[compiler][wip] Fix for ValidatePreserveMemo edge case w multiple return in useMemo

Potential fix for #34750. With a useMemo with multiple returns we can sometimes end up thinking the FinishMemoize value wasn't memoized, even though it definitely is. But the same case works if you rewrite to have a let binding, assign to the let instead of returning, and then have a single return with the let — which is the exact shape we convert to when we inline the useMemo! The only difference is that when we inline, we end up with an extra LoadLocal. Adding an extra LoadLocal "fixes" the issue but this is definitely a hack.

However, along the way this helped me find that we also have a bug in PruneAlwaysInvalidatingScopes, where we were forgetting to set FinishMemoize instructions to pruned if their declaration always invalidates.

Putting up for feedback, not necessarily expecting to land as-is.

.eslintrc.js

@@ -517,6 +517,14 @@ module.exports = {
         __IS_INTERNAL_VERSION__: 'readonly',
       },
     },
+    {
+      files: ['packages/react-devtools-*/**/*.js'],
+      excludedFiles: '**/__tests__/**/*.js',
+      plugins: ['eslint-plugin-react-hooks-published'],
+      rules: {
+        'react-hooks-published/rules-of-hooks': ERROR,
+      },
+    },
     {
       files: ['packages/eslint-plugin-react-hooks/src/**/*'],
       extends: ['plugin:@typescript-eslint/recommended'],

.github/workflows/compiler_prereleases.yml

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

.github/workflows/compiler_prereleases_manual.yml

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

.github/workflows/compiler_prereleases_nightly.yml

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

.github/workflows/runtime_prereleases.yml

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

CHANGELOG.md

@@ -1,3 +1,76 @@
+## 19.2.0 (October 1st, 2025)
+
+Below is a list of all new features, APIs, and bug fixes.
+
+Read the [React 19.2 release post](https://react.dev/blog/2025/10/01/react-19-2) for more information.
+
+### New React Features
+
+- [`<Activity>`](https://react.dev/reference/react/Activity): A new API to hide and restore the UI and internal state of its children.
+- [`useEffectEvent`](https://react.dev/reference/react/useEffectEvent) is a React Hook that lets you extract non-reactive logic into an [Effect Event](https://react.dev/learn/separating-events-from-effects#declaring-an-effect-event).
+- [`cacheSignal`](https://react.dev/reference/react/cacheSignal) (for RSCs) lets your know when the `cache()` lifetime is over.
+- [React Performance tracks](https://react.dev/reference/dev-tools/react-performance-tracks) appear on the Performance panel’s timeline in your browser developer tools
+
+### New React DOM Features
+
+- Added resume APIs for partial pre-rendering with Web Streams:
+  - [`resume`](https://react.dev/reference/react-dom/server/resume): to resume a prerender to a stream.
+  - [`resumeAndPrerender`](https://react.dev/reference/react-dom/static/resumeAndPrerender): to resume a prerender to HTML.
+- Added resume APIs for partial pre-rendering with Node Streams:
+  - [`resumeToPipeableStream`](https://react.dev/reference/react-dom/server/resumeToPipeableStream): to resume a prerender to a stream.
+  - [`resumeAndPrerenderToNodeStream`](https://react.dev/reference/react-dom/static/resumeAndPrerenderToNodeStream): to resume a prerender to HTML.
+- Updated [`prerender`](https://react.dev/reference/react-dom/static/prerender) APIs to return a `postponed` state that can be passed to the `resume` APIs.
+
+### Notable changes
+
+- React DOM now batches suspense boundary reveals, matching the behavior of client side rendering. This change is especially noticeable when animating the reveal of Suspense boundaries e.g. with the upcoming `<ViewTransition>` Component. React will batch as much reveals as possible before the first paint while trying to hit popular first-contentful paint metrics.
+- Add Node Web Streams (`prerender`, `renderToReadableStream`) to server-side-rendering APIs for Node.js
+- Use underscore instead of `:` IDs generated by useId
+
+### All Changes
+
+#### React
+
+- `<Activity />` was developed over many years, starting before `ClassComponent.setState` (@acdlite @sebmarkbage and many others)
+- Stringify context as "SomeContext" instead of "SomeContext.Provider" (@kassens [#33507](https://github.com/facebook/react/pull/33507))
+- Include stack of cause of React instrumentation errors with `%o` placeholder (@eps1lon [#34198](https://github.com/facebook/react/pull/34198))
+- Fix infinite `useDeferredValue` loop in popstate event (@acdlite [#32821](https://github.com/facebook/react/pull/32821))
+- Fix a bug when an initial value was passed to `useDeferredValue` (@acdlite [#34376](https://github.com/facebook/react/pull/34376))
+- Fix a crash when submitting forms with Client Actions (@sebmarkbage [#33055](https://github.com/facebook/react/pull/33055))
+- Hide/unhide the content of dehydrated suspense boundaries if they resuspend (@sebmarkbage [#32900](https://github.com/facebook/react/pull/32900))
+- Avoid stack overflow on wide trees during Hot Reload (@sophiebits [#34145](https://github.com/facebook/react/pull/34145))
+- Improve Owner and Component stacks in various places (@sebmarkbage, @eps1lon: [#33629](https://github.com/facebook/react/pull/33629), [#33724](https://github.com/facebook/react/pull/33724), [#32735](https://github.com/facebook/react/pull/32735), [#33723](https://github.com/facebook/react/pull/33723))
+- Add `cacheSignal` (@sebmarkbage [#33557](https://github.com/facebook/react/pull/33557))
+
+#### React DOM
+
+- Block on Suspensey Fonts during reveal of server-side-rendered content (@sebmarkbage [#33342](https://github.com/facebook/react/pull/33342))
+- Use underscore instead of `:` for IDs generated by `useId` (@sebmarkbage, @eps1lon: [#32001](https://github.com/facebook/react/pull/32001), [https://github.com/facebook/react/pull/33342](https://github.com/facebook/react/pull/33342)[#33099](https://github.com/facebook/react/pull/33099), [#33422](https://github.com/facebook/react/pull/33422))
+- Stop warning when ARIA 1.3 attributes are used (@Abdul-Omira [#34264](https://github.com/facebook/react/pull/34264))
+- Allow `nonce` to be used on hoistable styles (@Andarist [#32461](https://github.com/facebook/react/pull/32461))
+- Warn for using a React owned node as a Container if it also has text content (@sebmarkbage [#32774](https://github.com/facebook/react/pull/32774))
+- s/HTML/text for for error messages if text hydration mismatches (@rickhanlonii [#32763](https://github.com/facebook/react/pull/32763))
+- Fix a bug with `React.use` inside `React.lazy`\-ed Component (@hi-ogawa [#33941](https://github.com/facebook/react/pull/33941))
+- Enable the `progressiveChunkSize` option for server-side-rendering APIs (@sebmarkbage [#33027](https://github.com/facebook/react/pull/33027))
+- Fix a bug with deeply nested Suspense inside Suspense fallback when server-side-rendering (@gnoff [#33467](https://github.com/facebook/react/pull/33467))
+- Avoid hanging when suspending after aborting while rendering (@gnoff [#34192](https://github.com/facebook/react/pull/34192))
+- Add Node Web Streams to server-side-rendering APIs for Node.js (@sebmarkbage [#33475](https://github.com/facebook/react/pull/33475))
+
+#### React Server Components
+
+- Preload `<img>` and `<link>` using hints before they're rendered (@sebmarkbage [#34604](https://github.com/facebook/react/pull/34604))
+- Log error if production elements are rendered during development (@eps1lon [#34189](https://github.com/facebook/react/pull/34189))
+- Fix a bug when returning a Temporary reference (e.g. a Client Reference) from Server Functions (@sebmarkbage [#34084](https://github.com/facebook/react/pull/34084), @denk0403 [#33761](https://github.com/facebook/react/pull/33761))
+- Pass line/column to `filterStackFrame` (@eps1lon [#33707](https://github.com/facebook/react/pull/33707))
+- Support Async Modules in Turbopack Server References (@lubieowoce [#34531](https://github.com/facebook/react/pull/34531))
+- Add support for .mjs file extension in Webpack (@jennyscript [#33028](https://github.com/facebook/react/pull/33028))
+- Fix a wrong missing key warning (@unstubbable [#34350](https://github.com/facebook/react/pull/34350))
+- Make console log resolve in predictable order (@sebmarkbage [#33665](https://github.com/facebook/react/pull/33665))
+
+#### React Reconciler
+
+- [createContainer](https://github.com/facebook/react/blob/v19.2.0/packages/react-reconciler/src/ReactFiberReconciler.js#L255-L261) and [createHydrationContainer](https://github.com/facebook/react/blob/v19.2.0/packages/react-reconciler/src/ReactFiberReconciler.js#L305-L312) had their parameter order adjusted after `on*` handlers to account for upcoming experimental APIs
+
 ## 19.1.1 (July 28, 2025)
 
 ### React

ReactVersions.js

@@ -7,18 +7,18 @@
 //
 // The @latest channel uses the version as-is, e.g.:
 //
-//   19.1.0
+//   19.3.0
 //
 // The @canary channel appends additional information, with the scheme
 // <version>-<label>-<commit_sha>, e.g.:
 //
-//   19.1.0-canary-a1c2d3e4
+//   19.3.0-canary-a1c2d3e4
 //
 // The @experimental channel doesn't include a version, only a date and a sha, e.g.:
 //
 //   0.0.0-experimental-241c4467e-20200129
 
-const ReactVersion = '19.2.0';
+const ReactVersion = '19.3.0';
 
 // The label used by the @canary channel. Represents the upcoming release's
 // stability. Most of the time, this will be "canary", but we may temporarily
@@ -33,8 +33,8 @@ const canaryChannelLabel = 'canary';
 const rcNumber = 0;
 
 const stablePackages = {
-  'eslint-plugin-react-hooks': '6.1.0',
-  'jest-react': '0.17.0',
+  'eslint-plugin-react-hooks': '7.0.0',
+  'jest-react': '0.18.0',
   react: ReactVersion,
   'react-art': ReactVersion,
   'react-dom': ReactVersion,
@@ -42,12 +42,12 @@ const stablePackages = {
   'react-server-dom-turbopack': ReactVersion,
   'react-server-dom-parcel': ReactVersion,
   'react-is': ReactVersion,
-  'react-reconciler': '0.33.0',
-  'react-refresh': '0.18.0',
+  'react-reconciler': '0.34.0',
+  'react-refresh': '0.19.0',
   'react-test-renderer': ReactVersion,
-  'use-subscription': '1.12.0',
-  'use-sync-external-store': '1.6.0',
-  scheduler: '0.27.0',
+  'use-subscription': '1.13.0',
+  'use-sync-external-store': '1.7.0',
+  scheduler: '0.28.0',
 };
 
 // These packages do not exist in the @canary or @latest channel, only

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

@@ -23,7 +23,8 @@ function formatPrint(data: Array<string>): Promise<string> {
 
 async function expandConfigs(page: Page): Promise<void> {
   const expandButton = page.locator('[title="Expand config editor"]');
-  expandButton.click();
+  await expandButton.click();
+  await page.waitForSelector('.monaco-editor-config', {state: 'visible'});
 }
 
 const TEST_SOURCE = `export default function TestComponent({ x }) {
@@ -313,6 +314,36 @@ test('disableMemoizationForDebugging flag works as expected', async ({
   expect(output).toMatchSnapshot('disableMemoizationForDebugging-output.txt');
 });
 
+test('error is displayed when source has syntax error', async ({page}) => {
+  const syntaxErrorSource = `function TestComponent(props) {
+  const oops = props.
+  return (
+    <>{oops}</>
+  );
+}`;
+  const store: Store = {
+    source: syntaxErrorSource,
+    config: defaultConfig,
+    showInternals: false,
+  };
+  const hash = encodeStore(store);
+  await page.goto(`/#${hash}`);
+  await page.waitForFunction(isMonacoLoaded);
+  await expandConfigs(page);
+  await page.screenshot({
+    fullPage: true,
+    path: 'test-results/08-source-syntax-error.png',
+  });
+
+  const text =
+    (await page.locator('.monaco-editor-output').allInnerTexts()) ?? [];
+  const output = text.join('');
+
+  expect(output.replace(/\s+/g, ' ')).toContain(
+    'Expected identifier to be defined before being used',
+  );
+});
+
 TEST_CASE_INPUTS.forEach((t, idx) =>
   test(`playground compiles: ${t.name}`, async ({page}) => {
     const store: Store = {

compiler/apps/playground/components/AccordionWindow.tsx

@@ -6,7 +6,13 @@
  */
 
 import {Resizable} from 're-resizable';
-import React, {useCallback} from 'react';
+import React, {
+  useId,
+  unstable_ViewTransition as ViewTransition,
+  unstable_addTransitionType as addTransitionType,
+  startTransition,
+} from 'react';
+import {EXPAND_ACCORDION_TRANSITION} from '../lib/transitionTypes';
 
 type TabsRecord = Map<string, React.ReactNode>;
 
@@ -18,19 +24,21 @@ export default function AccordionWindow(props: {
   changedPasses: Set<string>;
 }): React.ReactElement {
   return (
-    <div className="flex flex-row h-full">
-      {Array.from(props.tabs.keys()).map(name => {
-        return (
-          <AccordionWindowItem
-            name={name}
-            key={name}
-            tabs={props.tabs}
-            tabsOpen={props.tabsOpen}
-            setTabsOpen={props.setTabsOpen}
-            hasChanged={props.changedPasses.has(name)}
-          />
-        );
-      })}
+    <div className="flex-1 min-w-[550px] sm:min-w-0">
+      <div className="flex flex-row h-full">
+        {Array.from(props.tabs.keys()).map(name => {
+          return (
+            <AccordionWindowItem
+              name={name}
+              key={name}
+              tabs={props.tabs}
+              tabsOpen={props.tabsOpen}
+              setTabsOpen={props.setTabsOpen}
+              hasChanged={props.changedPasses.has(name)}
+            />
+          );
+        })}
+      </div>
     </div>
   );
 }
@@ -47,18 +55,25 @@ function AccordionWindowItem({
   tabsOpen: Set<string>;
   setTabsOpen: (newTab: Set<string>) => void;
   hasChanged: boolean;
+  isFailure: boolean;
 }): React.ReactElement {
+  const id = useId();
   const isShow = tabsOpen.has(name);
 
-  const toggleTabs = useCallback(() => {
-    const nextState = new Set(tabsOpen);
-    if (nextState.has(name)) {
-      nextState.delete(name);
-    } else {
-      nextState.add(name);
-    }
-    setTabsOpen(nextState);
-  }, [tabsOpen, name, setTabsOpen]);
+  const transitionName = `accordion-window-item-${id}`;
+
+  const toggleTabs = (): void => {
+    startTransition(() => {
+      addTransitionType(EXPAND_ACCORDION_TRANSITION);
+      const nextState = new Set(tabsOpen);
+      if (nextState.has(name)) {
+        nextState.delete(name);
+      } else {
+        nextState.add(name);
+      }
+      setTabsOpen(nextState);
+    });
+  };
 
