it's safe to mutate globals in a function passed
+    // as a prop, because the prop could be used as an event handler or effect.
+    // But if the function returns JSX we can assume it's a render helper, ie
+    // called during render, and thus it's unsafe to mutate globals or call
+    // other impure code.
+    global.property = true;
+    return <Item item={item} value={rand} />;
+  };
+  return <ItemList renderItem={renderItem} />;
+}
+
+```
+
+
+## Error
+
+```
+Found 1 error:
+
+Error: This value cannot be modified
+
+Modifying a variable defined outside a component or hook is not allowed. Consider using an effect.
+
+error.invalid-mutate-global-in-render-helper-prop.ts:8:4
+   6 |     // called during render, and thus it's unsafe to mutate globals or call
+   7 |     // other impure code.
+>  8 |     global.property = true;
+     |     ^^^^^^ value cannot be modified
+   9 |     return <Item item={item} value={rand} />;
+  10 |   };
+  11 |   return <ItemList renderItem={renderItem} />;
+```
+          
+      

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.invalid-mutate-global-in-render-helper-prop.js

@@ -0,0 +1,12 @@
+function Component() {
+  const renderItem = item => {
+    // Normally we assume that it's safe to mutate globals in a function passed
+    // as a prop, because the prop could be used as an event handler or effect.
+    // But if the function returns JSX we can assume it's a render helper, ie
+    // called during render, and thus it's unsafe to mutate globals or call
+    // other impure code.
+    global.property = true;
+    return <Item item={item} value={rand} />;
+  };
+  return <ItemList renderItem={renderItem} />;
+}

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.invalid-optional-member-expression-as-memo-dep-non-optional-in-body.expect.md

@@ -20,7 +20,7 @@ function Component(props) {
 ```
 Found 1 error:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. The inferred dependencies did not match the manually specified dependencies, which could cause the value to change more or less frequently than expected. The inferred dependency was `props.items.edges.nodes`, but the source dependencies were [props.items?.edges?.nodes]. Inferred different dependency than source.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.invalid-useEffect-dep-not-memoized-bc-range-overlaps-hook.expect.md

@@ -25,7 +25,7 @@ function Component(props) {
 ```
 Found 1 error:
 
-Memoization: React Compiler has skipped optimizing this component because the effect dependencies could not be memoized. Unmemoized effect dependencies can trigger an infinite loop or other unexpected behavior
+Compilation Skipped: React Compiler has skipped optimizing this component because the effect dependencies could not be memoized. Unmemoized effect dependencies can trigger an infinite loop or other unexpected behavior
 
 error.invalid-useEffect-dep-not-memoized-bc-range-overlaps-hook.ts:9:2
    7 |

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.invalid-useEffect-dep-not-memoized.expect.md

@@ -22,7 +22,7 @@ function Component(props) {
 ```
 Found 1 error:
 
-Memoization: React Compiler has skipped optimizing this component because the effect dependencies could not be memoized. Unmemoized effect dependencies can trigger an infinite loop or other unexpected behavior
+Compilation Skipped: React Compiler has skipped optimizing this component because the effect dependencies could not be memoized. Unmemoized effect dependencies can trigger an infinite loop or other unexpected behavior
 
 error.invalid-useEffect-dep-not-memoized.ts:6:2
    4 | function Component(props) {

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.invalid-useInsertionEffect-dep-not-memoized.expect.md

@@ -22,7 +22,7 @@ function Component(props) {
 ```
 Found 1 error:
 
-Memoization: React Compiler has skipped optimizing this component because the effect dependencies could not be memoized. Unmemoized effect dependencies can trigger an infinite loop or other unexpected behavior
+Compilation Skipped: React Compiler has skipped optimizing this component because the effect dependencies could not be memoized. Unmemoized effect dependencies can trigger an infinite loop or other unexpected behavior
 
 error.invalid-useInsertionEffect-dep-not-memoized.ts:6:2
    4 | function Component(props) {

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.invalid-useLayoutEffect-dep-not-memoized.expect.md

