[compiler][wip] Extend ValidateNoDerivedComputationsInEffects for props derived effects

This PR adds infra to disambiguate between two types of derived state in effects:
  1. State derived from props
  2. State derived from other state

TODO:
- [ ] Props tracking through destructuring and property access does not seem to be propagated correctly inside of Functions' instructions (or i might be misunderstanding how we track aliasing effects)
- [ ] compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/useEffect/invalid-derived-state-from-props-computed.js should be failing
- [ ] Handle "mixed" case where deps flow from at least one prop AND state. Should probably have a different error reason, to aid with categorization

.eslintrc.js

@@ -608,6 +608,7 @@ module.exports = {
     symbol: 'readonly',
     SyntheticEvent: 'readonly',
     SyntheticMouseEvent: 'readonly',
+    SyntheticPointerEvent: 'readonly',
     Thenable: 'readonly',
     TimeoutID: 'readonly',
     WheelEventHandler: 'readonly',

CHANGELOG.md

@@ -1,3 +1,8 @@
+## 19.1.1 (July 28, 2025)
+
+### React
+* Fixed Owner Stacks to work with ES2015 function.name semantics ([#33680](https://github.com/facebook/react/pull/33680) by @hoxyq)
+
 ## 19.1.0 (March 28, 2025)
 
 ### Owner Stack

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

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

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

@@ -11,6 +11,7 @@ import * as t from '@babel/types';
 import BabelPluginReactCompiler, {
   CompilerError,
   CompilerErrorDetail,
+  CompilerDiagnostic,
   Effect,
   ErrorSeverity,
   parseConfigPragmaForTests,
@@ -141,10 +142,13 @@ const COMMON_HOOKS: Array<[string, Hook]> = [
   ],
 ];
 
-function compile(source: string): [CompilerOutput, 'flow' | 'typescript'] {
+function compile(
+  source: string,
+  mode: 'compiler' | 'linter',
+): [CompilerOutput, 'flow' | 'typescript'] {
   const results = new Map<string, Array<PrintedCompilerPipelineValue>>();
   const error = new CompilerError();
-  const otherErrors: Array<CompilerErrorDetail> = [];
+  const otherErrors: Array<CompilerErrorDetail | CompilerDiagnostic> = [];
   const upsert: (result: PrintedCompilerPipelineValue) => void = result => {
     const entry = results.get(result.name);
     if (Array.isArray(entry)) {
@@ -203,6 +207,23 @@ function compile(source: string): [CompilerOutput, 'flow' | 'typescript'] {
     };
     const parsedOptions = parseConfigPragmaForTests(pragma, {
       compilationMode: 'infer',
+      environment:
+        mode === 'linter'
+          ? {
+              // enabled in compiler
+              validateRefAccessDuringRender: false,
+              // enabled in linter
+              validateNoSetStateInRender: true,
+              validateNoSetStateInEffects: true,
+              validateNoJSXInTryStatements: true,
+              validateNoImpureFunctionsInRender: true,
+              validateStaticComponents: true,
+              validateNoFreezingKnownMutableFunctions: true,
+              validateNoVoidUseMemo: true,
+            }
+          : {
+              /* use defaults for compiler mode */
+            },
     });
     const opts: PluginOptions = parsePluginOptions({
       ...parsedOptions,
@@ -214,7 +235,7 @@ function compile(source: string): [CompilerOutput, 'flow' | 'typescript'] {
         debugLogIRs: logIR,
         logEvent: (_filename: string | null, event: LoggerEvent) => {
           if (event.kind === 'CompileError') {
-            otherErrors.push(new CompilerErrorDetail(event.detail));
+            otherErrors.push(event.detail);
           }
         },
       },
@@ -226,7 +247,7 @@ function compile(source: string): [CompilerOutput, 'flow' | 'typescript'] {
      * (i.e. object shape that is not CompilerError)
      */
     if (err instanceof CompilerError && err.details.length > 0) {
-      error.details.push(...err.details);
+      error.merge(err);
     } else {
       /**
        * Handle unexpected failures by logging (to get a stack trace)
@@ -245,12 +266,15 @@ function compile(source: string): [CompilerOutput, 'flow' | 'typescript'] {
   }
   // Only include logger errors if there weren't other errors
   if (!error.hasErrors() && otherErrors.length !== 0) {
-    otherErrors.forEach(e => error.push(e));
+    otherErrors.forEach(e => error.details.push(e));
   }
   if (error.hasErrors()) {
-    return [{kind: 'err', results, error: error}, language];
+    return [{kind: 'err', results, error}, language];
   }
-  return [{kind: 'ok', results, transformOutput}, language];
+  return [
+    {kind: 'ok', results, transformOutput, errors: error.details},
+    language,
+  ];
 }
 