   // Replace spaces with non-breaking spaces
   const displayName = name.replace(/ /g, '\u00A0');
@@ -66,31 +81,45 @@ function AccordionWindowItem({
   return (
     <div key={name} className="flex flex-row">
       {isShow ? (
-        <Resizable className="border-r" minWidth={550} enable={{right: true}}>
-          <h2
-            title="Minimize tab"
-            aria-label="Minimize tab"
-            onClick={toggleTabs}
-            className={`p-4 duration-150 ease-in border-b cursor-pointer border-grey-200 ${
-              hasChanged ? 'font-bold' : 'font-light'
-            } text-secondary hover:text-link`}>
-            - {displayName}
-          </h2>
-          {tabs.get(name) ?? <div>No output for {name}</div>}
-        </Resizable>
+        <ViewTransition
+          name={transitionName}
+          update={{
+            [EXPAND_ACCORDION_TRANSITION]: 'expand-accordion',
+            default: 'none',
+          }}>
+          <Resizable className="border-r" minWidth={550} enable={{right: true}}>
+            <h2
+              title="Minimize tab"
+              aria-label="Minimize tab"
+              onClick={toggleTabs}
+              className={`p-4 duration-150 ease-in border-b cursor-pointer border-grey-200 ${
+                hasChanged ? 'font-bold' : 'font-light'
+              } text-secondary hover:text-link`}>
+              - {displayName}
+            </h2>
+            {tabs.get(name) ?? <div>No output for {name}</div>}
+          </Resizable>
+        </ViewTransition>
       ) : (
-        <div className="relative items-center h-full px-1 py-6 align-middle border-r border-grey-200">
-          <button
-            title={`Expand compiler tab: ${name}`}
-            aria-label={`Expand compiler tab: ${name}`}
-            style={{transform: 'rotate(90deg) translate(-50%)'}}
-            onClick={toggleTabs}
-            className={`flex-grow-0 w-5 transition-colors duration-150 ease-in ${
-              hasChanged ? 'font-bold' : 'font-light'
-            } text-secondary hover:text-link`}>
-            {displayName}
-          </button>
-        </div>
+        <ViewTransition
+          name={transitionName}
+          update={{
+            [EXPAND_ACCORDION_TRANSITION]: 'expand-accordion',
+            default: 'none',
+          }}>
+          <div className="relative items-center h-full px-1 py-6 align-middle border-r border-grey-200">
+            <button
+              title={`Expand compiler tab: ${name}`}
+              aria-label={`Expand compiler tab: ${name}`}
+              style={{transform: 'rotate(90deg) translate(-50%)'}}
+              onClick={toggleTabs}
+              className={`flex-grow-0 w-5 transition-colors duration-150 ease-in ${
+                hasChanged ? 'font-bold' : 'font-light'
+              } text-secondary hover:text-link`}>
+              {displayName}
+            </button>
+          </div>
+        </ViewTransition>
       )}
     </div>
   );

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

@@ -6,15 +6,20 @@
  */
 
 import MonacoEditor, {loader, type Monaco} from '@monaco-editor/react';
-import {PluginOptions} from 'babel-plugin-react-compiler';
 import type {editor} from 'monaco-editor';
 import * as monaco from 'monaco-editor';
-import React, {useState, useRef} from 'react';
+import React, {
+  useState,
+  useRef,
+  unstable_ViewTransition as ViewTransition,
+  unstable_addTransitionType as addTransitionType,
+  startTransition,
+} from 'react';
 import {Resizable} from 're-resizable';
 import {useStore, useStoreDispatch} from '../StoreContext';
-import {monacoOptions} from './monacoOptions';
+import {monacoConfigOptions} from './monacoOptions';
 import {IconChevron} from '../Icons/IconChevron';
-import prettyFormat from 'pretty-format';
+import {CONFIG_PANEL_TRANSITION} from '../../lib/transitionTypes';
 
 // @ts-expect-error - webpack asset/source loader handles .d.ts files as strings
 import compilerTypeDefs from 'babel-plugin-react-compiler/dist/index.d.ts';
@@ -22,9 +27,9 @@ import compilerTypeDefs from 'babel-plugin-react-compiler/dist/index.d.ts';
 loader.config({monaco});
 
 export default function ConfigEditor({
-  appliedOptions,
+  formattedAppliedConfig,
 }: {
-  appliedOptions: PluginOptions | null;
+  formattedAppliedConfig: string;
 }): React.ReactElement {
   const [isExpanded, setIsExpanded] = useState(false);
 
@@ -36,15 +41,27 @@ export default function ConfigEditor({
           display: isExpanded ? 'block' : 'none',
         }}>
         <ExpandedEditor
-          onToggle={setIsExpanded}
-          appliedOptions={appliedOptions}
+          onToggle={() => {
+            startTransition(() => {
+              addTransitionType(CONFIG_PANEL_TRANSITION);
+              setIsExpanded(false);
+            });
+          }}
+          formattedAppliedConfig={formattedAppliedConfig}
         />
       </div>
       <div
         style={{
           display: !isExpanded ? 'block' : 'none',
         }}>
-        <CollapsedEditor onToggle={setIsExpanded} />
+        <CollapsedEditor
+          onToggle={() => {
+            startTransition(() => {
+              addTransitionType(CONFIG_PANEL_TRANSITION);
+              setIsExpanded(true);
+            });
+          }}
+        />
       </div>
     </>
   );
@@ -52,10 +69,10 @@ export default function ConfigEditor({
 
 function ExpandedEditor({
   onToggle,
-  appliedOptions,
+  formattedAppliedConfig,
 }: {
   onToggle: (expanded: boolean) => void;
-  appliedOptions: PluginOptions | null;
+  formattedAppliedConfig: string;
 }): React.ReactElement {
   const store = useStore();
   const dispatchStore = useStoreDispatch();
@@ -103,98 +120,78 @@ function ExpandedEditor({
     });
   };
 
-  const formattedAppliedOptions = appliedOptions
-    ? prettyFormat(appliedOptions, {
-        printFunctionName: false,
-        printBasicPrototype: false,
-      })
-    : 'Invalid configs';
-
   return (
-    <Resizable
-      minWidth={300}
-      maxWidth={600}
-      defaultSize={{width: 350}}
-      enable={{right: true, bottom: false}}>
-      <div className="bg-blue-10 relative h-full flex flex-col !h-[calc(100vh_-_3.5rem)] border border-gray-300">
-        <div
-          className="absolute w-8 h-16 bg-blue-10 rounded-r-full flex items-center justify-center z-[2] cursor-pointer border border-l-0 border-gray-300"
-          title="Minimize config editor"
-          onClick={() => onToggle(false)}
-          style={{
-            top: '50%',
-            marginTop: '-32px',
-            right: '-32px',
-            borderTopLeftRadius: 0,
-            borderBottomLeftRadius: 0,
-          }}>
-          <IconChevron displayDirection="left" className="text-blue-50" />
-        </div>
+    <ViewTransition
+      update={{[CONFIG_PANEL_TRANSITION]: 'slide-in', default: 'none'}}>
+      <Resizable
+        minWidth={300}
+        maxWidth={600}
+        defaultSize={{width: 350}}
+        enable={{right: true, bottom: false}}>
+        <div className="bg-blue-10 relative h-full flex flex-col !h-[calc(100vh_-_3.5rem)] border border-gray-300">
+          <div
+            className="absolute w-8 h-16 bg-blue-10 rounded-r-full flex items-center justify-center z-[2] cursor-pointer border border-l-0 border-gray-300"
+            title="Minimize config editor"
+            onClick={onToggle}
+            style={{
+              top: '50%',
+              marginTop: '-32px',
+              right: '-32px',
+              borderTopLeftRadius: 0,
+              borderBottomLeftRadius: 0,
+            }}>
+            <IconChevron displayDirection="left" className="text-blue-50" />
+          </div>
 
-        <div className="flex-1 flex flex-col m-2 mb-2">
-          <div className="pb-2">
-            <h2 className="inline-block text-blue-50 py-1.5 px-1.5 xs:px-3 sm:px-4 text-sm">
-              Config Overrides
-            </h2>
+          <div className="flex-1 flex flex-col m-2 mb-2">
+            <div className="pb-2">
+              <h2 className="inline-block text-blue-50 py-1.5 px-1.5 xs:px-3 sm:px-4 text-sm">
+                Config Overrides
+              </h2>
+            </div>
+            <div className="flex-1 border border-gray-300">
+              <MonacoEditor
+                path={'config.ts'}
+                language={'typescript'}
+                value={store.config}
+                onMount={handleMount}
+                onChange={handleChange}
+                loading={''}
+                className="monaco-editor-config"
+                options={monacoConfigOptions}
+              />
+            </div>
           </div>
-          <div className="flex-1 rounded-lg overflow-hidden border border-gray-300">
-            <MonacoEditor
-              path={'config.ts'}
-              language={'typescript'}
-              value={store.config}
-              onMount={handleMount}
-              onChange={handleChange}
-              loading={''}
-              className="monaco-editor-config"
-              options={{
-                ...monacoOptions,
-                lineNumbers: 'off',
-                renderLineHighlight: 'none',
-                overviewRulerBorder: false,
-                overviewRulerLanes: 0,
-                fontSize: 12,
-                scrollBeyondLastLine: false,
-                glyphMargin: false,
-              }}
-            />
+          <div className="flex-1 flex flex-col m-2">
+            <div className="pb-2">
+              <h2 className="inline-block text-blue-50 py-1.5 px-1.5 xs:px-3 sm:px-4 text-sm">
+                Applied Configs
+              </h2>
+            </div>
+            <div className="flex-1 border border-gray-300">
+              <MonacoEditor
+                path={'applied-config.js'}
+                language={'javascript'}
+                value={formattedAppliedConfig}
+                loading={''}
+                className="monaco-editor-applied-config"
+                options={{
+                  ...monacoConfigOptions,
+                  readOnly: true,
+                }}
+              />
+            </div>
           </div>
         </div>
-        <div className="flex-1 flex flex-col m-2">
-          <div className="pb-2">
-            <h2 className="inline-block text-blue-50 py-1.5 px-1.5 xs:px-3 sm:px-4 text-sm">
-              Applied Configs
-            </h2>
-          </div>
-          <div className="flex-1 rounded-lg overflow-hidden border border-gray-300">
-            <MonacoEditor
-              path={'applied-config.js'}
-              language={'javascript'}
-              value={formattedAppliedOptions}
-              loading={''}
-              className="monaco-editor-applied-config"
-              options={{
-                ...monacoOptions,
-                lineNumbers: 'off',
-                renderLineHighlight: 'none',
-                overviewRulerBorder: false,
-                overviewRulerLanes: 0,
-                fontSize: 12,
-                scrollBeyondLastLine: false,
-                readOnly: true,
-                glyphMargin: false,
-              }}
-            />
-          </div>
-        </div>
-      </div>
-    </Resizable>
+      </Resizable>
+    </ViewTransition>
   );
 }
 
 function CollapsedEditor({
   onToggle,
 }: {
-  onToggle: (expanded: boolean) => void;
+  onToggle: () => void;
 }): React.ReactElement {
   return (
     <div
@@ -203,7 +200,7 @@ function CollapsedEditor({
       <div
         className="absolute w-10 h-16 bg-blue-10 hover:translate-x-2 transition-transform rounded-r-full flex items-center justify-center z-[2] cursor-pointer border border-gray-300"
         title="Expand config editor"
-        onClick={() => onToggle(true)}
+        onClick={onToggle}
         style={{
           top: '50%',
           marginTop: '-32px',

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

@@ -5,312 +5,17 @@
  * LICENSE file in the root directory of this source tree.
  */
 
-import {parse as babelParse, ParseResult} from '@babel/parser';
-import * as HermesParser from 'hermes-parser';
-import * as t from '@babel/types';
-import BabelPluginReactCompiler, {
-  CompilerError,
+import {
   CompilerErrorDetail,
   CompilerDiagnostic,
-  Effect,
-  ErrorCategory,
-  parseConfigPragmaForTests,
-  ValueKind,
-  type Hook,
-  PluginOptions,
-  CompilerPipelineValue,
-  parsePluginOptions,
-  printReactiveFunctionWithOutlined,
-  printFunctionWithOutlined,
-  type LoggerEvent,
 } from 'babel-plugin-react-compiler';
-import {useDeferredValue, useMemo} from 'react';
+import {useDeferredValue, useMemo, useState} from 'react';
 import {useStore} from '../StoreContext';
 import ConfigEditor from './ConfigEditor';
 import Input from './Input';
-import {
-  CompilerOutput,
-  CompilerTransformOutput,
-  default as Output,
-  PrintedCompilerPipelineValue,
-} from './Output';
-import {transformFromAstSync} from '@babel/core';
-
-function parseInput(
-  input: string,
-  language: 'flow' | 'typescript',
-): ParseResult<t.File> {
-  // Extract the first line to quickly check for custom test directives
-  if (language === 'flow') {
-    return HermesParser.parse(input, {
-      babel: true,
-      flow: 'all',
-      sourceType: 'module',
-      enableExperimentalComponentSyntax: true,
-    });
-  } else {
-    return babelParse(input, {
-      plugins: ['typescript', 'jsx'],
-      sourceType: 'module',
-    }) as ParseResult<t.File>;
-  }
-}
-
-function invokeCompiler(
-  source: string,
-  language: 'flow' | 'typescript',
-  options: PluginOptions,
-): CompilerTransformOutput {
-  const ast = parseInput(source, language);
-  let result = transformFromAstSync(ast, source, {
-    filename: '_playgroundFile.js',
-    highlightCode: false,
-    retainLines: true,
-    plugins: [[BabelPluginReactCompiler, options]],
-    ast: true,
-    sourceType: 'module',
-    configFile: false,
-    sourceMaps: true,
-    babelrc: false,
-  });
-  if (result?.ast == null || result?.code == null || result?.map == null) {
-    throw new Error('Expected successful compilation');
-  }
-  return {
-    code: result.code,
-    sourceMaps: result.map,
-    language,
-  };
-}
-
-const COMMON_HOOKS: Array<[string, Hook]> = [
-  [
-    'useFragment',
-    {
-      valueKind: ValueKind.Frozen,
-      effectKind: Effect.Freeze,
-      noAlias: true,
-      transitiveMixedData: true,
-    },
-  ],
-  [
-    'usePaginationFragment',
-    {
-      valueKind: ValueKind.Frozen,
-      effectKind: Effect.Freeze,
-      noAlias: true,
-      transitiveMixedData: true,
-    },
-  ],
-  [
-    'useRefetchableFragment',
-    {
-      valueKind: ValueKind.Frozen,
-      effectKind: Effect.Freeze,
-      noAlias: true,
-      transitiveMixedData: true,
-    },
-  ],
-  [
-    'useLazyLoadQuery',
-    {
-      valueKind: ValueKind.Frozen,
-      effectKind: Effect.Freeze,
-      noAlias: true,
-      transitiveMixedData: true,
-    },
-  ],
-  [
-    'usePreloadedQuery',
-    {
-      valueKind: ValueKind.Frozen,
-      effectKind: Effect.Freeze,
-      noAlias: true,
-      transitiveMixedData: true,
-    },
-  ],
-];
-
-function parseOptions(
-  source: string,
-  mode: 'compiler' | 'linter',
-  configOverrides: string,
-): PluginOptions {
-  // Extract the first line to quickly check for custom test directives
-  const pragma = source.substring(0, source.indexOf('\n'));
-
-  const parsedPragmaOptions = parseConfigPragmaForTests(pragma, {
-    compilationMode: 'infer',
-    environment:
-      mode === 'linter'
-        ? {
-            // enabled in compiler
-            validateRefAccessDuringRender: false,
-            // enabled in linter
-            validateNoSetStateInRender: true,
-            validateNoSetStateInEffects: true,
-            validateNoJSXInTryStatements: true,
-            validateNoImpureFunctionsInRender: true,
-            validateStaticComponents: true,
-            validateNoFreezingKnownMutableFunctions: true,
-            validateNoVoidUseMemo: true,
-          }
-        : {
-            /* use defaults for compiler mode */
-          },
-  });
-
-  // Parse config overrides from config editor
-  let configOverrideOptions: any = {};
-  const configMatch = configOverrides.match(/^\s*import.*?\n\n\((.*)\)/s);
-  if (configOverrides.trim()) {
-    if (configMatch && configMatch[1]) {
-      const configString = configMatch[1].replace(/satisfies.*$/, '').trim();
-      configOverrideOptions = new Function(`return (${configString})`)();
-    } else {
-      throw new Error('Invalid override format');
-    }
-  }
-
-  const opts: PluginOptions = parsePluginOptions({
-    ...parsedPragmaOptions,
-    ...configOverrideOptions,
-    environment: {
-      ...parsedPragmaOptions.environment,
-      ...configOverrideOptions.environment,
-      customHooks: new Map([...COMMON_HOOKS]),
-    },
-  });
-
-  return opts;
-}
-
-function compile(
-  source: string,
-  mode: 'compiler' | 'linter',
-  configOverrides: string,
-): [CompilerOutput, 'flow' | 'typescript', PluginOptions | null] {
-  const results = new Map<string, Array<PrintedCompilerPipelineValue>>();
-  const error = new CompilerError();
-  const otherErrors: Array<CompilerErrorDetail | CompilerDiagnostic> = [];
-  const upsert: (result: PrintedCompilerPipelineValue) => void = result => {
-    const entry = results.get(result.name);
-    if (Array.isArray(entry)) {
-      entry.push(result);
-    } else {
-      results.set(result.name, [result]);
-    }
-  };
-  let language: 'flow' | 'typescript';
-  if (source.match(/\@flow/)) {
-    language = 'flow';
-  } else {
-    language = 'typescript';
-  }
-  let transformOutput;
-
-  let baseOpts: PluginOptions | null = null;
-  try {
-    baseOpts = parseOptions(source, mode, configOverrides);
-  } catch (err) {
-    error.details.push(
-      new CompilerErrorDetail({
-        category: ErrorCategory.Config,
-        reason: `Unexpected failure when transforming configs! \n${err}`,
-        loc: null,
-        suggestions: null,
-      }),
-    );
-  }
-  if (baseOpts) {
-    try {
-      const logIR = (result: CompilerPipelineValue): void => {
-        switch (result.kind) {
-          case 'ast': {
-            break;
-          }
-          case 'hir': {
-            upsert({
-              kind: 'hir',
-              fnName: result.value.id,
-              name: result.name,
-              value: printFunctionWithOutlined(result.value),
-            });
-            break;
-          }
-          case 'reactive': {
-            upsert({
-              kind: 'reactive',
-              fnName: result.value.id,
-              name: result.name,
-              value: printReactiveFunctionWithOutlined(result.value),
-            });
-            break;
-          }
-          case 'debug': {
-            upsert({
-              kind: 'debug',
-              fnName: null,
-              name: result.name,
-              value: result.value,
-            });
-            break;
-          }
-          default: {
-            const _: never = result;
-            throw new Error(`Unhandled result ${result}`);
-          }
-        }
-      };
-      // Add logger options to the parsed options
-      const opts = {
-        ...baseOpts,
-        logger: {
-          debugLogIRs: logIR,
-          logEvent: (_filename: string | null, event: LoggerEvent): void => {
-            if (event.kind === 'CompileError') {
-              otherErrors.push(event.detail);
-            }
-          },
-        },
-      };
-      transformOutput = invokeCompiler(source, language, opts);
-    } catch (err) {
-      /**
-       * error might be an invariant violation or other runtime error
-       * (i.e. object shape that is not CompilerError)
-       */
-      if (err instanceof CompilerError && err.details.length > 0) {
-        error.merge(err);
-      } else {
-        /**
-         * Handle unexpected failures by logging (to get a stack trace)
-         * and reporting
-         */
-        error.details.push(
-          new CompilerErrorDetail({
-            category: ErrorCategory.Invariant,
-            reason: `Unexpected failure when transforming input! \n${err}`,
-            loc: null,
-            suggestions: null,
-          }),
-        );
-      }
-    }
-  }
-  // Only include logger errors if there weren't other errors
-  if (!error.hasErrors() && otherErrors.length !== 0) {
-    otherErrors.forEach(e => error.details.push(e));
-  }
-  if (error.hasErrors()) {
-    return [{kind: 'err', results, error}, language, baseOpts];
-  }
-  return [
-    {kind: 'ok', results, transformOutput, errors: error.details},
-    language,
-    baseOpts,
-  ];
-}
+import {CompilerOutput, default as Output} from './Output';
+import {compile} from '../../lib/compilation';
+import prettyFormat from 'pretty-format';
 