@@ -22,7 +22,7 @@ function Component(props) {
 ```
 Found 1 error:
 
-Memoization: React Compiler has skipped optimizing this component because the effect dependencies could not be memoized. Unmemoized effect dependencies can trigger an infinite loop or other unexpected behavior
+Compilation Skipped: React Compiler has skipped optimizing this component because the effect dependencies could not be memoized. Unmemoized effect dependencies can trigger an infinite loop or other unexpected behavior
 
 error.invalid-useLayoutEffect-dep-not-memoized.ts:6:2
    4 | function Component(props) {

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.nested-component-in-normal-function.expect.md

@@ -0,0 +1,54 @@
+
+## Input
+
+```javascript
+// @validateNoDynamicallyCreatedComponentsOrHooks
+export function getInput(a) {
+  const Wrapper = () => {
+    const handleChange = () => {
+      a.onChange();
+    };
+
+    return <input onChange={handleChange} />;
+  };
+
+  return Wrapper;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: getInput,
+  isComponent: false,
+  params: [{onChange() {}}],
+};
+
+```
+
+
+## Error
+
+```
+Found 1 error:
+
+Error: Components and hooks cannot be created dynamically
+
+The function `Wrapper` appears to be a React component, but it's defined inside `getInput`. Components and Hooks should always be declared at module scope
+
+error.nested-component-in-normal-function.ts:2:16
+  1 | // @validateNoDynamicallyCreatedComponentsOrHooks
+> 2 | export function getInput(a) {
+    |                 ^^^^^^^^ this function dynamically created a component/hook
+  3 |   const Wrapper = () => {
+  4 |     const handleChange = () => {
+  5 |       a.onChange();
+
+error.nested-component-in-normal-function.ts:3:8
+  1 | // @validateNoDynamicallyCreatedComponentsOrHooks
+  2 | export function getInput(a) {
+> 3 |   const Wrapper = () => {
+    |         ^^^^^^^ the component is created here
+  4 |     const handleChange = () => {
+  5 |       a.onChange();
+  6 |     };
+```
+          
+      

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.nested-component-in-normal-function.js

@@ -0,0 +1,18 @@
+// @validateNoDynamicallyCreatedComponentsOrHooks
+export function getInput(a) {
+  const Wrapper = () => {
+    const handleChange = () => {
+      a.onChange();
+    };
+
+    return <input onChange={handleChange} />;
+  };
+
+  return Wrapper;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: getInput,
+  isComponent: false,
+  params: [{onChange() {}}],
+};

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.nested-hook-in-normal-function.expect.md

@@ -0,0 +1,59 @@
+
+## Input
+
+```javascript
+// @validateNoDynamicallyCreatedComponentsOrHooks
+import {useState} from 'react';
+
+function createCustomHook(config) {
+  function useConfiguredState() {
+    const [state, setState] = useState(0);
+
+    const increment = () => {
+      setState(state + config.step);
+    };
+
+    return [state, increment];
+  }
+
+  return useConfiguredState;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: createCustomHook,
+  isComponent: false,
+  params: [{step: 1}],
+};
+
+```
+
+
+## Error
+
+```
+Found 1 error:
+
+Error: Components and hooks cannot be created dynamically
+
+The function `useConfiguredState` appears to be a React hook, but it's defined inside `createCustomHook`. Components and Hooks should always be declared at module scope
+
+error.nested-hook-in-normal-function.ts:4:9
+  2 | import {useState} from 'react';
+  3 |
+> 4 | function createCustomHook(config) {
+    |          ^^^^^^^^^^^^^^^^ this function dynamically created a component/hook
+  5 |   function useConfiguredState() {
+  6 |     const [state, setState] = useState(0);
+  7 |
+
+error.nested-hook-in-normal-function.ts:5:11
+  3 |
+  4 | function createCustomHook(config) {
+> 5 |   function useConfiguredState() {
+    |            ^^^^^^^^^^^^^^^^^^ the component is created here
+  6 |     const [state, setState] = useState(0);
+  7 |
+  8 |     const increment = () => {
+```
+          
+      

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.nested-hook-in-normal-function.js

@@ -0,0 +1,22 @@
+// @validateNoDynamicallyCreatedComponentsOrHooks
+import {useState} from 'react';
+
+function createCustomHook(config) {
+  function useConfiguredState() {
+    const [state, setState] = useState(0);
+
+    const increment = () => {
+      setState(state + config.step);
+    };
+
+    return [state, increment];
+  }
+
+  return useConfiguredState;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: createCustomHook,
+  isComponent: false,
+  params: [{step: 1}],
+};

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.ref-like-name-not-Ref.expect.md

@@ -33,7 +33,7 @@ export const FIXTURE_ENTRYPOINT = {
 ```
 Found 1 error:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. The inferred dependencies did not match the manually specified dependencies, which could cause the value to change more or less frequently than expected. The inferred dependency was `Ref.current`, but the source dependencies were []. Inferred dependency not present in source.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.ref-like-name-not-a-ref.expect.md