 export default function Editor(): JSX.Element {
@@ -259,7 +283,11 @@ export default function Editor(): JSX.Element {
   const dispatchStore = useStoreDispatch();
   const {enqueueSnackbar} = useSnackbar();
   const [compilerOutput, language] = useMemo(
-    () => compile(deferredStore.source),
+    () => compile(deferredStore.source, 'compiler'),
+    [deferredStore.source],
+  );
+  const [linterOutput] = useMemo(
+    () => compile(deferredStore.source, 'linter'),
     [deferredStore.source],
   );
 
@@ -285,19 +313,26 @@ export default function Editor(): JSX.Element {
     });
   });
 
+  let mergedOutput: CompilerOutput;
+  let errors: Array<CompilerErrorDetail | CompilerDiagnostic>;
+  if (compilerOutput.kind === 'ok') {
+    errors = linterOutput.kind === 'ok' ? [] : linterOutput.error.details;
+    mergedOutput = {
+      ...compilerOutput,
+      errors,
+    };
+  } else {
+    mergedOutput = compilerOutput;
+    errors = compilerOutput.error.details;
+  }
   return (
     <>
       <div className="relative flex basis top-14">
         <div className={clsx('relative sm:basis-1/4')}>
-          <Input
-            language={language}
-            errors={
-              compilerOutput.kind === 'err' ? compilerOutput.error.details : []
-            }
-          />
+          <Input language={language} errors={errors} />
         </div>
         <div className={clsx('flex sm:flex flex-wrap')}>
-          <Output store={deferredStore} compilerOutput={compilerOutput} />
+          <Output store={deferredStore} compilerOutput={mergedOutput} />
         </div>
       </div>
     </>

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

@@ -36,13 +36,18 @@ export default function Input({errors, language}: Props): JSX.Element {
     const uri = monaco.Uri.parse(`file:///index.js`);
     const model = monaco.editor.getModel(uri);
     invariant(model, 'Model must exist for the selected input file.');
-    renderReactCompilerMarkers({monaco, model, details: errors});
+    renderReactCompilerMarkers({
+      monaco,
+      model,
+      details: errors,
+      source: store.source,
+    });
     /**
      * N.B. that `tabSize` is a model property, not an editor property.
      * So, the tab size has to be set per model.
      */
     model.updateOptions({tabSize: 2});
-  }, [monaco, errors]);
+  }, [monaco, errors, store.source]);
 