 export default function Editor(): JSX.Element {
   const store = useStore();
@@ -323,6 +28,7 @@ export default function Editor(): JSX.Element {
     () => compile(deferredStore.source, 'linter', deferredStore.config),
     [deferredStore.source, deferredStore.config],
   );
+  const [formattedAppliedConfig, setFormattedAppliedConfig] = useState('');
 
   let mergedOutput: CompilerOutput;
   let errors: Array<CompilerErrorDetail | CompilerDiagnostic>;
@@ -336,19 +42,26 @@ export default function Editor(): JSX.Element {
     mergedOutput = compilerOutput;
     errors = compilerOutput.error.details;
   }
+
+  if (appliedOptions) {
+    const formatted = prettyFormat(appliedOptions, {
+      printFunctionName: false,
+      printBasicPrototype: false,
+    });
+    if (formatted !== formattedAppliedConfig) {
+      setFormattedAppliedConfig(formatted);
+    }
+  }
+
   return (
     <>
       <div className="relative flex top-14">
         <div className="flex-shrink-0">
-          <ConfigEditor appliedOptions={appliedOptions} />
+          <ConfigEditor formattedAppliedConfig={formattedAppliedConfig} />
         </div>
         <div className="flex flex-1 min-w-0">
-          <div className="flex-1 min-w-[550px] sm:min-w-0">
-            <Input language={language} errors={errors} />
-          </div>
-          <div className="flex-1 min-w-[550px] sm:min-w-0">
-            <Output store={deferredStore} compilerOutput={mergedOutput} />
-          </div>
+          <Input language={language} errors={errors} />
+          <Output store={deferredStore} compilerOutput={mergedOutput} />
         </div>
       </div>
     </>

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

@@ -13,11 +13,17 @@ import {
 import invariant from 'invariant';
 import type {editor} from 'monaco-editor';
 import * as monaco from 'monaco-editor';
-import {useEffect, useState} from 'react';
+import {
+  useEffect,
+  useState,
+  unstable_ViewTransition as ViewTransition,
+} from 'react';
 import {renderReactCompilerMarkers} from '../../lib/reactCompilerMonacoDiagnostics';
 import {useStore, useStoreDispatch} from '../StoreContext';
 import TabbedWindow from '../TabbedWindow';
 import {monacoOptions} from './monacoOptions';
+import {CONFIG_PANEL_TRANSITION} from '../../lib/transitionTypes';
+
 // @ts-expect-error TODO: Make TS recognize .d.ts files, in addition to loading them with webpack.
 import React$Types from '../../node_modules/@types/react/index.d.ts';
 
@@ -155,9 +161,13 @@ export default function Input({errors, language}: Props): JSX.Element {
   const [activeTab, setActiveTab] = useState('Input');
 
   return (
-    <div className="relative flex flex-col flex-none border-r border-gray-200">
-      <div className="!h-[calc(100vh_-_3.5rem)]">
-        <div className="flex flex-col h-full">
+    <ViewTransition
+      update={{
+        [CONFIG_PANEL_TRANSITION]: 'container',
+        default: 'none',
+      }}>
+      <div className="flex-1 min-w-[550px] sm:min-w-0">
+        <div className="flex flex-col h-full !h-[calc(100vh_-_3.5rem)] border-r border-gray-200">
           <TabbedWindow
             tabs={tabs}
             activeTab={activeTab}
@@ -165,6 +175,6 @@ export default function Input({errors, language}: Props): JSX.Element {
           />
         </div>
       </div>
-    </div>
+    </ViewTransition>
   );
 }

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

@@ -20,11 +20,25 @@ import parserBabel from 'prettier/plugins/babel';
 import * as prettierPluginEstree from 'prettier/plugins/estree';
 import * as prettier from 'prettier/standalone';
 import {type Store} from '../../lib/stores';
-import {memo, ReactNode, use, useState, Suspense} from 'react';
+import {
+  memo,
+  ReactNode,
+  use,
+  useState,
+  Suspense,
+  unstable_ViewTransition as ViewTransition,
+  unstable_addTransitionType as addTransitionType,
+  startTransition,
+} from 'react';
 import AccordionWindow from '../AccordionWindow';
 import TabbedWindow from '../TabbedWindow';
 import {monacoOptions} from './monacoOptions';
 import {BabelFileResult} from '@babel/core';
+import {
+  CONFIG_PANEL_TRANSITION,
+  TOGGLE_INTERNALS_TRANSITION,
+  EXPAND_ACCORDION_TRANSITION,
+} from '../../lib/transitionTypes';
 import {LRUCache} from 'lru-cache';
 
 const MemoizedOutput = memo(Output);
@@ -257,11 +271,19 @@ function OutputContent({store, compilerOutput}: Props): JSX.Element {
   const [previousOutputKind, setPreviousOutputKind] = useState(
     compilerOutput.kind,
   );
+  const isFailure = compilerOutput.kind !== 'ok';
+
   if (compilerOutput.kind !== previousOutputKind) {
     setPreviousOutputKind(compilerOutput.kind);
-    setTabsOpen(new Set(['Output']));
-    setActiveTab('Output');
+    if (isFailure) {
+      startTransition(() => {
+        addTransitionType(EXPAND_ACCORDION_TRANSITION);
+        setTabsOpen(prev => new Set(prev).add('Output'));
+        setActiveTab('Output');
+      });
+    }
   }
+
   const changedPasses: Set<string> = new Set(['Output', 'HIR']); // Initial and final passes should always be bold
   let lastResult: string = '';
   for (const [passName, results] of compilerOutput.results) {
@@ -280,22 +302,36 @@ function OutputContent({store, compilerOutput}: Props): JSX.Element {
 
   if (!store.showInternals) {
     return (
-      <TabbedWindow
-        tabs={tabs}
-        activeTab={activeTab}
-        onTabChange={setActiveTab}
-      />
+      <ViewTransition
+        update={{
+          [CONFIG_PANEL_TRANSITION]: 'container',
+          [TOGGLE_INTERNALS_TRANSITION]: '',
+          default: 'none',
+        }}>
+        <TabbedWindow
+          tabs={tabs}
+          activeTab={activeTab}
+          onTabChange={setActiveTab}
+        />
+      </ViewTransition>
     );
   }
 
   return (
-    <AccordionWindow
-      defaultTab={store.showInternals ? 'HIR' : 'Output'}
-      setTabsOpen={setTabsOpen}
-      tabsOpen={tabsOpen}
-      tabs={tabs}
-      changedPasses={changedPasses}
-    />
+    <ViewTransition
+      update={{
+        [CONFIG_PANEL_TRANSITION]: 'accordion-container',
+        [TOGGLE_INTERNALS_TRANSITION]: '',
+        default: 'none',
+      }}>
+      <AccordionWindow
+        defaultTab={store.showInternals ? 'HIR' : 'Output'}
+        setTabsOpen={setTabsOpen}
+        tabsOpen={tabsOpen}
+        tabs={tabs}
+        changedPasses={changedPasses}
+      />
+    </ViewTransition>
   );
 }
 
@@ -350,12 +386,18 @@ function TextTabContent({
           loading={''}
           options={{
             ...monacoOptions,
+            scrollbar: {
+              vertical: 'hidden',
+            },
+            dimension: {
+              width: 0,
+              height: 0,
+            },
             readOnly: true,
             lineNumbers: 'off',
             glyphMargin: false,
             // Undocumented see https://github.com/Microsoft/vscode/issues/30795#issuecomment-410998882
-            lineDecorationsWidth: 0,
-            lineNumbersMinChars: 0,
+            overviewRulerLanes: 0,
           }}
         />
       ) : (

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

@@ -32,3 +32,14 @@ export const monacoOptions: Partial<EditorProps['options']> = {
 
   tabSize: 2,
 };
+
+export const monacoConfigOptions: Partial<EditorProps['options']> = {
+  ...monacoOptions,
+  lineNumbers: 'off',
+  renderLineHighlight: 'none',
+  overviewRulerBorder: false,
+  overviewRulerLanes: 0,
+  fontSize: 12,
+  scrollBeyondLastLine: false,
+  glyphMargin: false,
+};

compiler/apps/playground/components/Header.tsx

@@ -10,11 +10,16 @@ import {CheckIcon} from '@heroicons/react/solid';
 import clsx from 'clsx';
 import Link from 'next/link';
 import {useSnackbar} from 'notistack';
-import {useState} from 'react';
+import {
+  useState,
+  startTransition,
+  unstable_addTransitionType as addTransitionType,
+} from 'react';
 import {defaultStore} from '../lib/defaultStore';
 import {IconGitHub} from './Icons/IconGitHub';
 import Logo from './Logo';
 import {useStore, useStoreDispatch} from './StoreContext';
+import {TOGGLE_INTERNALS_TRANSITION} from '../lib/transitionTypes';
 
 export default function Header(): JSX.Element {
   const [showCheck, setShowCheck] = useState(false);
@@ -62,7 +67,12 @@ export default function Header(): JSX.Element {
             <input
               type="checkbox"
               checked={store.showInternals}
-              onChange={() => dispatchStore({type: 'toggleInternals'})}
+              onChange={() =>
+                startTransition(() => {
+                  addTransitionType(TOGGLE_INTERNALS_TRANSITION);
+                  dispatchStore({type: 'toggleInternals'});
+                })
+              }
               className="absolute opacity-0 cursor-pointer h-full w-full m-0"
             />
             <span

compiler/apps/playground/components/TabbedWindow.tsx

@@ -4,8 +4,14 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  */
-import React from 'react';
+import React, {
+  startTransition,
+  useId,
+  unstable_ViewTransition as ViewTransition,
+  unstable_addTransitionType as addTransitionType,
+} from 'react';
 import clsx from 'clsx';
+import {TOGGLE_TAB_TRANSITION} from '../lib/transitionTypes';
 
 export default function TabbedWindow({
   tabs,
@@ -16,27 +22,59 @@ export default function TabbedWindow({
   activeTab: string;
   onTabChange: (tab: string) => void;
 }): React.ReactElement {
+  const id = useId();
+  const transitionName = `tab-highlight-${id}`;
+
+  const handleTabChange = (tab: string): void => {
+    startTransition(() => {
+      addTransitionType(TOGGLE_TAB_TRANSITION);
+      onTabChange(tab);
+    });
+  };
+
   return (
-    <div className="flex flex-col h-full max-w-full">
-      <div className="flex p-2 flex-shrink-0">
-        {Array.from(tabs.keys()).map(tab => {
-          const isActive = activeTab === tab;
-          return (
-            <button
-              key={tab}
-              onClick={() => onTabChange(tab)}
-              className={clsx(
-                'active:scale-95 transition-transform py-1.5 px-1.5 xs:px-3 sm:px-4 rounded-full text-sm',
-                !isActive && 'hover:bg-primary/5',
-                isActive && 'bg-highlight text-link',
-              )}>
-              {tab}
-            </button>
-          );
-        })}
-      </div>
-      <div className="flex-1 overflow-hidden w-full h-full">
-        {tabs.get(activeTab)}
+    <div className="flex-1 min-w-[550px] sm:min-w-0">
+      <div className="flex flex-col h-full max-w-full">
+        <div className="flex p-2 flex-shrink-0">
+          {Array.from(tabs.keys()).map(tab => {
+            const isActive = activeTab === tab;
+            return (
+              <button
+                key={tab}
+                onClick={() => handleTabChange(tab)}
+                className={clsx(
+                  'transition-transform py-1.5 px-1.5 xs:px-3 sm:px-4 rounded-full text-sm relative',
+                  isActive ? 'text-link' : 'hover:bg-primary/5',
+                )}>
+                {isActive && (
+                  <ViewTransition
+                    name={transitionName}
+                    enter={{default: 'none'}}
+                    exit={{default: 'none'}}
+                    share={{
+                      [TOGGLE_TAB_TRANSITION]: 'tab-highlight',
+                      default: 'none',
+                    }}
+                    update={{default: 'none'}}>
+                    <div className="absolute inset-0 bg-highlight rounded-full" />
+                  </ViewTransition>
+                )}
+                <ViewTransition
+                  enter={{default: 'none'}}
+                  exit={{default: 'none'}}
+                  update={{
+                    [TOGGLE_TAB_TRANSITION]: 'tab-text',
+                    default: 'none',
+                  }}>
+                  <span className="relative z-1">{tab}</span>
+                </ViewTransition>
+              </button>
+            );
+          })}
+        </div>
+        <div className="flex-1 overflow-hidden w-full h-full">
+          {tabs.get(activeTab)}
+        </div>
       </div>
     </div>
   );

compiler/apps/playground/lib/compilation.ts

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

compiler/apps/playground/lib/transitionTypes.ts

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

compiler/apps/playground/next.config.js

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

compiler/apps/playground/styles/globals.css

@@ -69,3 +69,66 @@
     scrollbar-width: none; /* Firefox */
   }
 }
+
+::view-transition-old(.slide-in) {
+  animation-name: slideOutLeft;
+}
+::view-transition-new(.slide-in) {
+  animation-name: slideInLeft;
+}
+::view-transition-group(.slide-in) {
+  z-index: 1;
+}
+
+@keyframes slideOutLeft {
+  from {
+    transform: translateX(0);
+  }
+  to {
+    transform: translateX(-100%);
+  }
+}
+@keyframes slideInLeft {
+  from {
+    transform: translateX(-100%);
+  }
+  to {
+    transform: translateX(0);
+  }
+}
+
+::view-transition-old(.container),
+::view-transition-new(.container) {
+  height: 100%;
+}
+
+::view-transition-old(.accordion-container),
+::view-transition-new(.accordion-container) {
+  height: 100%;
+  object-fit: none;
+  object-position: left;
+}
+
+::view-transition-old(.tab-highlight),
+::view-transition-new(.tab-highlight) {
+  height: 100%;
+}
+::view-transition-group(.tab-text) {
+	z-index: 1;
+}
+
+::view-transition-old(.expand-accordion),
+::view-transition-new(.expand-accordion) {
+  width: auto;
+}
+::view-transition-group(.expand-accordion) {
+  overflow: clip;
+}
+
+/**
+ * For some reason, the original Monaco editor is still visible to the
+ * left of the DiffEditor. This is a workaround for better visual clarity.
+ */
+.monaco-diff-editor .editor.original{
+  visibility: hidden !important;
+}

compiler/packages/babel-plugin-react-compiler/package.json

@@ -52,8 +52,8 @@
     "react-dom": "0.0.0-experimental-4beb1fd8-20241118",
     "ts-jest": "^29.1.1",
     "ts-node": "^10.9.2",
-    "zod": "^3.22.4",
-    "zod-validation-error": "^2.1.0"
+    "zod": "^3.25.0 || ^4.0.0",
+    "zod-validation-error": "^3.5.0 || ^4.0.0"
   },
   "resolutions": {
     "./**/@babel/parser": "7.7.4",

compiler/packages/babel-plugin-react-compiler/src/CompilerError.ts

@@ -536,7 +536,8 @@ function printErrorSummary(category: ErrorCategory, message: string): string {
     case ErrorCategory.StaticComponents:
     case ErrorCategory.Suppression:
     case ErrorCategory.Syntax:
-    case ErrorCategory.UseMemo: {
+    case ErrorCategory.UseMemo:
+    case ErrorCategory.VoidUseMemo: {
       heading = 'Error';
       break;
     }
@@ -582,6 +583,10 @@ export enum ErrorCategory {
    * Checking for valid usage of manual memoization
    */
   UseMemo = 'UseMemo',
+  /**
+   * Checking that useMemos always return a value
+   */
+  VoidUseMemo = 'VoidUseMemo',
   /**
    * Checking for higher order functions acting as factories for components/hooks
    */
@@ -669,6 +674,21 @@ export enum ErrorCategory {
   FBT = 'FBT',
 }
 