@@ -33,7 +33,7 @@ export const FIXTURE_ENTRYPOINT = {
 ```
 Found 1 error:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. The inferred dependencies did not match the manually specified dependencies, which could cause the value to change more or less frequently than expected. The inferred dependency was `notaref.current`, but the source dependencies were []. Inferred dependency not present in source.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.todo-repro-missed-memoization-from-capture-in-invoked-function-inferred-as-mutation.expect.md

@@ -44,7 +44,7 @@ component Component() {
 ```
 Found 1 error:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. This value was memoized in source but not in compilation output.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.todo-repro-missed-memoization-from-inferred-mutation-in-logger.expect.md

@@ -54,7 +54,7 @@ component Component(id) {
 ```
 Found 3 errors:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. This value was memoized in source but not in compilation output.
 
@@ -76,7 +76,7 @@ React Compiler has skipped optimizing this component because the existing manual
   18 |   const setCurrentIndex = useCallback(
   19 |     (index: number) => {
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. This dependency may be mutated later, which could cause the value to change unexpectedly.
 
@@ -88,7 +88,7 @@ React Compiler has skipped optimizing this component because the existing manual
   30 |
   31 |   if (prevId !== id) {
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. This value was memoized in source but not in compilation output.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.todo-repro-unmemoized-callback-captured-in-context-variable.expect.md

@@ -52,7 +52,7 @@ export const FIXTURE_ENTRYPOINT = {
 ```
 Found 1 error:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. This value was memoized in source but not in compilation output.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.validate-memoized-effect-deps-invalidated-dep-value.expect.md

@@ -30,7 +30,7 @@ export const FIXTURE_ENTRYPOINT = {
 ```
 Found 1 error:
 
-Memoization: React Compiler has skipped optimizing this component because the effect dependencies could not be memoized. Unmemoized effect dependencies can trigger an infinite loop or other unexpected behavior
+Compilation Skipped: React Compiler has skipped optimizing this component because the effect dependencies could not be memoized. Unmemoized effect dependencies can trigger an infinite loop or other unexpected behavior
 
 error.validate-memoized-effect-deps-invalidated-dep-value.ts:11:2
    9 |   const y = [x];

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.validate-object-entries-mutation.expect.md

@@ -27,7 +27,7 @@ export const FIXTURE_ENTRYPOINT = {
 ```
 Found 2 errors:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. This dependency may be mutated later, which could cause the value to change unexpectedly.
 
@@ -40,7 +40,7 @@ error.validate-object-entries-mutation.ts:6:57
   8 |     value.updated = true;
   9 |   });
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. This value was memoized in source but not in compilation output.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.validate-object-values-mutation.expect.md

@@ -27,7 +27,7 @@ export const FIXTURE_ENTRYPOINT = {
 ```
 Found 2 errors:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. This dependency may be mutated later, which could cause the value to change unexpectedly.
 
@@ -40,7 +40,7 @@ error.validate-object-values-mutation.ts:6:55
   8 |     value.updated = true;
   9 |   });
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. This value was memoized in source but not in compilation output.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/gating/dynamic-gating-bailout-nopanic.expect.md

@@ -58,7 +58,7 @@ export const FIXTURE_ENTRYPOINT = {
 ## Logs
 