   useEffect(() => {
     /**

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

@@ -11,7 +11,11 @@ import {
   InformationCircleIcon,
 } from '@heroicons/react/outline';
 import MonacoEditor, {DiffEditor} from '@monaco-editor/react';
-import {type CompilerError} from 'babel-plugin-react-compiler';
+import {
+  CompilerErrorDetail,
+  CompilerDiagnostic,
+  type CompilerError,
+} from 'babel-plugin-react-compiler';
 import parserBabel from 'prettier/plugins/babel';
 import * as prettierPluginEstree from 'prettier/plugins/estree';
 import * as prettier from 'prettier/standalone';
@@ -44,6 +48,7 @@ export type CompilerOutput =
       kind: 'ok';
       transformOutput: CompilerTransformOutput;
       results: Map<string, Array<PrintedCompilerPipelineValue>>;
+      errors: Array<CompilerErrorDetail | CompilerDiagnostic>;
     }
   | {
       kind: 'err';
@@ -123,10 +128,36 @@ async function tabify(
       parser: transformOutput.language === 'flow' ? 'babel-flow' : 'babel-ts',
       plugins: [parserBabel, prettierPluginEstree],
     });
+
+    let output: string;
+    let language: string;
+    if (compilerOutput.errors.length === 0) {
+      output = code;
+      language = 'javascript';
+    } else {
+      language = 'markdown';
+      output = `
+# Summary
+
+React Compiler compiled this function successfully, but there are lint errors that indicate potential issues with the original code.
+
+## ${compilerOutput.errors.length} Lint Errors
+
+${compilerOutput.errors.map(e => e.printErrorMessage(source, {eslint: false})).join('\n\n')}
+
+## Output
+
+\`\`\`js
+${code}
+\`\`\`
+`.trim();
+    }
+
     reorderedTabs.set(
-      'JS',
+      'Output',
       <TextTabContent
-        output={code}
+        output={output}
+        language={language}
         diff={null}
         showInfoPanel={false}></TextTabContent>,
     );
@@ -142,6 +173,18 @@ async function tabify(
         </>,
       );
     }
+  } else if (compilerOutput.kind === 'err') {
+    const errors = compilerOutput.error.printErrorMessage(source, {
+      eslint: false,
+    });
+    reorderedTabs.set(
+      'Output',
+      <TextTabContent
+        output={errors}
+        language="markdown"
+        diff={null}
+        showInfoPanel={false}></TextTabContent>,
+    );
   }
   tabs.forEach((tab, name) => {
     reorderedTabs.set(name, tab);
@@ -162,17 +205,32 @@ function getSourceMapUrl(code: string, map: string): string | null {
 }
 
 function Output({store, compilerOutput}: Props): JSX.Element {
-  const [tabsOpen, setTabsOpen] = useState<Set<string>>(() => new Set(['JS']));
+  const [tabsOpen, setTabsOpen] = useState<Set<string>>(
+    () => new Set(['Output']),
+  );
   const [tabs, setTabs] = useState<Map<string, React.ReactNode>>(
     () => new Map(),
   );
+
+  /*
+   * Update the active tab back to the output or errors tab when the compilation state
+   * changes between success/failure.
+   */
+  const [previousOutputKind, setPreviousOutputKind] = useState(
+    compilerOutput.kind,
+  );
+  if (compilerOutput.kind !== previousOutputKind) {
+    setPreviousOutputKind(compilerOutput.kind);
+    setTabsOpen(new Set(['Output']));
+  }
+
   useEffect(() => {
     tabify(store.source, compilerOutput).then(tabs => {
       setTabs(tabs);
     });
   }, [store.source, compilerOutput]);
 
-  const changedPasses: Set<string> = new Set(['JS', 'HIR']); // Initial and final passes should always be bold
+  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) {
     for (const result of results) {
@@ -196,20 +254,6 @@ function Output({store, compilerOutput}: Props): JSX.Element {
         tabs={tabs}
         changedPasses={changedPasses}
       />
-      {compilerOutput.kind === 'err' ? (
-        <div
-          className="flex flex-wrap absolute bottom-0 bg-white grow border-y border-grey-200 transition-all ease-in"
-          style={{width: 'calc(100vw - 650px)'}}>
-          <div className="w-full p-4 basis-full border-b">
-            <h2>COMPILER ERRORS</h2>
-          </div>
-          <pre
-            className="p-4 basis-full text-red-600 overflow-y-scroll whitespace-pre-wrap"
-            style={{width: 'calc(100vw - 650px)', height: '150px'}}>
-            <code>{compilerOutput.error.toString()}</code>
-          </pre>
-        </div>
-      ) : null}
     </>
   );
 }
@@ -218,10 +262,12 @@ function TextTabContent({
   output,
   diff,
   showInfoPanel,
+  language,
 }: {
   output: string;
   diff: string | null;
   showInfoPanel: boolean;
+  language: string;
 }): JSX.Element {
   const [diffMode, setDiffMode] = useState(false);
   return (
@@ -272,7 +318,7 @@ function TextTabContent({
         />
       ) : (
         <MonacoEditor
-          defaultLanguage="javascript"
+          language={language ?? 'javascript'}
           value={output}
           options={{
             ...monacoOptions,