+export enum LintRulePreset {
+  /**
+   * Rules that are stable and included in the `recommended` preset.
+   */
+  Recommended = 'recommended',
+  /**
+   * Rules that are more experimental and only included in the `recommended-latest` preset.
+   */
+  RecommendedLatest = 'recommended-latest',
+  /**
+   * Rules that are disabled.
+   */
+  Off = 'off',
+}
+
 export type LintRule = {
   // Stores the category the rule corresponds to, used to filter errors when reporting
   category: ErrorCategory;
@@ -689,15 +709,14 @@ export type LintRule = {
   description: string;
 
   /**
-   * If true, this rule will automatically appear in the default, "recommended" ESLint
-   * rule set. Otherwise it will be part of an `allRules` export that developers can
-   * use to opt-in to showing output of all possible rules.
+   * Configures the preset in which the rule is enabled. If 'off', the rule will not be included in
+   * any preset.
    *
    * NOTE: not all validations are enabled by default! Setting this flag only affects
    * whether a given rule is part of the recommended set. The corresponding validation
    * also should be enabled by default if you want the error to actually show up!
    */
-  recommended: boolean;
+  preset: LintRulePreset;
 };
 
 const RULE_NAME_PATTERN = /^[a-z]+(-[a-z]+)*$/;
@@ -720,7 +739,7 @@ function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
         name: 'automatic-effect-dependencies',
         description:
           'Verifies that automatic effect dependencies are compiled if opted-in',
-        recommended: false,
+        preset: LintRulePreset.Off,
       };
     }
     case ErrorCategory.CapitalizedCalls: {
@@ -730,7 +749,7 @@ function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
         name: 'capitalized-calls',
         description:
           'Validates against calling capitalized functions/methods instead of using JSX',
-        recommended: false,
+        preset: LintRulePreset.Off,
       };
     }
     case ErrorCategory.Config: {
@@ -739,7 +758,7 @@ function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
         severity: ErrorSeverity.Error,
         name: 'config',
         description: 'Validates the compiler configuration options',
-        recommended: true,
+        preset: LintRulePreset.Recommended,
       };
     }
     case ErrorCategory.EffectDependencies: {
@@ -748,7 +767,7 @@ function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
         severity: ErrorSeverity.Error,
         name: 'memoized-effect-dependencies',
         description: 'Validates that effect dependencies are memoized',
-        recommended: false,
+        preset: LintRulePreset.Off,
       };
     }
     case ErrorCategory.EffectDerivationsOfState: {
@@ -758,7 +777,7 @@ function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
         name: 'no-deriving-state-in-effects',
         description:
           'Validates against deriving values from state in an effect',
-        recommended: false,
+        preset: LintRulePreset.Off,
       };
     }
     case ErrorCategory.EffectSetState: {
@@ -768,7 +787,7 @@ function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
         name: 'set-state-in-effect',
         description:
           'Validates against calling setState synchronously in an effect, which can lead to re-renders that degrade performance',
-        recommended: true,
+        preset: LintRulePreset.Recommended,
       };
     }
     case ErrorCategory.ErrorBoundaries: {
@@ -778,7 +797,7 @@ function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
         name: 'error-boundaries',
         description:
           'Validates usage of error boundaries instead of try/catch for errors in child components',
-        recommended: true,
+        preset: LintRulePreset.Recommended,
       };
     }
     case ErrorCategory.Factories: {
@@ -789,7 +808,7 @@ function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
         description:
           'Validates against higher order functions defining nested components or hooks. ' +
           'Components and hooks should be defined at the module level',
-        recommended: true,
+        preset: LintRulePreset.Recommended,
       };
     }
     case ErrorCategory.FBT: {
@@ -798,7 +817,7 @@ function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
         severity: ErrorSeverity.Error,
         name: 'fbt',
         description: 'Validates usage of fbt',
-        recommended: false,
+        preset: LintRulePreset.Off,
       };
     }
     case ErrorCategory.Fire: {
@@ -807,7 +826,7 @@ function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
         severity: ErrorSeverity.Error,
         name: 'fire',
         description: 'Validates usage of `fire`',
-        recommended: false,
+        preset: LintRulePreset.Off,
       };
     }
     case ErrorCategory.Gating: {
@@ -817,7 +836,7 @@ function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
         name: 'gating',
         description:
           'Validates configuration of [gating mode](https://react.dev/reference/react-compiler/gating)',
-        recommended: true,
+        preset: LintRulePreset.Recommended,
       };
     }
     case ErrorCategory.Globals: {
@@ -828,7 +847,7 @@ function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
         description:
           '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,
+        preset: LintRulePreset.Recommended,
       };
     }
     case ErrorCategory.Hooks: {
@@ -842,7 +861,7 @@ function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
          * We need to dedeupe these (moving the remaining bits into the compiler) and then enable
          * this rule.
          */
-        recommended: false,
+        preset: LintRulePreset.Off,
       };
     }
     case ErrorCategory.Immutability: {
@@ -852,7 +871,7 @@ function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
         name: 'immutability',
         description:
           '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,
+        preset: LintRulePreset.Recommended,
       };
     }
     case ErrorCategory.Invariant: {
@@ -861,7 +880,7 @@ function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
         severity: ErrorSeverity.Error,
         name: 'invariant',
         description: 'Internal invariants',
-        recommended: false,
+        preset: LintRulePreset.Off,
       };
     }
     case ErrorCategory.PreserveManualMemo: {
@@ -873,7 +892,7 @@ function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
           '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,
+        preset: LintRulePreset.Recommended,
       };
     }
     case ErrorCategory.Purity: {
@@ -883,7 +902,7 @@ function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
         name: 'purity',
         description:
           '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,
+        preset: LintRulePreset.Recommended,
       };
     }
     case ErrorCategory.Refs: {
@@ -893,7 +912,7 @@ function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
         name: 'refs',
         description:
           '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,
+        preset: LintRulePreset.Recommended,
       };
     }
     case ErrorCategory.RenderSetState: {
@@ -903,7 +922,7 @@ function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
         name: 'set-state-in-render',
         description:
           'Validates against setting state during render, which can trigger additional renders and potential infinite render loops',
-        recommended: true,
+        preset: LintRulePreset.Recommended,
       };
     }
     case ErrorCategory.StaticComponents: {
@@ -913,7 +932,7 @@ function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
         name: 'static-components',
         description:
           'Validates that components are static, not recreated every render. Components that are recreated dynamically can reset state and trigger excessive re-rendering',
-        recommended: true,
+        preset: LintRulePreset.Recommended,
       };
     }
     case ErrorCategory.Suppression: {
@@ -922,7 +941,7 @@ function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
         severity: ErrorSeverity.Error,
         name: 'rule-suppression',
         description: 'Validates against suppression of other rules',
-        recommended: false,
+        preset: LintRulePreset.Off,
       };
     }
     case ErrorCategory.Syntax: {
@@ -931,7 +950,7 @@ function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
         severity: ErrorSeverity.Error,
         name: 'syntax',
         description: 'Validates against invalid syntax',
-        recommended: false,
+        preset: LintRulePreset.Off,
       };
     }
     case ErrorCategory.Todo: {
@@ -940,7 +959,7 @@ function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
         severity: ErrorSeverity.Hint,
         name: 'todo',
         description: 'Unimplemented features',
-        recommended: false,
+        preset: LintRulePreset.Off,
       };
     }
     case ErrorCategory.UnsupportedSyntax: {
@@ -950,7 +969,7 @@ function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
         name: 'unsupported-syntax',
         description:
           'Validates against syntax that we do not plan to support in React Compiler',
-        recommended: true,
+        preset: LintRulePreset.Recommended,
       };
     }
     case ErrorCategory.UseMemo: {
@@ -960,7 +979,17 @@ function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
         name: 'use-memo',
         description:
           'Validates usage of the useMemo() hook against common mistakes. See [`useMemo()` docs](https://react.dev/reference/react/useMemo) for more information.',
-        recommended: true,
+        preset: LintRulePreset.Recommended,
+      };
+    }
+    case ErrorCategory.VoidUseMemo: {
+      return {
+        category,
+        severity: ErrorSeverity.Error,
+        name: 'void-use-memo',
+        description:
+          'Validates that useMemos always return a value and that the result of the useMemo is used by the component/hook. See [`useMemo()` docs](https://react.dev/reference/react/useMemo) for more information.',
+        preset: LintRulePreset.RecommendedLatest,
       };
     }
     case ErrorCategory.IncompatibleLibrary: {
@@ -970,7 +999,7 @@ function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
         name: 'incompatible-library',
         description:
           'Validates against usage of libraries which are incompatible with memoization (manual or automatic)',
-        recommended: true,
+        preset: LintRulePreset.Recommended,
       };
     }
     default: {

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

@@ -6,7 +6,7 @@
  */
 
 import * as t from '@babel/types';
-import {z} from 'zod';
+import {z} from 'zod/v4';
 import {
   CompilerDiagnostic,
   CompilerError,
@@ -20,7 +20,7 @@ import {
   tryParseExternalFunction,
 } from '../HIR/Environment';
 import {hasOwnProperty} from '../Utils/utils';
-import {fromZodError} from 'zod-validation-error';
+import {fromZodError} from 'zod-validation-error/v4';
 import {CompilerPipelineValue} from './Pipeline';
 
 const PanicThresholdOptionsSchema = z.enum([

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

@@ -454,6 +454,32 @@ function collectNonNullsInBlocks(
             assumedNonNullObjects.add(entry);
           }
         }
+      } else if (
+        fn.env.config.enablePreserveExistingMemoizationGuarantees &&
+        instr.value.kind === 'StartMemoize' &&
+        instr.value.deps != null
+      ) {
+        for (const dep of instr.value.deps) {
+          if (dep.root.kind === 'NamedLocal') {
+            if (
+              !isImmutableAtInstr(dep.root.value.identifier, instr.id, context)
+            ) {
+              continue;
+            }
+            for (let i = 0; i < dep.path.length; i++) {
+              const pathEntry = dep.path[i]!;
+              if (pathEntry.optional) {
+                break;
+              }
+              const depNode = context.registry.getOrCreateProperty({
+                identifier: dep.root.value.identifier,
+                path: dep.path.slice(0, i),
+                reactive: dep.root.value.reactive,
+              });
+              assumedNonNullObjects.add(depNode);
+            }
+          }
+        }
       }
     }
 

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

@@ -6,8 +6,8 @@
  */
 
 import * as t from '@babel/types';
-import {ZodError, z} from 'zod';
-import {fromZodError} from 'zod-validation-error';
+import {ZodError, z} from 'zod/v4';
+import {fromZodError} from 'zod-validation-error/v4';
 import {CompilerError} from '../CompilerError';
 import {Logger, ProgramContext} from '../Entrypoint';
 import {Err, Ok, Result} from '../Utils/Result';
@@ -159,7 +159,7 @@ export const EnvironmentConfigSchema = z.object({
    * A function that, given the name of a module, can optionally return a description
    * of that module's type signature.
    */
-  moduleTypeProvider: z.nullable(z.function().args(z.string())).default(null),
+  moduleTypeProvider: z.nullable(z.any()).default(null),
 
   /**
    * A list of functions which the application compiles as macros, where
@@ -210,7 +210,7 @@ export const EnvironmentConfigSchema = z.object({
    * that if a useEffect or useCallback references a function value, that function value will be
    * considered frozen, and in turn all of its referenced variables will be considered frozen as well.
    */
-  enablePreserveExistingMemoizationGuarantees: z.boolean().default(false),
+  enablePreserveExistingMemoizationGuarantees: z.boolean().default(true),
 
   /**
    * Validates that all useMemo/useCallback values are also memoized by Forget. This mode can be
@@ -249,7 +249,7 @@ export const EnvironmentConfigSchema = z.object({
    * Allows specifying a function that can populate HIR with type information from
    * Flow
    */
-  flowTypeProvider: z.nullable(z.function().args(z.string())).default(null),
+  flowTypeProvider: z.nullable(z.any()).default(null),
 
   /**
    * Enables inference of optional dependency chains. Without this flag
@@ -659,7 +659,7 @@ export const EnvironmentConfigSchema = z.object({
    * Invalid:
    *   useMemo(() => { ... }, [...]);
    */
-  validateNoVoidUseMemo: z.boolean().default(false),
+  validateNoVoidUseMemo: z.boolean().default(true),
 
   /**
    * Validates that Components/Hooks are always defined at module level. This prevents scope
@@ -906,6 +906,12 @@ export class Environment {
       if (moduleTypeProvider == null) {
         return null;
       }
+      if (typeof moduleTypeProvider !== 'function') {
+        CompilerError.throwInvalidConfig({
+          reason: `Expected a function for \`moduleTypeProvider\``,
+          loc,
+        });
+      }
       const unparsedModuleConfig = moduleTypeProvider(moduleName);
       if (unparsedModuleConfig != null) {
         const parsedModuleConfig = TypeSchema.safeParse(unparsedModuleConfig);

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

@@ -16,7 +16,7 @@ import {assertExhaustive} from '../Utils/utils';
 import {Environment, ReactFunctionType} from './Environment';
 import type {HookKind} from './ObjectShape';
 import {Type, makeType} from './Types';
-import {z} from 'zod';
+import {z} from 'zod/v4';
 import type {AliasingEffect} from '../Inference/AliasingEffects';
 import {isReservedWord} from '../Utils/Keyword';
 import {Err, Ok, Result} from '../Utils/Result';

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

@@ -988,7 +988,7 @@ export function createTemporaryPlace(
     identifier: makeTemporaryIdentifier(env.nextIdentifierId, loc),
     reactive: false,
     effect: Effect.Unknown,
-    loc: GeneratedSource,
+    loc,
   };
 }
 

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

@@ -6,7 +6,7 @@
  */
 
 import {isValidIdentifier} from '@babel/types';
-import {z} from 'zod';
+import {z} from 'zod/v4';
 import {Effect, ValueKind} from '..';
 import {
   EffectSchema,