 ```
-{"kind":"CompileError","fnLoc":{"start":{"line":6,"column":0,"index":206},"end":{"line":16,"column":1,"index":433},"filename":"dynamic-gating-bailout-nopanic.ts"},"detail":{"options":{"category":"PreserveManualMemo","severity":"CannotPreserveMemoization","reason":"Compilation skipped because existing memoization could not be preserved","description":"React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. The inferred dependencies did not match the manually specified dependencies, which could cause the value to change more or less frequently than expected. The inferred dependency was `value`, but the source dependencies were []. Inferred dependency not present in source.","suggestions":null,"details":[{"kind":"error","loc":{"start":{"line":9,"column":31,"index":288},"end":{"line":9,"column":52,"index":309},"filename":"dynamic-gating-bailout-nopanic.ts"},"message":"Could not preserve existing manual memoization"}]}}}
+{"kind":"CompileError","fnLoc":{"start":{"line":6,"column":0,"index":206},"end":{"line":16,"column":1,"index":433},"filename":"dynamic-gating-bailout-nopanic.ts"},"detail":{"options":{"category":"PreserveManualMemo","severity":"CannotPreserveMemoization","reason":"Existing memoization could not be preserved","description":"React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. The inferred dependencies did not match the manually specified dependencies, which could cause the value to change more or less frequently than expected. The inferred dependency was `value`, but the source dependencies were []. Inferred dependency not present in source.","suggestions":null,"details":[{"kind":"error","loc":{"start":{"line":9,"column":31,"index":288},"end":{"line":9,"column":52,"index":309},"filename":"dynamic-gating-bailout-nopanic.ts"},"message":"Could not preserve existing manual memoization"}]}}}
 ```
       
 ### Eval output

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/memoize-primitive-function-calls.expect.md

@@ -0,0 +1,107 @@
+
+## Input
+
+```javascript
+// @compilationMode:"infer" @enablePreserveExistingMemoizationGuarantees @validatePreserveExistingMemoizationGuarantees
+import {useMemo} from 'react';
+import {makeObject_Primitives, ValidateMemoization} from 'shared-runtime';
+
+function Component(props) {
+  const result = useMemo(() => {
+    return makeObject(props.value).value + 1;
+  }, [props.value]);
+  return <ValidateMemoization inputs={[props.value]} output={result} />;
+}
+
+function makeObject(value) {
+  console.log(value);
+  return {value};
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{value: 42}],
+  sequentialRenders: [
+    {value: 42},
+    {value: 42},
+    {value: 3.14},
+    {value: 3.14},
+    {value: 42},
+    {value: 3.14},
+    {value: 42},
+    {value: 3.14},
+  ],
+};
+
+```
+
+## Code
+
+```javascript
+import { c as _c } from "react/compiler-runtime"; // @compilationMode:"infer" @enablePreserveExistingMemoizationGuarantees @validatePreserveExistingMemoizationGuarantees
+import { useMemo } from "react";
+import { makeObject_Primitives, ValidateMemoization } from "shared-runtime";
+
+function Component(props) {
+  const $ = _c(7);
+  let t0;
+  if ($[0] !== props.value) {
+    t0 = makeObject(props.value);
+    $[0] = props.value;
+    $[1] = t0;
+  } else {
+    t0 = $[1];
+  }
+  const result = t0.value + 1;
+  let t1;
+  if ($[2] !== props.value) {
+    t1 = [props.value];
+    $[2] = props.value;
+    $[3] = t1;
+  } else {
+    t1 = $[3];
+  }
+  let t2;
+  if ($[4] !== result || $[5] !== t1) {
+    t2 = <ValidateMemoization inputs={t1} output={result} />;
+    $[4] = result;
+    $[5] = t1;
+    $[6] = t2;
+  } else {
+    t2 = $[6];
+  }
+  return t2;
+}
+
+function makeObject(value) {
+  console.log(value);
+  return { value };
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ value: 42 }],
+  sequentialRenders: [
+    { value: 42 },
+    { value: 42 },
+    { value: 3.14 },
+    { value: 3.14 },
+    { value: 42 },
+    { value: 3.14 },
+    { value: 42 },
+    { value: 3.14 },
+  ],
+};
+
+```
+      
+### Eval output
+(kind: ok) <div>{"inputs":[42],"output":43}</div>
+<div>{"inputs":[42],"output":43}</div>
+<div>{"inputs":[3.14],"output":4.140000000000001}</div>
+<div>{"inputs":[3.14],"output":4.140000000000001}</div>
+<div>{"inputs":[42],"output":43}</div>
+<div>{"inputs":[3.14],"output":4.140000000000001}</div>
+<div>{"inputs":[42],"output":43}</div>
+<div>{"inputs":[3.14],"output":4.140000000000001}</div>
+logs: [42,3.14,42,3.14,42,3.14]

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/memoize-primitive-function-calls.js

@@ -0,0 +1,30 @@
+// @compilationMode:"infer" @enablePreserveExistingMemoizationGuarantees @validatePreserveExistingMemoizationGuarantees
+import {useMemo} from 'react';
+import {makeObject_Primitives, ValidateMemoization} from 'shared-runtime';
+
+function Component(props) {
+  const result = useMemo(() => {
+    return makeObject(props.value).value + 1;
+  }, [props.value]);
+  return <ValidateMemoization inputs={[props.value]} output={result} />;
+}
+
+function makeObject(value) {
+  console.log(value);
+  return {value};
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{value: 42}],
+  sequentialRenders: [
+    {value: 42},
+    {value: 42},
+    {value: 3.14},
+    {value: 3.14},
+    {value: 42},
+    {value: 3.14},
+    {value: 42},
+    {value: 3.14},
+  ],
+};

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/meta-isms/repro-cx-namespace-nesting.expect.md

@@ -2,6 +2,7 @@
 ## Input
 