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

@@ -28,5 +28,5 @@ export const monacoOptions: Partial<EditorProps['options']> = {
 
   automaticLayout: true,
   wordWrap: 'on',
-  wrappingIndent: 'deepIndent',
+  wrappingIndent: 'same',
 };

compiler/apps/playground/lib/reactCompilerMonacoDiagnostics.ts

@@ -6,7 +6,11 @@
  */
 
 import {Monaco} from '@monaco-editor/react';
-import {CompilerErrorDetail, ErrorSeverity} from 'babel-plugin-react-compiler';
+import {
+  CompilerDiagnostic,
+  CompilerErrorDetail,
+  ErrorSeverity,
+} from 'babel-plugin-react-compiler';
 import {MarkerSeverity, type editor} from 'monaco-editor';
 
 function mapReactCompilerSeverityToMonaco(
@@ -22,38 +26,46 @@ function mapReactCompilerSeverityToMonaco(
 }
 
 function mapReactCompilerDiagnosticToMonacoMarker(
-  detail: CompilerErrorDetail,
+  detail: CompilerErrorDetail | CompilerDiagnostic,
   monaco: Monaco,
+  source: string,
 ): editor.IMarkerData | null {
-  if (detail.loc == null || typeof detail.loc === 'symbol') {
+  const loc = detail.primaryLocation();
+  if (loc == null || typeof loc === 'symbol') {
     return null;
   }
   const severity = mapReactCompilerSeverityToMonaco(detail.severity, monaco);
-  let message = detail.printErrorMessage();
+  let message = detail.printErrorMessage(source, {eslint: true});
   return {
     severity,
     message,
-    startLineNumber: detail.loc.start.line,
-    startColumn: detail.loc.start.column + 1,
-    endLineNumber: detail.loc.end.line,
-    endColumn: detail.loc.end.column + 1,
+    startLineNumber: loc.start.line,
+    startColumn: loc.start.column + 1,
+    endLineNumber: loc.end.line,
+    endColumn: loc.end.column + 1,
   };
 }
 
 type ReactCompilerMarkerConfig = {
   monaco: Monaco;
   model: editor.ITextModel;
-  details: Array<CompilerErrorDetail>;
+  details: Array<CompilerErrorDetail | CompilerDiagnostic>;
+  source: string;
 };
 let decorations: Array<string> = [];
 export function renderReactCompilerMarkers({
   monaco,
   model,
   details,
+  source,
 }: ReactCompilerMarkerConfig): void {
   const markers: Array<editor.IMarkerData> = [];
   for (const detail of details) {
-    const marker = mapReactCompilerDiagnosticToMonacoMarker(detail, monaco);
+    const marker = mapReactCompilerDiagnosticToMonacoMarker(
+      detail,
+      monaco,
+      source,
+    );
     if (marker == null) {
       continue;
     }

compiler/packages/babel-plugin-react-compiler/src/Babel/BabelPlugin.ts

@@ -84,7 +84,7 @@ export default function BabelPluginReactCompiler(
             }
           } catch (e) {
             if (e instanceof CompilerError) {
-              throw new Error(e.printErrorMessage(pass.file.code));
+              throw e.withPrintedMessage(pass.file.code, {eslint: false});
             }
             throw e;
           }

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

@@ -5,6 +5,7 @@
  * LICENSE file in the root directory of this source tree.
  */
 
+import * as t from '@babel/types';
 import {codeFrameColumns} from '@babel/code-frame';
 import type {SourceLocation} from './HIR';
 import {Err, Ok, Result} from './Utils/Result';
@@ -93,6 +94,14 @@ export type CompilerErrorDetailOptions = {
   suggestions?: Array<CompilerSuggestion> | null | undefined;
 };
 
+export type PrintErrorMessageOptions = {
+  /**
+   * ESLint uses 1-indexed columns and prints one error at a time
+   * So it doesn't require the "Found # error(s)" text
+   */
+  eslint: boolean;
+};
+
 export class CompilerDiagnostic {
   options: CompilerDiagnosticOptions;
 
@@ -128,7 +137,7 @@ export class CompilerDiagnostic {
     return this.options.details.filter(d => d.kind === 'error')[0]?.loc ?? null;
   }
 