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

@@ -11,7 +11,6 @@ import {
   CallExpression,
   Effect,
   Environment,
-  FinishMemoize,
   FunctionExpression,
   HIRFunction,
   IdentifierId,
@@ -25,7 +24,6 @@ import {
   Place,
   PropertyLoad,
   SpreadPattern,
-  StartMemoize,
   TInstruction,
   getHookKindForType,
   makeInstructionId,
@@ -184,36 +182,52 @@ function makeManualMemoizationMarkers(
   depsList: Array<ManualMemoDependency> | null,
   memoDecl: Place,
   manualMemoId: number,
-): [TInstruction<StartMemoize>, TInstruction<FinishMemoize>] {
+): [Array<Instruction>, Array<Instruction>] {
+  const temp = createTemporaryPlace(env, memoDecl.loc);
   return [
-    {
-      id: makeInstructionId(0),
-      lvalue: createTemporaryPlace(env, fnExpr.loc),
-      value: {
-        kind: 'StartMemoize',
-        manualMemoId,
-        /*
-         * Use deps list from source instead of inferred deps
-         * as dependencies
-         */
-        deps: depsList,
+    [
+      {
+        id: makeInstructionId(0),
+        lvalue: createTemporaryPlace(env, fnExpr.loc),
+        value: {
+          kind: 'StartMemoize',
+          manualMemoId,
+          /*
+           * Use deps list from source instead of inferred deps
+           * as dependencies
+           */
+          deps: depsList,
+          loc: fnExpr.loc,
+        },
+        effects: null,
         loc: fnExpr.loc,
       },
-      effects: null,
-      loc: fnExpr.loc,
-    },
-    {
-      id: makeInstructionId(0),
-      lvalue: createTemporaryPlace(env, fnExpr.loc),
-      value: {
-        kind: 'FinishMemoize',
-        manualMemoId,
-        decl: {...memoDecl},
-        loc: fnExpr.loc,
+    ],
+    [
+      {
+        id: makeInstructionId(0),
+        lvalue: {...temp},
+        value: {
+          kind: 'LoadLocal',
+          place: {...memoDecl},
+          loc: memoDecl.loc,
+        },
+        effects: null,
+        loc: memoDecl.loc,
       },
-      effects: null,
-      loc: fnExpr.loc,
-    },
+      {
+        id: makeInstructionId(0),
+        lvalue: createTemporaryPlace(env, memoDecl.loc),
+        value: {
+          kind: 'FinishMemoize',
+          manualMemoId,
+          decl: {...temp},
+          loc: memoDecl.loc,
+        },
+        effects: null,
+        loc: memoDecl.loc,
+      },
+    ],
   ];
 }
 
@@ -409,10 +423,7 @@ export function dropManualMemoization(
    *   LoadLocal fnArg
    * - (if validation is enabled) collect manual memoization markers
    */
-  const queuedInserts: Map<
-    InstructionId,
-    TInstruction<StartMemoize> | TInstruction<FinishMemoize>
-  > = new Map();
+  const queuedInserts: Map<InstructionId, Array<Instruction>> = new Map();
   for (const [_, block] of func.body.blocks) {
     for (let i = 0; i < block.instructions.length; i++) {
       const instr = block.instructions[i]!;
@@ -438,40 +449,6 @@ export function dropManualMemoization(
             continue;
           }
 
-          /**
-           * Bailout on void return useMemos. This is an anti-pattern where code might be using
-           * useMemo like useEffect: running arbirtary side-effects synced to changes in specific
-           * values.
-           */
-          if (
-            func.env.config.validateNoVoidUseMemo &&
-            manualMemo.kind === 'useMemo'
-          ) {
-            const funcToCheck = sidemap.functions.get(
-              fnPlace.identifier.id,
-            )?.value;
-            if (funcToCheck !== undefined && funcToCheck.loweredFunc.func) {
-              if (!hasNonVoidReturn(funcToCheck.loweredFunc.func)) {
-                errors.pushDiagnostic(
-                  CompilerDiagnostic.create({
-                    category: ErrorCategory.UseMemo,
-                    reason: 'useMemo() callbacks must return a value',
-                    description: `This ${
-                      manualMemo.loadInstr.value.kind === 'PropertyLoad'
-                        ? 'React.useMemo'
-                        : 'useMemo'
-                    } callback doesn't return a value. useMemo is for computing and caching values, not for arbitrary side effects`,
-                    suggestions: null,
-                  }).withDetails({
-                    kind: 'error',
-                    loc: instr.value.loc,
-                    message: 'useMemo() callbacks must return a value',
-                  }),
-                );
-              }
-            }
-          }
-
           instr.value = getManualMemoizationReplacement(
             fnPlace,
             instr.value.loc,
@@ -557,11 +534,11 @@ export function dropManualMemoization(
       let nextInstructions: Array<Instruction> | null = null;
       for (let i = 0; i < block.instructions.length; i++) {
         const instr = block.instructions[i];
-        const insertInstr = queuedInserts.get(instr.id);
-        if (insertInstr != null) {
+        const insertInstructions = queuedInserts.get(instr.id);
+        if (insertInstructions != null) {
           nextInstructions = nextInstructions ?? block.instructions.slice(0, i);
           nextInstructions.push(instr);
-          nextInstructions.push(insertInstr);
+          nextInstructions.push(...insertInstructions);
         } else if (nextInstructions != null) {
           nextInstructions.push(instr);
         }
@@ -629,17 +606,3 @@ function findOptionalPlaces(fn: HIRFunction): Set<IdentifierId> {
   }
   return optionals;
 }
-
-function hasNonVoidReturn(func: HIRFunction): boolean {
-  for (const [, block] of func.body.blocks) {
-    if (block.terminal.kind === 'return') {
-      if (
-        block.terminal.returnVariant === 'Explicit' ||
-        block.terminal.returnVariant === 'Implicit'
-      ) {
-        return true;
-      }
-    }
-  }
-  return false;
-}

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

@@ -7,14 +7,17 @@
 
 import {
   HIRFunction,
+  Identifier,
   IdentifierId,
+  InstructionValue,
   makeInstructionId,
   MutableRange,
   Place,
-  ReactiveValue,
+  ReactiveScope,
 } from '../HIR';
 import {Macro, MacroMethod} from '../HIR/Environment';
-import {eachReactiveValueOperand} from './visitors';
+import {eachInstructionValueOperand} from '../HIR/visitors';
+import {Iterable_some} from '../Utils/utils';
 
 /**
  * This pass supports the `fbt` translation system (https://facebook.github.io/fbt/)
@@ -48,24 +51,49 @@ export function memoizeFbtAndMacroOperandsInSameScope(
     ...Array.from(FBT_TAGS).map((tag): Macro => [tag, []]),
     ...(fn.env.config.customMacros ?? []),
   ]);
-  const fbtValues: Set<IdentifierId> = new Set();
+  /**
+   * Set of all identifiers that load fbt or other macro functions or their nested
+   * properties, as well as values known to be the results of invoking macros
+   */
+  const macroTagsCalls: Set<IdentifierId> = new Set();
+  /**
+   * Mapping of lvalue => list of operands for all expressions where either
+   * the lvalue is a known fbt/macro call and/or the operands transitively
+   * contain fbt/macro calls.
+   *
+   * This is the key data structure that powers the scope merging: we start
+   * at the lvalues and merge operands into the lvalue's scope.
+   */
+  const macroValues: Map<Identifier, Array<Identifier>> = new Map();
+  // Tracks methods loaded from macros, like fbt.param or idx.foo
   const macroMethods = new Map<IdentifierId, Array<Array<MacroMethod>>>();
-  while (true) {
-    let vsize = fbtValues.size;
-    let msize = macroMethods.size;
-    visit(fn, fbtMacroTags, fbtValues, macroMethods);
-    if (vsize === fbtValues.size && msize === macroMethods.size) {
-      break;
+
+  visit(fn, fbtMacroTags, macroTagsCalls, macroMethods, macroValues);
+
+  for (const root of macroValues.keys()) {
+    const scope = root.scope;
+    if (scope == null) {
+      continue;
     }
+    // Merge the operands into the same scope if this is a known macro invocation
+    if (!macroTagsCalls.has(root.id)) {
+      continue;
+    }
+    mergeScopes(root, scope, macroValues, macroTagsCalls);
   }
-  return fbtValues;
+
+  return macroTagsCalls;
 }
 
 export const FBT_TAGS: Set<string> = new Set([
   'fbt',
   'fbt:param',
+  'fbt:enum',
+  'fbt:plural',
   'fbs',
   'fbs:param',
+  'fbs:enum',
+  'fbs:plural',
 ]);
 export const SINGLE_CHILD_FBT_TAGS: Set<string> = new Set([
   'fbt:param',
@@ -75,10 +103,22 @@ export const SINGLE_CHILD_FBT_TAGS: Set<string> = new Set([
 function visit(
   fn: HIRFunction,
   fbtMacroTags: Set<Macro>,
-  fbtValues: Set<IdentifierId>,
+  macroTagsCalls: Set<IdentifierId>,
   macroMethods: Map<IdentifierId, Array<Array<MacroMethod>>>,
+  macroValues: Map<Identifier, Array<Identifier>>,
 ): void {
   for (const [, block] of fn.body.blocks) {
+    for (const phi of block.phis) {
+      const macroOperands: Array<Identifier> = [];
+      for (const operand of phi.operands.values()) {
+        if (macroValues.has(operand.identifier)) {
+          macroOperands.push(operand.identifier);
+        }
+      }
+      if (macroOperands.length !== 0) {
+        macroValues.set(phi.place.identifier, macroOperands);
+      }
+    }
     for (const instruction of block.instructions) {
       const {lvalue, value} = instruction;
       if (lvalue === null) {
@@ -93,13 +133,13 @@ function visit(
          * We don't distinguish between tag names and strings, so record
          * all `fbt` string literals in case they are used as a jsx tag.
          */
-        fbtValues.add(lvalue.identifier.id);
+        macroTagsCalls.add(lvalue.identifier.id);
       } else if (
         value.kind === 'LoadGlobal' &&
         matchesExactTag(value.binding.name, fbtMacroTags)
       ) {
         // Record references to `fbt` as a global
-        fbtValues.add(lvalue.identifier.id);
+        macroTagsCalls.add(lvalue.identifier.id);
       } else if (
         value.kind === 'LoadGlobal' &&
         matchTagRoot(value.binding.name, fbtMacroTags) !== null
@@ -121,84 +161,66 @@ function visit(
             if (method.length > 1) {
               newMethods.push(method.slice(1));
             } else {
-              fbtValues.add(lvalue.identifier.id);
+              macroTagsCalls.add(lvalue.identifier.id);
             }
           }
         }
         if (newMethods.length > 0) {
           macroMethods.set(lvalue.identifier.id, newMethods);
         }
-      } else if (isFbtCallExpression(fbtValues, value)) {
-        const fbtScope = lvalue.identifier.scope;
-        if (fbtScope === null) {
-          continue;
-        }
-
-        /*
-         * if the JSX element's tag was `fbt`, mark all its operands
-         * to ensure that they end up in the same scope as the jsx element
-         * itself.
-         */
-        for (const operand of eachReactiveValueOperand(value)) {
-          operand.identifier.scope = fbtScope;
-
-          // Expand the jsx element's range to account for its operands
-          expandFbtScopeRange(fbtScope.range, operand.identifier.mutableRange);
-          fbtValues.add(operand.identifier.id);
-        }
       } else if (
-        isFbtJsxExpression(fbtMacroTags, fbtValues, value) ||
-        isFbtJsxChild(fbtValues, lvalue, value)
+        value.kind === 'PropertyLoad' &&
+        macroTagsCalls.has(value.object.identifier.id)
       ) {
-        const fbtScope = lvalue.identifier.scope;
-        if (fbtScope === null) {
-          continue;
-        }
-
-        /*
-         * if the JSX element's tag was `fbt`, mark all its operands
-         * to ensure that they end up in the same scope as the jsx element
-         * itself.
-         */
-        for (const operand of eachReactiveValueOperand(value)) {
-          operand.identifier.scope = fbtScope;
-
-          // Expand the jsx element's range to account for its operands
-          expandFbtScopeRange(fbtScope.range, operand.identifier.mutableRange);
-
-          /*
-           * NOTE: we add the operands as fbt values so that they are also
-           * grouped with this expression
-           */
-          fbtValues.add(operand.identifier.id);
-        }
-      } else if (fbtValues.has(lvalue.identifier.id)) {
-        const fbtScope = lvalue.identifier.scope;
-        if (fbtScope === null) {
-          return;
-        }
-
-        for (const operand of eachReactiveValueOperand(value)) {
-          if (
-            operand.identifier.name !== null &&
-            operand.identifier.name.kind === 'named'
-          ) {
-            /*
-             * named identifiers were already locals, we only have to force temporaries
-             * into the same scope
-             */
-            continue;
+        macroTagsCalls.add(lvalue.identifier.id);
+      } else if (
+        isFbtJsxExpression(fbtMacroTags, macroTagsCalls, value) ||
+        isFbtJsxChild(macroTagsCalls, lvalue, value) ||
+        isFbtCallExpression(macroTagsCalls, value)
+      ) {
+        macroTagsCalls.add(lvalue.identifier.id);
+        macroValues.set(
+          lvalue.identifier,
+          Array.from(
+            eachInstructionValueOperand(value),
+            operand => operand.identifier,
+          ),
+        );
+      } else if (
+        Iterable_some(eachInstructionValueOperand(value), operand =>
+          macroValues.has(operand.identifier),
+        )
+      ) {
+        const macroOperands: Array<Identifier> = [];
+        for (const operand of eachInstructionValueOperand(value)) {
+          if (macroValues.has(operand.identifier)) {
+            macroOperands.push(operand.identifier);
           }
-          operand.identifier.scope = fbtScope;
-
-          // Expand the jsx element's range to account for its operands
-          expandFbtScopeRange(fbtScope.range, operand.identifier.mutableRange);
         }
+        macroValues.set(lvalue.identifier, macroOperands);
       }
     }
   }
 }
 
+function mergeScopes(
+  root: Identifier,
+  scope: ReactiveScope,
+  macroValues: Map<Identifier, Array<Identifier>>,
+  macroTagsCalls: Set<IdentifierId>,
+): void {
+  const operands = macroValues.get(root);
+  if (operands == null) {
+    return;
+  }
+  for (const operand of operands) {
+    operand.scope = scope;
+    expandFbtScopeRange(scope.range, operand.mutableRange);
+    macroTagsCalls.add(operand.id);
+    mergeScopes(operand, scope, macroValues, macroTagsCalls);
+  }
+}
+
 function matchesExactTag(s: string, tags: Set<Macro>): boolean {
   return Array.from(tags).some(macro =>
     typeof macro === 'string'
@@ -229,39 +251,40 @@ function matchTagRoot(
 }
 
 function isFbtCallExpression(
-  fbtValues: Set<IdentifierId>,
-  value: ReactiveValue,
+  macroTagsCalls: Set<IdentifierId>,
+  value: InstructionValue,
 ): boolean {
   return (
     (value.kind === 'CallExpression' &&
-      fbtValues.has(value.callee.identifier.id)) ||
-    (value.kind === 'MethodCall' && fbtValues.has(value.property.identifier.id))
+      macroTagsCalls.has(value.callee.identifier.id)) ||
+    (value.kind === 'MethodCall' &&
+      macroTagsCalls.has(value.property.identifier.id))
   );
 }
 
 function isFbtJsxExpression(
   fbtMacroTags: Set<Macro>,
-  fbtValues: Set<IdentifierId>,
-  value: ReactiveValue,
+  macroTagsCalls: Set<IdentifierId>,
+  value: InstructionValue,
 ): boolean {
   return (
     value.kind === 'JsxExpression' &&
     ((value.tag.kind === 'Identifier' &&
-      fbtValues.has(value.tag.identifier.id)) ||
+      macroTagsCalls.has(value.tag.identifier.id)) ||
       (value.tag.kind === 'BuiltinTag' &&
         matchesExactTag(value.tag.name, fbtMacroTags)))
   );
 }
 
 function isFbtJsxChild(
-  fbtValues: Set<IdentifierId>,
+  macroTagsCalls: Set<IdentifierId>,
   lvalue: Place | null,
-  value: ReactiveValue,
+  value: InstructionValue,
 ): boolean {
   return (
     (value.kind === 'JsxExpression' || value.kind === 'JsxFragment') &&
     lvalue !== null &&
-    fbtValues.has(lvalue.identifier.id)
+    macroTagsCalls.has(lvalue.identifier.id)
   );
 }
 