 ```javascript
+// @compilationMode:"infer"
 import {makeArray} from 'shared-runtime';
 
 function Component() {
@@ -30,7 +31,7 @@ export const FIXTURE_ENTRYPOINT = {
 ## Code
 
 ```javascript
-import { c as _c } from "react/compiler-runtime";
+import { c as _c } from "react/compiler-runtime"; // @compilationMode:"infer"
 import { makeArray } from "shared-runtime";
 
 function Component() {

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/meta-isms/repro-cx-namespace-nesting.js

@@ -1,3 +1,4 @@
+// @compilationMode:"infer"
 import {makeArray} from 'shared-runtime';
 
 function Component() {

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/mutation-within-jsx-and-break.expect.md

@@ -49,12 +49,12 @@ import {
 } from "shared-runtime";
 
 function useFoo(t0) {
-  const $ = _c(3);
+  const $ = _c(4);
   const { data } = t0;
   let obj;
   let myDiv = null;
-  bb0: if (data.cond) {
-    if ($[0] !== data.cond1) {
+  if ($[0] !== data.cond || $[1] !== data.cond1) {
+    bb0: if (data.cond) {
       obj = makeObject_Primitives();
       if (data.cond1) {
         myDiv = <Stringify value={mutateAndReturn(obj)} />;
@@ -62,13 +62,14 @@ function useFoo(t0) {
       }
 
       mutate(obj);
-      $[0] = data.cond1;
-      $[1] = obj;
-      $[2] = myDiv;
-    } else {
-      obj = $[1];
-      myDiv = $[2];
     }
+    $[0] = data.cond;
+    $[1] = data.cond1;
+    $[2] = obj;
+    $[3] = myDiv;
+  } else {
+    obj = $[2];
+    myDiv = $[3];
   }
   return myDiv;
 }

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/new-mutability/error.invalid-useCallback-captures-reassigned-context.expect.md

@@ -31,7 +31,7 @@ export const FIXTURE_ENTRYPOINT = {
 ```
 Found 2 errors:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. This dependency may be mutated later, which could cause the value to change unexpectedly.
 
@@ -44,7 +44,7 @@ error.invalid-useCallback-captures-reassigned-context.ts:11:37
   13 |   x = makeArray();
   14 |
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. This value was memoized in source but not in compilation output.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/preserve-existing-memoization-guarantees/lambda-with-fbt-preserve-memoization.expect.md