-  printErrorMessage(source: string): string {
+  printErrorMessage(source: string, options: PrintErrorMessageOptions): string {
     const buffer = [
       printErrorSummary(this.severity, this.category),
       '\n\n',
@@ -143,28 +152,18 @@ export class CompilerDiagnostic {
           }
           let codeFrame: string;
           try {
-            codeFrame = codeFrameColumns(
-              source,
-              {
-                start: {
-                  line: loc.start.line,
-                  column: loc.start.column + 1,
-                },
-                end: {
-                  line: loc.end.line,
-                  column: loc.end.column + 1,
-                },
-              },
-              {
-                message: detail.message,
-              },
-            );
+            codeFrame = printCodeFrame(source, loc, detail.message);
           } catch (e) {
             codeFrame = detail.message;
           }
-          buffer.push(
-            `\n\n${loc.filename}:${loc.start.line}:${loc.start.column}\n`,
-          );
+          buffer.push('\n\n');
+          if (loc.filename != null) {
+            const line = loc.start.line;
+            const column = options.eslint
+              ? loc.start.column + 1
+              : loc.start.column;
+            buffer.push(`${loc.filename}:${line}:${column}\n`);
+          }
           buffer.push(codeFrame);
           break;
         }
@@ -223,7 +222,7 @@ export class CompilerErrorDetail {
     return this.loc;
   }
 
-  printErrorMessage(source: string): string {
+  printErrorMessage(source: string, options: PrintErrorMessageOptions): string {
     const buffer = [printErrorSummary(this.severity, this.reason)];
     if (this.description != null) {
       buffer.push(`\n\n${this.description}.`);
@@ -232,28 +231,16 @@ export class CompilerErrorDetail {
     if (loc != null && typeof loc !== 'symbol') {
       let codeFrame: string;
       try {
-        codeFrame = codeFrameColumns(
-          source,
-          {
-            start: {
-              line: loc.start.line,
-              column: loc.start.column + 1,
-            },
-            end: {
-              line: loc.end.line,
-              column: loc.end.column + 1,
-            },
-          },
-          {
-            message: this.reason,
-          },
-        );
+        codeFrame = printCodeFrame(source, loc, this.reason);
       } catch (e) {
         codeFrame = '';
       }
-      buffer.push(
-        `\n\n${loc.filename}:${loc.start.line}:${loc.start.column}\n`,
-      );
+      buffer.push(`\n\n`);
+      if (loc.filename != null) {
+        const line = loc.start.line;
+        const column = options.eslint ? loc.start.column + 1 : loc.start.column;
+        buffer.push(`${loc.filename}:${line}:${column}\n`);
+      }
       buffer.push(codeFrame);
       buffer.push('\n\n');
     }
@@ -275,6 +262,7 @@ export class CompilerErrorDetail {
 
 export class CompilerError extends Error {
   details: Array<CompilerErrorDetail | CompilerDiagnostic> = [];
+  printedMessage: string | null = null;
 
   static invariant(
     condition: unknown,
@@ -360,22 +348,38 @@ export class CompilerError extends Error {
   }
 
   override get message(): string {
-    return this.toString();
+    return this.printedMessage ?? this.toString();
   }
 
   override set message(_message: string) {}
 
   override toString(): string {
+    if (this.printedMessage) {
+      return this.printedMessage;
+    }
     if (Array.isArray(this.details)) {
       return this.details.map(detail => detail.toString()).join('\n\n');
     }
     return this.name;
   }
 
-  printErrorMessage(source: string): string {
+  withPrintedMessage(
+    source: string,
+    options: PrintErrorMessageOptions,
+  ): CompilerError {
+    this.printedMessage = this.printErrorMessage(source, options);
+    return this;
+  }
+
+  printErrorMessage(source: string, options: PrintErrorMessageOptions): string {
+    if (options.eslint && this.details.length === 1) {
+      return this.details[0].printErrorMessage(source, options);
+    }
     return (
-      `Found ${this.details.length} error${this.details.length === 1 ? '' : 's'}:\n` +
-      this.details.map(detail => detail.printErrorMessage(source)).join('\n')
+      `Found ${this.details.length} error${this.details.length === 1 ? '' : 's'}:\n\n` +
+      this.details
+        .map(detail => detail.printErrorMessage(source, options).trim())
+        .join('\n\n')
     );
   }
 