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

@@ -77,6 +77,15 @@ class Transform extends ReactiveFunctionTransform<boolean> {
         }
         break;
       }
+      case 'FinishMemoize': {
+        if (
+          !withinScope &&
+          this.alwaysInvalidatingValues.has(value.decl.identifier)
+        ) {
+          value.pruned = true;
+        }
+        break;
+      }
     }
     return {kind: 'keep'};
   }

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

@@ -393,7 +393,7 @@ function* generateInstructionTypes(
               shapeId: BuiltInArrayId,
             });
           } else {
-            break;
+            continue;
           }
         }
       } else {

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

@@ -5,7 +5,7 @@
  * LICENSE file in the root directory of this source tree.
  */
 
-import {fromZodError} from 'zod-validation-error';
+import {fromZodError} from 'zod-validation-error/v4';
 import {CompilerError} from '../CompilerError';
 import {
   CompilationMode,

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

@@ -10,16 +10,37 @@ import {
   CompilerError,
   ErrorCategory,
 } from '../CompilerError';
-import {FunctionExpression, HIRFunction, IdentifierId} from '../HIR';
+import {
+  FunctionExpression,
+  HIRFunction,
+  IdentifierId,
+  SourceLocation,
+} from '../HIR';
+import {
+  eachInstructionValueOperand,
+  eachTerminalOperand,
+} from '../HIR/visitors';
 import {Result} from '../Utils/Result';
 
 export function validateUseMemo(fn: HIRFunction): Result<void, CompilerError> {
   const errors = new CompilerError();
+  const voidMemoErrors = new CompilerError();
   const useMemos = new Set<IdentifierId>();
   const react = new Set<IdentifierId>();
   const functions = new Map<IdentifierId, FunctionExpression>();
+  const unusedUseMemos = new Map<IdentifierId, SourceLocation>();
   for (const [, block] of fn.body.blocks) {
     for (const {lvalue, value} of block.instructions) {
+      if (unusedUseMemos.size !== 0) {
+        /**
+         * Most of the time useMemo results are referenced immediately. Don't bother
+         * scanning instruction operands for useMemos unless there is an as-yet-unused
+         * useMemo.
+         */
+        for (const operand of eachInstructionValueOperand(value)) {
+          unusedUseMemos.delete(operand.identifier.id);
+        }
+      }
       switch (value.kind) {
         case 'LoadGlobal': {
           if (value.binding.name === 'useMemo') {
@@ -45,10 +66,8 @@ export function validateUseMemo(fn: HIRFunction): Result<void, CompilerError> {
         case 'CallExpression': {
           // Is the function being called useMemo, with at least 1 argument?
           const callee =
-            value.kind === 'CallExpression'
-              ? value.callee.identifier.id
-              : value.property.identifier.id;
-          const isUseMemo = useMemos.has(callee);
+            value.kind === 'CallExpression' ? value.callee : value.property;
+          const isUseMemo = useMemos.has(callee.identifier.id);
           if (!isUseMemo || value.args.length === 0) {
             continue;
           }
@@ -104,10 +123,103 @@ export function validateUseMemo(fn: HIRFunction): Result<void, CompilerError> {
             );
           }
 
+          validateNoContextVariableAssignment(body.loweredFunc.func, errors);
+
+          if (fn.env.config.validateNoVoidUseMemo) {
+            if (!hasNonVoidReturn(body.loweredFunc.func)) {
+              voidMemoErrors.pushDiagnostic(
+                CompilerDiagnostic.create({
+                  category: ErrorCategory.VoidUseMemo,
+                  reason: 'useMemo() callbacks must return a value',
+                  description: `This useMemo() callback doesn't return a value. useMemo() is for computing and caching values, not for arbitrary side effects`,
+                  suggestions: null,
+                }).withDetails({
+                  kind: 'error',
+                  loc: body.loc,
+                  message: 'useMemo() callbacks must return a value',
+                }),
+              );
+            } else {
+              unusedUseMemos.set(lvalue.identifier.id, callee.loc);
+            }
+          }
+          break;
+        }
+      }
+    }
+    if (unusedUseMemos.size !== 0) {
+      for (const operand of eachTerminalOperand(block.terminal)) {
+        unusedUseMemos.delete(operand.identifier.id);
+      }
+    }
+  }
+  if (unusedUseMemos.size !== 0) {
+    /**
+     * Basic check for unused memos, where the result of the call is never referenced. This runs
+     * before DCE so it's more of an AST-level check that something, _anything_, cares about the value.
+     *
+     * This is easy to defeat with e.g. `const _ = useMemo(...)` but it at least gives us something to teach.
+     * Even a DCE-based version could be bypassed with `noop(useMemo(...))`.
+     */
+    for (const loc of unusedUseMemos.values()) {
+      voidMemoErrors.pushDiagnostic(
+        CompilerDiagnostic.create({
+          category: ErrorCategory.VoidUseMemo,
+          reason: 'useMemo() result is unused',
+          description: `This useMemo() value is unused. useMemo() is for computing and caching values, not for arbitrary side effects`,
+          suggestions: null,
+        }).withDetails({
+          kind: 'error',
+          loc,
+          message: 'useMemo() result is unused',
+        }),
+      );
+    }
+  }
+  fn.env.logErrors(voidMemoErrors.asResult());
+  return errors.asResult();
+}
+
+function validateNoContextVariableAssignment(
+  fn: HIRFunction,
+  errors: CompilerError,
+): void {
+  for (const block of fn.body.blocks.values()) {
+    for (const instr of block.instructions) {
+      const value = instr.value;
+      switch (value.kind) {
+        case 'StoreContext': {
+          errors.pushDiagnostic(
+            CompilerDiagnostic.create({
+              category: ErrorCategory.UseMemo,
+              reason:
+                'useMemo() callbacks may not reassign variables declared outside of the callback',
+              description:
+                'useMemo() callbacks must be pure functions and cannot reassign variables defined outside of the callback function',
+              suggestions: null,
+            }).withDetails({
+              kind: 'error',
+              loc: value.lvalue.place.loc,
+              message: 'Cannot reassign variable',
+            }),
+          );
           break;
         }
       }
     }
   }
-  return errors.asResult();
+}
+
+function hasNonVoidReturn(func: HIRFunction): boolean {
+  for (const [, block] of func.body.blocks) {
+    if (block.terminal.kind === 'return') {
+      if (
+        block.terminal.returnVariant === 'Explicit' ||
+        block.terminal.returnVariant === 'Implicit'
+      ) {
+        return true;
+      }
+    }
+  }
+  return false;
 }

compiler/packages/babel-plugin-react-compiler/src/__tests__/envConfig-test.ts

@@ -20,7 +20,7 @@ describe('parseConfigPragma()', () => {
         validateHooksUsage: 1,
       } as any);
     }).toThrowErrorMatchingInlineSnapshot(
-      `"Error: Could not validate environment config. Update React Compiler config to fix the error. Validation error: Expected boolean, received number at "validateHooksUsage"."`,
+      `"Error: Could not validate environment config. Update React Compiler config to fix the error. Validation error: Invalid input: expected boolean, received number at "validateHooksUsage"."`,
     );
   });
 
@@ -38,7 +38,7 @@ describe('parseConfigPragma()', () => {
         ],
       } as any);
     }).toThrowErrorMatchingInlineSnapshot(
-      `"Error: Could not validate environment config. Update React Compiler config to fix the error. Validation error: autodepsIndex must be > 0 at "inferEffectDependencies[0].autodepsIndex"."`,
+      `"Error: Could not validate environment config. Update React Compiler config to fix the error. Validation error: AutodepsIndex must be > 0 at "inferEffectDependencies[0].autodepsIndex"."`,
     );
   });
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/allocating-primitive-as-dep.expect.md

@@ -2,6 +2,7 @@
 ## Input
 
 ```javascript
+// @enablePreserveExistingMemoizationGuarantees:false
 // bar(props.b) is an allocating expression that produces a primitive, which means
 // that Forget should memoize it.
 // Correctness:
@@ -16,7 +17,8 @@ function AllocatingPrimitiveAsDep(props) {
 ## Code
 
 ```javascript
-import { c as _c } from "react/compiler-runtime"; // bar(props.b) is an allocating expression that produces a primitive, which means
+import { c as _c } from "react/compiler-runtime"; // @enablePreserveExistingMemoizationGuarantees:false
+// bar(props.b) is an allocating expression that produces a primitive, which means
 // that Forget should memoize it.
 // Correctness:
 //   - y depends on either bar(props.b) or bar(props.b) + 1

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/allocating-primitive-as-dep.js

@@ -1,3 +1,4 @@
+// @enablePreserveExistingMemoizationGuarantees:false
 // bar(props.b) is an allocating expression that produces a primitive, which means
 // that Forget should memoize it.
 // Correctness:

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/allow-modify-global-in-callback-jsx.expect.md

@@ -2,6 +2,7 @@
 ## Input
 
 ```javascript
+// @enablePreserveExistingMemoizationGuarantees:false
 import {useMemo} from 'react';
 
 const someGlobal = {value: 0};
@@ -32,7 +33,7 @@ export const FIXTURE_ENTRYPOINT = {
 ## Code
 
 ```javascript
-import { c as _c } from "react/compiler-runtime";
+import { c as _c } from "react/compiler-runtime"; // @enablePreserveExistingMemoizationGuarantees:false
 import { useMemo } from "react";
 
 const someGlobal = { value: 0 };

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/allow-modify-global-in-callback-jsx.js

@@ -1,3 +1,4 @@
+// @enablePreserveExistingMemoizationGuarantees:false
 import {useMemo} from 'react';
 
 const someGlobal = {value: 0};

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/computed-load-primitive-as-dependency.expect.md

@@ -2,6 +2,7 @@
 ## Input
 
 ```javascript
+// @enablePreserveExistingMemoizationGuarantees:false
 function Component(props) {
   let a = foo();
   // freeze `a` so we know the next line cannot mutate it
@@ -17,7 +18,7 @@ function Component(props) {
 ## Code
 
 ```javascript
-import { c as _c } from "react/compiler-runtime";
+import { c as _c } from "react/compiler-runtime"; // @enablePreserveExistingMemoizationGuarantees:false
 function Component(props) {
   const $ = _c(2);
   const a = foo();

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/computed-load-primitive-as-dependency.js

@@ -1,3 +1,4 @@
+// @enablePreserveExistingMemoizationGuarantees:false
 function Component(props) {
   let a = foo();
   // freeze `a` so we know the next line cannot mutate it

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/constant-propagation-template-literal.expect.md

@@ -2,6 +2,7 @@
 ## Input
 
 ```javascript
+// @enablePreserveExistingMemoizationGuarantees:false
 import {Stringify, identity} from 'shared-runtime';
 
 function foo() {
@@ -64,7 +65,7 @@ export const FIXTURE_ENTRYPOINT = {
 ## Code
 
 ```javascript
-import { c as _c } from "react/compiler-runtime";
+import { c as _c } from "react/compiler-runtime"; // @enablePreserveExistingMemoizationGuarantees:false
 import { Stringify, identity } from "shared-runtime";
 
 function foo() {

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/constant-propagation-template-literal.js

@@ -1,3 +1,4 @@
+// @enablePreserveExistingMemoizationGuarantees:false
 import {Stringify, identity} from 'shared-runtime';
 
 function foo() {

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/context-variable-as-jsx-element-tag.expect.md

@@ -2,6 +2,7 @@
 ## Input
 
 ```javascript
+// @enablePreserveExistingMemoizationGuarantees:false
 import {useMemo} from 'react';
 import {Stringify} from 'shared-runtime';
 
@@ -25,7 +26,7 @@ export const FIXTURE_ENTRYPOINT = {
 ## Code
 
 ```javascript
-import { c as _c } from "react/compiler-runtime";
+import { c as _c } from "react/compiler-runtime"; // @enablePreserveExistingMemoizationGuarantees:false
 import { useMemo } from "react";
 import { Stringify } from "shared-runtime";
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/context-variable-as-jsx-element-tag.js

@@ -1,3 +1,4 @@
+// @enablePreserveExistingMemoizationGuarantees:false
 import {useMemo} from 'react';
 import {Stringify} from 'shared-runtime';
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/destructure-direct-reassignment.expect.md

@@ -2,6 +2,7 @@
 ## Input
 
 ```javascript
+// @enablePreserveExistingMemoizationGuarantees:false
 function foo(props) {
   let x, y;
   ({x, y} = {x: props.a, y: props.b});
@@ -21,6 +22,7 @@ export const FIXTURE_ENTRYPOINT = {
 ## Code
 
 ```javascript
+// @enablePreserveExistingMemoizationGuarantees:false
 function foo(props) {
   let x;
   let y;

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/destructure-direct-reassignment.js

@@ -1,3 +1,4 @@
+// @enablePreserveExistingMemoizationGuarantees:false
 function foo(props) {
   let x, y;
   ({x, y} = {x: props.a, y: props.b});

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.bug-invariant-expected-consistent-destructuring.expect.md

@@ -29,7 +29,7 @@ Found 1 error:
 
 Invariant: Expected consistent kind for destructuring
 
-Other places were `Reassign` but 'mutate? #t8$46[7:9]{reactive}' is const.
+Other places were `Reassign` but 'mutate? #t8$47[7:9]{reactive}' is const.
 
 error.bug-invariant-expected-consistent-destructuring.ts:9:9
    7 |

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.invalid-reassign-variable-in-usememo.expect.md

@@ -0,0 +1,38 @@
+
+## Input
+
+```javascript
+function Component() {
+  let x;
+  const y = useMemo(() => {
+    let z;
+    x = [];
+    z = true;
+    return z;
+  }, []);
+  return [x, y];
+}
+
+```
+
+
+## Error
+
+```
+Found 1 error:
+
+Error: useMemo() callbacks may not reassign variables declared outside of the callback
+
+useMemo() callbacks must be pure functions and cannot reassign variables defined outside of the callback function.
+
+error.invalid-reassign-variable-in-usememo.ts:5:4
+  3 |   const y = useMemo(() => {
+  4 |     let z;
+> 5 |     x = [];
+    |     ^ Cannot reassign variable
+  6 |     z = true;
+  7 |     return z;
+  8 |   }, []);
+```
+          
+      

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.invalid-reassign-variable-in-usememo.js

@@ -0,0 +1,10 @@
+function Component() {
+  let x;
+  const y = useMemo(() => {
+    let z;
+    x = [];
+    z = true;
+    return z;
+  }, []);
+  return [x, y];
+}

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.invalid-setState-in-render-unbound-state.expect.md

@@ -0,0 +1,41 @@
+
+## Input
+
+```javascript
+function Component(props) {
+  // Intentionally don't bind state, this repros a bug where we didn't
+  // infer the type of destructured properties after a hole in the array
+  let [, setState] = useState();
+  setState(1);
+  return props.foo;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: ['TodoAdd'],
+  isComponent: 'TodoAdd',
+};
+
+```
+
+
+## Error
+
+```
+Found 1 error:
+
+Error: Calling setState during render may trigger an infinite loop
+
+Calling setState during render will trigger another render, and can lead to infinite loops. (https://react.dev/reference/react/useState).
+
+error.invalid-setState-in-render-unbound-state.ts:5:2
+  3 |   // infer the type of destructured properties after a hole in the array
+  4 |   let [, setState] = useState();
+> 5 |   setState(1);
+    |   ^^^^^^^^ Found setState() in render
+  6 |   return props.foo;
+  7 | }
+  8 |
+```
+          
+      

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.invalid-setState-in-render-unbound-state.js

@@ -0,0 +1,13 @@
+function Component(props) {
+  // Intentionally don't bind state, this repros a bug where we didn't
+  // infer the type of destructured properties after a hole in the array
+  let [, setState] = useState();
+  setState(1);
+  return props.foo;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: ['TodoAdd'],
+  isComponent: 'TodoAdd',
+};

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.invalid-uncalled-function-capturing-mutable-values-memoizes-with-captures-values.expect.md