@@ -0,0 +1,86 @@
+
+## Input
+
+```javascript
+// @enablePreserveExistingMemoizationGuarantees
+import {fbt} from 'fbt';
+
+function Component() {
+  const buttonLabel = () => {
+    if (!someCondition) {
+      return <fbt desc="My label">{'Purchase as a gift'}</fbt>;
+    } else if (
+      !iconOnly &&
+      showPrice &&
+      item?.current_gift_offer?.price?.formatted != null
+    ) {
+      return (
+        <fbt desc="Gift button's label">
+          {'Gift | '}
+          <fbt:param name="price">
+            {item?.current_gift_offer?.price?.formatted}
+          </fbt:param>
+        </fbt>
+      );
+    } else if (!iconOnly && !showPrice) {
+      return <fbt desc="Gift button's label">{'Gift'}</fbt>;
+    }
+  };
+
+  return (
+    <View>
+      <Button text={buttonLabel()} />
+    </View>
+  );
+}
+
+```
+
+## Code
+
+```javascript
+import { c as _c } from "react/compiler-runtime"; // @enablePreserveExistingMemoizationGuarantees
+import { fbt } from "fbt";
+
+function Component() {
+  const $ = _c(1);
+  const buttonLabel = _temp;
+  let t0;
+  if ($[0] === Symbol.for("react.memo_cache_sentinel")) {
+    t0 = (
+      <View>
+        <Button text={buttonLabel()} />
+      </View>
+    );
+    $[0] = t0;
+  } else {
+    t0 = $[0];
+  }
+  return t0;
+}
+function _temp() {
+  if (!someCondition) {
+    return fbt._("Purchase as a gift", null, { hk: "1gHj4g" });
+  } else {
+    if (
+      !iconOnly &&
+      showPrice &&
+      item?.current_gift_offer?.price?.formatted != null
+    ) {
+      return fbt._(
+        "Gift | {price}",
+        [fbt._param("price", item?.current_gift_offer?.price?.formatted)],
+        { hk: "3GTnGE" },
+      );
+    } else {
+      if (!iconOnly && !showPrice) {
+        return fbt._("Gift", null, { hk: "3fqfrk" });
+      }
+    }
+  }
+}
+
+```
+      
+### Eval output
+(kind: exception) Fixture not implemented

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/preserve-existing-memoization-guarantees/lambda-with-fbt-preserve-memoization.js

@@ -0,0 +1,31 @@
+// @enablePreserveExistingMemoizationGuarantees
+import {fbt} from 'fbt';
+
+function Component() {
+  const buttonLabel = () => {
+    if (!someCondition) {
+      return <fbt desc="My label">{'Purchase as a gift'}</fbt>;
+    } else if (
+      !iconOnly &&
+      showPrice &&
+      item?.current_gift_offer?.price?.formatted != null
+    ) {
+      return (
+        <fbt desc="Gift button's label">
+          {'Gift | '}
+          <fbt:param name="price">
+            {item?.current_gift_offer?.price?.formatted}
+          </fbt:param>
+        </fbt>
+      );
+    } else if (!iconOnly && !showPrice) {
+      return <fbt desc="Gift button's label">{'Gift'}</fbt>;
+    }
+  };
+
+  return (
+    <View>
+      <Button text={buttonLabel()} />
+    </View>
+  );
+}

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/preserve-memo-validation/error.false-positive-useMemo-dropped-infer-always-invalidating.expect.md

@@ -32,7 +32,7 @@ export const FIXTURE_ENTRYPOINT = {
 ```
 Found 1 error:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. This value was memoized in source but not in compilation output.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/preserve-memo-validation/error.false-positive-useMemo-infer-mutate-deps.expect.md

@@ -31,7 +31,7 @@ export const FIXTURE_ENTRYPOINT = {
 ```
 Found 1 error:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. This dependency may be mutated later, which could cause the value to change unexpectedly.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/preserve-memo-validation/error.false-positive-useMemo-overlap-scopes.expect.md

@@ -42,7 +42,7 @@ export const FIXTURE_ENTRYPOINT = {
 ```
 Found 1 error:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. This dependency may be mutated later, which could cause the value to change unexpectedly.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/preserve-memo-validation/error.hoist-useCallback-conditional-access-own-scope.expect.md

@@ -28,7 +28,7 @@ export const FIXTURE_ENTRYPOINT = {
 ```
 Found 1 error:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. The inferred dependencies did not match the manually specified dependencies, which could cause the value to change more or less frequently than expected. The inferred dependency was `propB`, but the source dependencies were [propA, propB.x.y]. Inferred less specific property than source.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/preserve-memo-validation/error.hoist-useCallback-infer-conditional-value-block.expect.md

@@ -31,7 +31,7 @@ export const FIXTURE_ENTRYPOINT = {
 ```
 Found 2 errors:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. The inferred dependencies did not match the manually specified dependencies, which could cause the value to change more or less frequently than expected. The inferred dependency was `propA`, but the source dependencies were [propA.a, propB.x.y]. Inferred less specific property than source.
 
@@ -60,7 +60,7 @@ error.hoist-useCallback-infer-conditional-value-block.ts:6:21
   16 |
   17 | export const FIXTURE_ENTRYPOINT = {
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. The inferred dependencies did not match the manually specified dependencies, which could cause the value to change more or less frequently than expected. The inferred dependency was `propB`, but the source dependencies were [propA.a, propB.x.y]. Inferred less specific property than source.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/preserve-memo-validation/error.invalid-useCallback-captures-reassigned-context.expect.md

@@ -32,7 +32,7 @@ export const FIXTURE_ENTRYPOINT = {
 ```
 Found 2 errors:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. This dependency may be mutated later, which could cause the value to change unexpectedly.
 
@@ -45,7 +45,7 @@ error.invalid-useCallback-captures-reassigned-context.ts:12:37
   14 |   x = makeArray();
   15 |
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. This value was memoized in source but not in compilation output.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/preserve-memo-validation/error.preserve-use-memo-ref-missing-reactive.expect.md

@@ -30,7 +30,7 @@ export const FIXTURE_ENTRYPOINT = {
 ```
 Found 1 error:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. The inferred dependencies did not match the manually specified dependencies, which could cause the value to change more or less frequently than expected. The inferred dependency was `ref`, but the source dependencies were []. Inferred dependency not present in source.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/preserve-memo-validation/error.todo-useCallback-captures-invalidating-value.expect.md

@@ -30,7 +30,7 @@ export const FIXTURE_ENTRYPOINT = {
 ```
 Found 1 error:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. This value was memoized in source but not in compilation output.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/preserve-memo-validation/error.useCallback-aliased-var.expect.md

@@ -21,7 +21,7 @@ function useHook(x) {
 ```
 Found 1 error:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. The inferred dependencies did not match the manually specified dependencies, which could cause the value to change more or less frequently than expected. The inferred dependency was `aliasedX`, but the source dependencies were [x, aliasedProp]. Inferred different dependency than source.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/preserve-memo-validation/error.useCallback-conditional-access-noAlloc.expect.md

@@ -27,7 +27,7 @@ export const FIXTURE_ENTRYPOINT = {
 ```
 Found 1 error:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. The inferred dependencies did not match the manually specified dependencies, which could cause the value to change more or less frequently than expected. The inferred dependency was `propB?.x.y`, but the source dependencies were [propA, propB.x.y]. Inferred different dependency than source.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/preserve-memo-validation/error.useCallback-infer-less-specific-conditional-access.expect.md

@@ -26,7 +26,7 @@ function Component({propA, propB}) {
 ```
 Found 1 error:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. The inferred dependencies did not match the manually specified dependencies, which could cause the value to change more or less frequently than expected. The inferred dependency was `propB`, but the source dependencies were [propA?.a, propB.x.y]. Inferred less specific property than source.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/preserve-memo-validation/error.useCallback-property-call-dep.expect.md

@@ -19,7 +19,7 @@ function Component({propA}) {
 ```
 Found 1 error:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. The inferred dependencies did not match the manually specified dependencies, which could cause the value to change more or less frequently than expected. The inferred dependency was `propA`, but the source dependencies were [propA.x]. Inferred less specific property than source.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/preserve-memo-validation/error.useMemo-aliased-var.expect.md

@@ -21,7 +21,7 @@ function useHook(x) {
 ```
 Found 1 error:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. The inferred dependencies did not match the manually specified dependencies, which could cause the value to change more or less frequently than expected. The inferred dependency was `x`, but the source dependencies were [aliasedX, aliasedProp]. Inferred different dependency than source.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/preserve-memo-validation/error.useMemo-infer-less-specific-conditional-access.expect.md

@@ -26,7 +26,7 @@ function Component({propA, propB}) {
 ```
 Found 1 error:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. The inferred dependencies did not match the manually specified dependencies, which could cause the value to change more or less frequently than expected. The inferred dependency was `propB`, but the source dependencies were [propA?.a, propB.x.y]. Inferred less specific property than source.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/preserve-memo-validation/error.useMemo-infer-less-specific-conditional-value-block.expect.md