@@ -2,7 +2,7 @@
 ## Input
 
 ```javascript
-// @flow @enableNewMutationAliasingModel
+// @flow @enableNewMutationAliasingModel @enablePreserveExistingMemoizationGuarantees:false
 /**
  * This hook returns a function that when called with an input object,
  * will return the result of mapping that input with the supplied map

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.invalid-uncalled-function-capturing-mutable-values-memoizes-with-captures-values.js

@@ -1,4 +1,4 @@
-// @flow @enableNewMutationAliasingModel
+// @flow @enableNewMutationAliasingModel @enablePreserveExistingMemoizationGuarantees:false
 /**
  * This hook returns a function that when called with an input object,
  * will return the result of mapping that input with the supplied map

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.repro-preserve-memoization-inner-destructured-value-mistaken-as-dependency-later-mutation.expect.md

@@ -2,7 +2,7 @@
 ## Input
 
 ```javascript
-// @validatePreserveExistingMemoizationGuarantees
+// @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 
 /**
  * Repro from https://github.com/facebook/react/issues/34262

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.repro-preserve-memoization-inner-destructured-value-mistaken-as-dependency-later-mutation.js

@@ -1,4 +1,4 @@
-// @validatePreserveExistingMemoizationGuarantees
+// @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 
 /**
  * Repro from https://github.com/facebook/react/issues/34262

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.repro-preserve-memoization-inner-destructured-value-mistaken-as-dependency-mutated-dep.expect.md

@@ -2,7 +2,7 @@
 ## Input
 
 ```javascript
-// @validatePreserveExistingMemoizationGuarantees
+// @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 
 import {identity, Stringify, useHook} from 'shared-runtime';
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.repro-preserve-memoization-inner-destructured-value-mistaken-as-dependency-mutated-dep.js

@@ -1,4 +1,4 @@
-// @validatePreserveExistingMemoizationGuarantees
+// @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 
 import {identity, Stringify, useHook} from 'shared-runtime';
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.todo-preserve-memo-deps-mixed-optional-nonoptional-property-chain.expect.md

@@ -0,0 +1,91 @@
+
+## Input
+
+```javascript
+// @enablePreserveExistingMemoizationGuarantees @validatePreserveExistingMemoizationGuarantees @enableOptionalDependencies @enableTreatFunctionDepsAsConditional:false
+
+import {useMemo} from 'react';
+import {identity, ValidateMemoization} from 'shared-runtime';
+
+function Component({x}) {
+  const object = useMemo(() => {
+    return identity({
+      callback: () => {
+        // This is a bug in our dependency inference: we stop capturing dependencies
+        // after x.a.b?.c. But what this dependency is telling us is that if `x.a.b`
+        // was non-nullish, then we can access `.c.d?.e`. Thus we should take the
+        // full property chain, exactly as-is with optionals/non-optionals, as a
+        // dependency
+        return identity(x.a.b?.c.d?.e);
+      },
+    });
+  }, [x.a.b?.c.d?.e]);
+  const result = useMemo(() => {
+    return [object.callback()];
+  }, [object]);
+  return <Inner x={x} result={result} />;
+}
+
+function Inner({x, result}) {
+  'use no memo';
+  return <ValidateMemoization inputs={[x.y.z]} output={result} />;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{x: {y: {z: 42}}}],
+  sequentialRenders: [
+    {x: {y: {z: 42}}},
+    {x: {y: {z: 42}}},
+    {x: {y: {z: 3.14}}},
+    {x: {y: {z: 42}}},
+    {x: {y: {z: 3.14}}},
+    {x: {y: {z: 42}}},
+  ],
+};
+
+```
+
+
+## Error
+
+```
+Found 1 error:
+
+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.a.b?.c`, but the source dependencies were [x.a.b?.c.d?.e]. Inferred less specific property than source.
+
+error.todo-preserve-memo-deps-mixed-optional-nonoptional-property-chain.ts:7:25
+   5 |
+   6 | function Component({x}) {
+>  7 |   const object = useMemo(() => {
+     |                          ^^^^^^^
+>  8 |     return identity({
+     | ^^^^^^^^^^^^^^^^^^^^^
+>  9 |       callback: () => {
+     | ^^^^^^^^^^^^^^^^^^^^^
+> 10 |         // This is a bug in our dependency inference: we stop capturing dependencies
+     | ^^^^^^^^^^^^^^^^^^^^^
+> 11 |         // after x.a.b?.c. But what this dependency is telling us is that if `x.a.b`
+     | ^^^^^^^^^^^^^^^^^^^^^
+> 12 |         // was non-nullish, then we can access `.c.d?.e`. Thus we should take the
+     | ^^^^^^^^^^^^^^^^^^^^^
+> 13 |         // full property chain, exactly as-is with optionals/non-optionals, as a
+     | ^^^^^^^^^^^^^^^^^^^^^
+> 14 |         // dependency
+     | ^^^^^^^^^^^^^^^^^^^^^
+> 15 |         return identity(x.a.b?.c.d?.e);
+     | ^^^^^^^^^^^^^^^^^^^^^
+> 16 |       },
+     | ^^^^^^^^^^^^^^^^^^^^^
+> 17 |     });
+     | ^^^^^^^^^^^^^^^^^^^^^
+> 18 |   }, [x.a.b?.c.d?.e]);
+     | ^^^^ Could not preserve existing manual memoization
+  19 |   const result = useMemo(() => {
+  20 |     return [object.callback()];
+  21 |   }, [object]);
+```
+          
+      

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.todo-preserve-memo-deps-mixed-optional-nonoptional-property-chain.js

@@ -0,0 +1,41 @@
+// @enablePreserveExistingMemoizationGuarantees @validatePreserveExistingMemoizationGuarantees @enableOptionalDependencies @enableTreatFunctionDepsAsConditional:false
+
+import {useMemo} from 'react';
+import {identity, ValidateMemoization} from 'shared-runtime';
+
+function Component({x}) {
+  const object = useMemo(() => {
+    return identity({
+      callback: () => {
+        // This is a bug in our dependency inference: we stop capturing dependencies
+        // after x.a.b?.c. But what this dependency is telling us is that if `x.a.b`
+        // was non-nullish, then we can access `.c.d?.e`. Thus we should take the
+        // full property chain, exactly as-is with optionals/non-optionals, as a
+        // dependency
+        return identity(x.a.b?.c.d?.e);
+      },
+    });
+  }, [x.a.b?.c.d?.e]);
+  const result = useMemo(() => {
+    return [object.callback()];
+  }, [object]);
+  return <Inner x={x} result={result} />;
+}
+
+function Inner({x, result}) {
+  'use no memo';
+  return <ValidateMemoization inputs={[x.y.z]} output={result} />;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{x: {y: {z: 42}}}],
+  sequentialRenders: [
+    {x: {y: {z: 42}}},
+    {x: {y: {z: 42}}},
+    {x: {y: {z: 3.14}}},
+    {x: {y: {z: 42}}},
+    {x: {y: {z: 3.14}}},
+    {x: {y: {z: 42}}},
+  ],
+};

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

@@ -2,7 +2,7 @@
 ## Input
 
 ```javascript
-// @flow @validatePreserveExistingMemoizationGuarantees
+// @flow @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 import {useMemo} from 'react';
 import {logValue, useFragment, useHook, typedLog} from 'shared-runtime';
 

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

@@ -1,4 +1,4 @@
-// @flow @validatePreserveExistingMemoizationGuarantees
+// @flow @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 import {useMemo} from 'react';
 import {logValue, useFragment, useHook, typedLog} from 'shared-runtime';
 

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

@@ -2,7 +2,7 @@
 ## Input
 
 ```javascript
-// @flow @validatePreserveExistingMemoizationGuarantees
+// @flow @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 import {useFragment} from 'react-relay';
 import LogEvent from 'LogEvent';
 import {useCallback, useMemo} from 'react';

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

@@ -1,4 +1,4 @@
-// @flow @validatePreserveExistingMemoizationGuarantees
+// @flow @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 import {useFragment} from 'react-relay';
 import LogEvent from 'LogEvent';
 import {useCallback, useMemo} from 'react';

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

@@ -2,7 +2,7 @@
 ## Input
 
 ```javascript
-// @validatePreserveExistingMemoizationGuarantees
+// @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 import {useMemo} from 'react';
 import {ValidateMemoization, useHook} from 'shared-runtime';
 

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

@@ -1,4 +1,4 @@
-// @validatePreserveExistingMemoizationGuarantees
+// @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 import {useMemo} from 'react';
 import {ValidateMemoization, useHook} from 'shared-runtime';
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.useMemo-no-return-value.expect.md

@@ -1,64 +0,0 @@
-
-## Input
-
-```javascript
-// @validateNoVoidUseMemo
-function Component() {
-  const value = useMemo(() => {
-    console.log('computing');
-  }, []);
-  const value2 = React.useMemo(() => {
-    console.log('computing');
-  }, []);
-  return (
-    <div>
-      {value}
-      {value2}
-    </div>
-  );
-}
-
-```
-
-
-## Error
-
-```
-Found 2 errors:
-
-Error: useMemo() callbacks must return a value
-
-This useMemo callback doesn't return a value. useMemo is for computing and caching values, not for arbitrary side effects.
-
-error.useMemo-no-return-value.ts:3:16
-  1 | // @validateNoVoidUseMemo
-  2 | function Component() {
-> 3 |   const value = useMemo(() => {
-    |                 ^^^^^^^^^^^^^^^
-> 4 |     console.log('computing');
-    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-> 5 |   }, []);
-    | ^^^^^^^^^ useMemo() callbacks must return a value
-  6 |   const value2 = React.useMemo(() => {
-  7 |     console.log('computing');
-  8 |   }, []);
-
-Error: useMemo() callbacks must return a value
-
-This React.useMemo callback doesn't return a value. useMemo is for computing and caching values, not for arbitrary side effects.
-
-error.useMemo-no-return-value.ts:6:17
-   4 |     console.log('computing');
-   5 |   }, []);
->  6 |   const value2 = React.useMemo(() => {
-     |                  ^^^^^^^^^^^^^^^^^^^^^
->  7 |     console.log('computing');
-     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
->  8 |   }, []);
-     | ^^^^^^^^^ useMemo() callbacks must return a value
-   9 |   return (
-  10 |     <div>
-  11 |       {value}
-```
-          
-      

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

@@ -2,7 +2,7 @@
 ## Input
 
 ```javascript
-// @validatePreserveExistingMemoizationGuarantees
+// @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 import {makeObject_Primitives, Stringify} from 'shared-runtime';
 
 function Component(props) {

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

@@ -1,4 +1,4 @@
-// @validatePreserveExistingMemoizationGuarantees
+// @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 import {makeObject_Primitives, Stringify} from 'shared-runtime';
 
 function Component(props) {

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

@@ -2,7 +2,7 @@
 ## Input
 
 ```javascript
-// @validatePreserveExistingMemoizationGuarantees
+// @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 import {makeObject_Primitives, Stringify} from 'shared-runtime';
 
 function Component(props) {

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

@@ -1,4 +1,4 @@
-// @validatePreserveExistingMemoizationGuarantees
+// @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 import {makeObject_Primitives, Stringify} from 'shared-runtime';
 
 function Component(props) {

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/existing-variables-with-c-name.expect.md

@@ -2,6 +2,7 @@
 ## Input
 
 ```javascript
+// @enablePreserveExistingMemoizationGuarantees:false
 import {useMemo, useState} from 'react';
 import {ValidateMemoization} from 'shared-runtime';
 
@@ -28,7 +29,7 @@ export const FIXTURE_ENTRYPOINT = {
 ## Code
 
 ```javascript
-import { c as _c2 } from "react/compiler-runtime";
+import { c as _c2 } from "react/compiler-runtime"; // @enablePreserveExistingMemoizationGuarantees:false
 import { useMemo, useState } from "react";
 import { ValidateMemoization } from "shared-runtime";
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/existing-variables-with-c-name.js

@@ -1,3 +1,4 @@
+// @enablePreserveExistingMemoizationGuarantees:false
 import {useMemo, useState} from 'react';
 import {ValidateMemoization} from 'shared-runtime';
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/fbt/bug-fbt-plural-multiple-function-calls.expect.md

@@ -2,6 +2,7 @@
 ## Input
 
 ```javascript
+// @enablePreserveExistingMemoizationGuarantees:false
 import fbt from 'fbt';
 
 /**
@@ -35,7 +36,7 @@ export const FIXTURE_ENTRYPOINT = {
 ## Code
 
 ```javascript
-import { c as _c } from "react/compiler-runtime";
+import { c as _c } from "react/compiler-runtime"; // @enablePreserveExistingMemoizationGuarantees:false
 import fbt from "fbt";
 
 /**

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/fbt/bug-fbt-plural-multiple-function-calls.ts

@@ -1,3 +1,4 @@
+// @enablePreserveExistingMemoizationGuarantees:false
 import fbt from 'fbt';
 
 /**

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/fbt/error.todo-fbt-param-nested-fbt.expect.md

@@ -1,56 +0,0 @@
-
-## Input
-
-```javascript
-import fbt from 'fbt';
-import {Stringify} from 'shared-runtime';
-
-/**
- * MemoizeFbtAndMacroOperands needs to account for nested fbt calls.
- * Expected fixture `fbt-param-call-arguments` to succeed but it failed with error:
- *   /fbt-param-call-arguments.ts: Line 19 Column 11: fbt: unsupported babel node: Identifier
- *   ---
- *   t3
- *   ---
- */
-function Component({firstname, lastname}) {
-  'use memo';
-  return (
-    <Stringify>
-      {fbt(
-        [
-          'Name: ',
-          fbt.param('firstname', <Stringify key={0} name={firstname} />),
-          ', ',
-          fbt.param(
-            'lastname',
-            <Stringify key={0} name={lastname}>
-              {fbt('(inner fbt)', 'Inner fbt value')}
-            </Stringify>
-          ),
-        ],
-        'Name'
-      )}
-    </Stringify>
-  );
-}
-
-export const FIXTURE_ENTRYPOINT = {
-  fn: Component,
-  params: [{firstname: 'first', lastname: 'last'}],
-  sequentialRenders: [{firstname: 'first', lastname: 'last'}],
-};
-
-```
-
-
-## Error
-
-```
-Line 19 Column 11: fbt: unsupported babel node: Identifier
----
-t3
----
-```
-          
-      

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/fbt/fbt-preserve-whitespace-two-subtrees.expect.md

@@ -37,27 +37,31 @@ import { c as _c } from "react/compiler-runtime";
 import fbt from "fbt";
 
 function Foo(t0) {
-  const $ = _c(3);
+  const $ = _c(7);
   const { name1, name2 } = t0;
   let t1;
   if ($[0] !== name1 || $[1] !== name2) {
+    let t2;
+    if ($[3] !== name1) {
+      t2 = <b>{name1}</b>;
+      $[3] = name1;
+      $[4] = t2;
+    } else {
+      t2 = $[4];
+    }
+    let t3;
+    if ($[5] !== name2) {
+      t3 = <b>{name2}</b>;
+      $[5] = name2;
+      $[6] = t3;
+    } else {
+      t3 = $[6];
+    }
     t1 = fbt._(
       "{user1} and {user2} accepted your PR!",
       [
-        fbt._param(
-          "user1",
-
-          <span key={name1}>
-            <b>{name1}</b>
-          </span>,
-        ),
-        fbt._param(
-          "user2",
-
-          <span key={name2}>
-            <b>{name2}</b>
-          </span>,
-        ),
+        fbt._param("user1", <span key={name1}>{t2}</span>),
+        fbt._param("user2", <span key={name2}>{t3}</span>),
       ],
       { hk: "2PxMie" },
     );

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/fbt/repro-fbt-param-nested-fbt.expect.md

@@ -0,0 +1,111 @@
+
+## Input
+
+```javascript
+import fbt from 'fbt';
+import {Stringify} from 'shared-runtime';
+
+/**
+ * MemoizeFbtAndMacroOperands needs to account for nested fbt calls.
+ * Expected fixture `fbt-param-call-arguments` to succeed but it failed with error:
+ *   /fbt-param-call-arguments.ts: Line 19 Column 11: fbt: unsupported babel node: Identifier
+ *   ---
+ *   t3
+ *   ---
+ */
+function Component({firstname, lastname}) {
+  'use memo';
+  return (
+    <Stringify>
+      {fbt(
+        [
+          'Name: ',
+          fbt.param('firstname', <Stringify key={0} name={firstname} />),
+          ', ',
+          fbt.param(
+            'lastname',
+            <Stringify key={0} name={lastname}>
+              {fbt('(inner fbt)', 'Inner fbt value')}
+            </Stringify>
+          ),
+        ],
+        'Name'
+      )}
+    </Stringify>
+  );
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{firstname: 'first', lastname: 'last'}],
+  sequentialRenders: [{firstname: 'first', lastname: 'last'}],
+};
+
+```
+
+## Code
+
+```javascript
+import { c as _c } from "react/compiler-runtime";
+import fbt from "fbt";
+import { Stringify } from "shared-runtime";
+
+/**
+ * MemoizeFbtAndMacroOperands needs to account for nested fbt calls.
+ * Expected fixture `fbt-param-call-arguments` to succeed but it failed with error:
+ *   /fbt-param-call-arguments.ts: Line 19 Column 11: fbt: unsupported babel node: Identifier
+ *   ---
+ *   t3
+ *   ---
+ */
+function Component(t0) {
+  "use memo";
+  const $ = _c(5);
+  const { firstname, lastname } = t0;
+  let t1;
+  if ($[0] !== firstname || $[1] !== lastname) {
+    t1 = fbt._(
+      "Name: {firstname}, {lastname}",
+      [
+        fbt._param(
+          "firstname",
+
+          <Stringify key={0} name={firstname} />,
+        ),
+        fbt._param(
+          "lastname",
+
+          <Stringify key={0} name={lastname}>
+            {fbt._("(inner fbt)", null, { hk: "36qNwF" })}
+          </Stringify>,
+        ),
+      ],
+      { hk: "3AiIf8" },
+    );
+    $[0] = firstname;
+    $[1] = lastname;
+    $[2] = t1;
+  } else {
+    t1 = $[2];
+  }
+  let t2;
+  if ($[3] !== t1) {
+    t2 = <Stringify>{t1}</Stringify>;
+    $[3] = t1;
+    $[4] = t2;
+  } else {
+    t2 = $[4];
+  }
+  return t2;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ firstname: "first", lastname: "last" }],
+  sequentialRenders: [{ firstname: "first", lastname: "last" }],
+};
+
+```
+      
+### Eval output
+(kind: ok) <div>{"children":"Name: , "}</div>

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/fbt/repro-separately-memoized-fbt-param.expect.md

@@ -0,0 +1,78 @@
+
+## Input
+
+```javascript
+import {fbt} from 'fbt';
+import {useState} from 'react';
+
+const MIN = 10;
+
+function Component() {
+  const [count, setCount] = useState(0);
+
+  return fbt(
+    'Expected at least ' +
+      fbt.param('min', MIN, {number: true}) +
+      ' items, but got ' +
+      fbt.param('count', count, {number: true}) +
+      ' items.',
+    'Error description'
+  );
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{}],
+};
+
+```
+
+## Code
+
+```javascript
+import { c as _c } from "react/compiler-runtime";
+import { fbt } from "fbt";
+import { useState } from "react";
+
+const MIN = 10;
+
+function Component() {
+  const $ = _c(2);
+  const [count] = useState(0);
+  let t0;
+  if ($[0] !== count) {
+    t0 = fbt._(
+      { "*": { "*": "Expected at least {min} items, but got {count} items." } },
+      [
+        fbt._param(
+          "min",
+
+          MIN,
+          [0],
+        ),
+        fbt._param(
+          "count",
+
+          count,
+          [0],
+        ),
+      ],
+      { hk: "36gbz8" },
+    );
+    $[0] = count;
+    $[1] = t0;
+  } else {
+    t0 = $[1];
+  }
+  return t0;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{}],
+};
+
+```
+      
+### Eval output
+(kind: ok) Expected at least 10 items, but got 0 items.

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/fbt/repro-separately-memoized-fbt-param.js

@@ -0,0 +1,22 @@
+import {fbt} from 'fbt';
+import {useState} from 'react';
+
+const MIN = 10;
+
+function Component() {
+  const [count, setCount] = useState(0);
+
+  return fbt(
+    'Expected at least ' +
+      fbt.param('min', MIN, {number: true}) +
+      ' items, but got ' +
+      fbt.param('count', count, {number: true}) +
+      ' items.',
+    'Error description'
+  );
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{}],
+};

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/for-in-statement-type-inference.expect.md

@@ -2,6 +2,7 @@
 ## Input
 
 ```javascript
+// @enablePreserveExistingMemoizationGuarantees:false
 const {identity, mutate} = require('shared-runtime');
 
 function Component(props) {
@@ -24,6 +25,7 @@ export const FIXTURE_ENTRYPOINT = {
 ## Code
 
 ```javascript
+// @enablePreserveExistingMemoizationGuarantees:false
 const { identity, mutate } = require("shared-runtime");
 
 function Component(props) {

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/for-in-statement-type-inference.js

@@ -1,3 +1,4 @@
+// @enablePreserveExistingMemoizationGuarantees:false
 const {identity, mutate} = require('shared-runtime');
 
 function Component(props) {

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/holey-array.expect.md

@@ -1,35 +0,0 @@
-
-## Input
-
-```javascript
-function t(props) {
-  let [, setstate] = useState();
-  setstate(1);
-  return props.foo;
-}
-
-export const FIXTURE_ENTRYPOINT = {
-  fn: t,
-  params: ['TodoAdd'],
-  isComponent: 'TodoAdd',
-};
-
-```
-
-## Code
-
-```javascript
-function t(props) {
-  const [, setstate] = useState();
-  setstate(1);
-  return props.foo;
-}
-
-export const FIXTURE_ENTRYPOINT = {
-  fn: t,
-  params: ["TodoAdd"],
-  isComponent: "TodoAdd",
-};
-
-```
-      

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/holey-array.js

@@ -1,11 +0,0 @@
-function t(props) {
-  let [, setstate] = useState();
-  setstate(1);
-  return props.foo;
-}
-
-export const FIXTURE_ENTRYPOINT = {
-  fn: t,
-  params: ['TodoAdd'],
-  isComponent: 'TodoAdd',
-};

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/hooks-with-prefix.expect.md

@@ -2,7 +2,7 @@
 ## Input
 
 ```javascript
-// @hookPattern:".*\b(use[^$]+)$"
+// @hookPattern:".*\b(use[^$]+)$" @enablePreserveExistingMemoizationGuarantees:false
 
 import * as React from 'react';
 import {makeArray, useHook} from 'shared-runtime';
@@ -36,7 +36,7 @@ export const FIXTURE_ENTRYPOINT = {
 ## Code
 
 ```javascript
-import { c as _c } from "react/compiler-runtime"; // @hookPattern:".*\b(use[^$]+)$"
+import { c as _c } from "react/compiler-runtime"; // @hookPattern:".*\b(use[^$]+)$" @enablePreserveExistingMemoizationGuarantees:false
 
 import * as React from "react";
 import { makeArray, useHook } from "shared-runtime";

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/hooks-with-prefix.js

@@ -1,4 +1,4 @@
-// @hookPattern:".*\b(use[^$]+)$"
+// @hookPattern:".*\b(use[^$]+)$" @enablePreserveExistingMemoizationGuarantees:false
 
 import * as React from 'react';
 import {makeArray, useHook} from 'shared-runtime';

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/idx-method-no-outlining-wildcard.expect.md

@@ -73,7 +73,7 @@ function Component(props) {
   const groupName4 = t3;
   let t4;
   if ($[8] !== props) {
-    t4 = idx.hello_world.b.c(props, _temp3);
+    t4 = idx.hello_world.b.c(props, (__3) => __3.group.label);
     $[8] = props;
     $[9] = t4;
   } else {
@@ -108,9 +108,6 @@ function Component(props) {
   }
   return t5;
 }
-function _temp3(__3) {
-  return __3.group.label;
-}
 function _temp2(__0) {
   return __0.group.label;
 }

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/idx-method-no-outlining.expect.md

@@ -49,7 +49,7 @@ function Component(props) {
   const groupName2 = t1;
   let t2;
   if ($[4] !== props) {
-    t2 = idx.a.b(props, _temp2);
+    t2 = idx.a.b(props, (__1) => __1.group.label);
     $[4] = props;
     $[5] = t2;
   } else {
@@ -74,9 +74,6 @@ function Component(props) {
   }
   return t3;
 }
-function _temp2(__1) {
-  return __1.group.label;
-}
 function _temp(_) {
   return _.group.label;
 }

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/infer-computed-delete.expect.md

@@ -2,7 +2,7 @@
 ## Input
 
 ```javascript
-// @debug
+// @debug @enablePreserveExistingMemoizationGuarantees:false
 function Component(props) {
   const x = makeObject();
   const y = delete x[props.value];
@@ -14,7 +14,7 @@ function Component(props) {
 ## Code
 
 ```javascript
-// @debug
+// @debug @enablePreserveExistingMemoizationGuarantees:false
 function Component(props) {
   const x = makeObject();
   const y = delete x[props.value];

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/infer-computed-delete.js

@@ -1,4 +1,4 @@
-// @debug
+// @debug @enablePreserveExistingMemoizationGuarantees:false
 function Component(props) {
   const x = makeObject();
   const y = delete x[props.value];

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/infer-property-delete.expect.md

@@ -2,6 +2,7 @@
 ## Input
 
 ```javascript
+// @enablePreserveExistingMemoizationGuarantees:false
 function Component(props) {
   const x = makeObject();
   const y = delete x.value;
@@ -13,6 +14,7 @@ function Component(props) {
 ## Code
 
 ```javascript
+// @enablePreserveExistingMemoizationGuarantees:false
 function Component(props) {
   const x = makeObject();
   const y = delete x.value;

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/infer-property-delete.js

@@ -1,3 +1,4 @@
+// @enablePreserveExistingMemoizationGuarantees:false
 function Component(props) {
   const x = makeObject();
   const y = delete x.value;

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/invalid-unused-usememo.expect.md

@@ -0,0 +1,41 @@
+
+## Input
+
+```javascript
+// @validateNoVoidUseMemo @loggerTestOnly
+function Component() {
+  useMemo(() => {
+    return [];
+  }, []);
+  return <div />;
+}
+
+```
+
+## Code
+
+```javascript
+import { c as _c } from "react/compiler-runtime"; // @validateNoVoidUseMemo @loggerTestOnly
+function Component() {
+  const $ = _c(1);
+  let t0;
+  if ($[0] === Symbol.for("react.memo_cache_sentinel")) {
+    t0 = <div />;
+    $[0] = t0;
+  } else {
+    t0 = $[0];
+  }
+  return t0;
+}
+
+```
+
+## Logs
+
+```
+{"kind":"CompileError","detail":{"options":{"category":"VoidUseMemo","reason":"useMemo() result is unused","description":"This useMemo() value is unused. useMemo() is for computing and caching values, not for arbitrary side effects","suggestions":null,"details":[{"kind":"error","loc":{"start":{"line":3,"column":2,"index":67},"end":{"line":3,"column":9,"index":74},"filename":"invalid-unused-usememo.ts","identifierName":"useMemo"},"message":"useMemo() result is unused"}]}},"fnLoc":null}
+{"kind":"CompileSuccess","fnLoc":{"start":{"line":2,"column":0,"index":42},"end":{"line":7,"column":1,"index":127},"filename":"invalid-unused-usememo.ts"},"fnName":"Component","memoSlots":1,"memoBlocks":1,"memoValues":1,"prunedMemoBlocks":0,"prunedMemoValues":0}
+```
+      
+### Eval output
+(kind: exception) Fixture not implemented

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/invalid-unused-usememo.js

@@ -0,0 +1,7 @@
+// @validateNoVoidUseMemo @loggerTestOnly
+function Component() {
+  useMemo(() => {
+    return [];
+  }, []);
+  return <div />;
+}

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/invalid-useMemo-no-return-value.expect.md

@@ -0,0 +1,59 @@
+
+## Input
+
+```javascript
+// @validateNoVoidUseMemo @loggerTestOnly
+function Component() {
+  const value = useMemo(() => {
+    console.log('computing');
+  }, []);
+  const value2 = React.useMemo(() => {
+    console.log('computing');
+  }, []);
+  return (
+    <div>
+      {value}
+      {value2}
+    </div>
+  );
+}
+
+```
+
+## Code
+
+```javascript
+import { c as _c } from "react/compiler-runtime"; // @validateNoVoidUseMemo @loggerTestOnly
+function Component() {
+  const $ = _c(1);
+
+  console.log("computing");
+
+  console.log("computing");
+  let t0;
+  if ($[0] === Symbol.for("react.memo_cache_sentinel")) {
+    t0 = (
+      <div>
+        {undefined}
+        {undefined}
+      </div>
+    );
+    $[0] = t0;
+  } else {
+    t0 = $[0];
+  }
+  return t0;
+}
+
+```
+
+## Logs
+
+```
+{"kind":"CompileError","detail":{"options":{"category":"VoidUseMemo","reason":"useMemo() callbacks must return a value","description":"This useMemo() callback doesn't return a value. useMemo() is for computing and caching values, not for arbitrary side effects","suggestions":null,"details":[{"kind":"error","loc":{"start":{"line":3,"column":24,"index":89},"end":{"line":5,"column":3,"index":130},"filename":"invalid-useMemo-no-return-value.ts"},"message":"useMemo() callbacks must return a value"}]}},"fnLoc":null}
+{"kind":"CompileError","detail":{"options":{"category":"VoidUseMemo","reason":"useMemo() callbacks must return a value","description":"This useMemo() callback doesn't return a value. useMemo() is for computing and caching values, not for arbitrary side effects","suggestions":null,"details":[{"kind":"error","loc":{"start":{"line":6,"column":31,"index":168},"end":{"line":8,"column":3,"index":209},"filename":"invalid-useMemo-no-return-value.ts"},"message":"useMemo() callbacks must return a value"}]}},"fnLoc":null}
+{"kind":"CompileSuccess","fnLoc":{"start":{"line":2,"column":0,"index":42},"end":{"line":15,"column":1,"index":283},"filename":"invalid-useMemo-no-return-value.ts"},"fnName":"Component","memoSlots":1,"memoBlocks":1,"memoValues":1,"prunedMemoBlocks":0,"prunedMemoValues":0}
+```
+      
+### Eval output
+(kind: exception) Fixture not implemented

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/invalid-useMemo-no-return-value.js

@@ -1,4 +1,4 @@
-// @validateNoVoidUseMemo
+// @validateNoVoidUseMemo @loggerTestOnly
 function Component() {
   const value = useMemo(() => {
     console.log('computing');

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/invalid-useMemo-return-empty.expect.md

@@ -0,0 +1,33 @@
+
+## Input
+
+```javascript
+// @loggerTestOnly
+function component(a) {
+  let x = useMemo(() => {
+    mutate(a);
+  }, []);
+  return x;
+}
+
+```
+
+## Code
+
+```javascript
+// @loggerTestOnly
+function component(a) {
+  mutate(a);
+}
+
+```
+
+## Logs
+
+```
+{"kind":"CompileError","detail":{"options":{"category":"VoidUseMemo","reason":"useMemo() callbacks must return a value","description":"This useMemo() callback doesn't return a value. useMemo() is for computing and caching values, not for arbitrary side effects","suggestions":null,"details":[{"kind":"error","loc":{"start":{"line":3,"column":18,"index":61},"end":{"line":5,"column":3,"index":87},"filename":"invalid-useMemo-return-empty.ts"},"message":"useMemo() callbacks must return a value"}]}},"fnLoc":null}
+{"kind":"CompileSuccess","fnLoc":{"start":{"line":2,"column":0,"index":19},"end":{"line":7,"column":1,"index":107},"filename":"invalid-useMemo-return-empty.ts"},"fnName":"component","memoSlots":0,"memoBlocks":0,"memoValues":0,"prunedMemoBlocks":1,"prunedMemoValues":0}
+```
+      
+### Eval output
+(kind: exception) Fixture not implemented

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/invalid-useMemo-return-empty.js

@@ -1,3 +1,4 @@
+// @loggerTestOnly
 function component(a) {
   let x = useMemo(() => {
     mutate(a);

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

@@ -2,7 +2,7 @@
 ## Input
 
 ```javascript
-// @validatePreserveExistingMemoizationGuarantees @enableNewMutationAliasingModel
+// @validatePreserveExistingMemoizationGuarantees @enableNewMutationAliasingModel @enablePreserveExistingMemoizationGuarantees:false
 import {useCallback} from 'react';
 import {makeArray} from 'shared-runtime';