@@ -26,7 +26,7 @@ function Component({propA, propB}) {
 ```
 Found 2 errors:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. The inferred dependencies did not match the manually specified dependencies, which could cause the value to change more or less frequently than expected. The inferred dependency was `propA`, but the source dependencies were [propA.a, propB.x.y]. Inferred less specific property than source.
 
@@ -54,7 +54,7 @@ error.useMemo-infer-less-specific-conditional-value-block.ts:6:17
   15 | }
   16 |
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. The inferred dependencies did not match the manually specified dependencies, which could cause the value to change more or less frequently than expected. The inferred dependency was `propB`, but the source dependencies were [propA.a, propB.x.y]. Inferred less specific property than source.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/preserve-memo-validation/error.useMemo-property-call-chained-object.expect.md

@@ -21,7 +21,7 @@ function Component({propA}) {
 ```
 Found 1 error:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. The inferred dependencies did not match the manually specified dependencies, which could cause the value to change more or less frequently than expected. The inferred dependency was `propA`, but the source dependencies were [propA.x]. Inferred less specific property than source.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/preserve-memo-validation/error.useMemo-property-call-dep.expect.md

@@ -19,7 +19,7 @@ function Component({propA}) {
 ```
 Found 1 error:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. The inferred dependencies did not match the manually specified dependencies, which could cause the value to change more or less frequently than expected. The inferred dependency was `propA`, but the source dependencies were [propA.x]. Inferred less specific property than source.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/preserve-memo-validation/error.useMemo-unrelated-mutation-in-depslist.expect.md

@@ -32,7 +32,7 @@ function useFoo(input1) {
 ```
 Found 1 error:
 
-Memoization: Compilation skipped because existing memoization could not be preserved
+Compilation Skipped: Existing memoization could not be preserved
 
 React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. The inferred dependencies did not match the manually specified dependencies, which could cause the value to change more or less frequently than expected. The inferred dependency was `input1`, but the source dependencies were [y]. Inferred different dependency than source.
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/primitive-reassigned-loop-force-scopes-enabled.expect.md

@@ -2,7 +2,7 @@
 ## Input
 
 ```javascript
-// @enableForest
+// @enablePreserveExistingMemoizationGuarantees
 function Component({base, start, increment, test}) {
   let value = base;
   for (let i = start; i < test; i += increment) {
@@ -27,25 +27,23 @@ export const FIXTURE_ENTRYPOINT = {
 ## Code
 
 ```javascript
-import { c as _c } from "react/compiler-runtime"; // @enableForest
+import { c as _c } from "react/compiler-runtime"; // @enablePreserveExistingMemoizationGuarantees
 function Component(t0) {
-  const $ = _c(5);
+  const $ = _c(2);
   const { base, start, increment, test } = t0;
-  let value;
-  if ($[0] !== base || $[1] !== increment || $[2] !== start || $[3] !== test) {
-    value = base;
-    for (let i = start; i < test; i = i + increment, i) {
-      value = value + i;
-    }
-    $[0] = base;
-    $[1] = increment;
-    $[2] = start;
-    $[3] = test;
-    $[4] = value;
-  } else {
-    value = $[4];
+  let value = base;
+  for (let i = start; i < test; i = i + increment, i) {
+    value = value + i;
   }
-  return <div>{value}</div>;
+  let t1;
+  if ($[0] !== value) {
+    t1 = <div>{value}</div>;
+    $[0] = value;
+    $[1] = t1;
+  } else {
+    t1 = $[1];
+  }
+  return t1;
 }
 
 export const FIXTURE_ENTRYPOINT = {

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/primitive-reassigned-loop-force-scopes-enabled.js

@@ -1,4 +1,4 @@
-// @enableForest
+// @enablePreserveExistingMemoizationGuarantees
 function Component({base, start, increment, test}) {
   let value = base;
   for (let i = start; i < test; i += increment) {