@@ -438,6 +442,29 @@ export class CompilerError extends Error {
   }
 }
 
+function printCodeFrame(
+  source: string,
+  loc: t.SourceLocation,
+  message: string,
+): string {
+  return codeFrameColumns(
+    source,
+    {
+      start: {
+        line: loc.start.line,
+        column: loc.start.column + 1,
+      },
+      end: {
+        line: loc.end.line,
+        column: loc.end.column + 1,
+      },
+    },
+    {
+      message,
+    },
+  );
+}
+
 function printErrorSummary(severity: ErrorSeverity, message: string): string {
   let severityCategory: string;
   switch (severity) {

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

@@ -92,7 +92,6 @@ import {
 } from '../Validation';
 import {validateLocalsNotReassignedAfterRender} from '../Validation/ValidateLocalsNotReassignedAfterRender';
 import {outlineFunctions} from '../Optimization/OutlineFunctions';
-import {propagatePhiTypes} from '../TypeInference/PropagatePhiTypes';
 import {lowerContextAccess} from '../Optimization/LowerContextAccess';
 import {validateNoSetStateInEffects} from '../Validation/ValidateNoSetStateInEffects';
 import {validateNoJSXInTryStatement} from '../Validation/ValidateNoJSXInTryStatement';
@@ -106,6 +105,7 @@ import {validateStaticComponents} from '../Validation/ValidateStaticComponents';
 import {validateNoFreezingKnownMutableFunctions} from '../Validation/ValidateNoFreezingKnownMutableFunctions';
 import {inferMutationAliasingEffects} from '../Inference/InferMutationAliasingEffects';
 import {inferMutationAliasingRanges} from '../Inference/InferMutationAliasingRanges';
+import {validateNoDerivedComputationsInEffects} from '../Validation/ValidateNoDerivedComputationsInEffects';
 
 export type CompilerPipelineValue =
   | {kind: 'ast'; name: string; value: CodegenFunction}
@@ -174,7 +174,7 @@ function runWithEnvironment(
     !env.config.disableMemoizationForDebugging &&
     !env.config.enableChangeDetectionForDebugging
   ) {
-    dropManualMemoization(hir);
+    dropManualMemoization(hir).unwrap();
     log({kind: 'hir', name: 'DropManualMemoization', value: hir});
   }
 
@@ -292,6 +292,10 @@ function runWithEnvironment(
       validateNoSetStateInRender(hir).unwrap();
     }
 
+    if (env.config.validateNoDerivedComputationsInEffects) {
+      validateNoDerivedComputationsInEffects(hir);
+    }
+
     if (env.config.validateNoSetStateInEffects) {
       env.logErrors(validateNoSetStateInEffects(hir));
     }
@@ -322,13 +326,6 @@ function runWithEnvironment(
     value: hir,
   });
 
-  propagatePhiTypes(hir);
-  log({
-    kind: 'hir',
-    name: 'PropagatePhiTypes',
-    value: hir,
-  });
-
   if (env.isInferredMemoEnabled) {
     if (env.config.validateStaticComponents) {
       env.logErrors(validateStaticComponents(hir));

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

@@ -8,8 +8,8 @@
 import {NodePath} from '@babel/core';
 import * as t from '@babel/types';
 import {
+  CompilerDiagnostic,
   CompilerError,
-  CompilerErrorDetail,
   CompilerSuggestionOperation,
   ErrorSeverity,
 } from '../CompilerError';
@@ -181,12 +181,11 @@ export function suppressionsToCompilerError(
           'Unhandled suppression source',
         );
     }
-    error.pushErrorDetail(
-      new CompilerErrorDetail({
-        reason: `${reason}. React Compiler only works when your components follow all the rules of React, disabling them may result in unexpected or incorrect behavior`,
-        description: suppressionRange.disableComment.value.trim(),
+    error.pushDiagnostic(
+      CompilerDiagnostic.create({
+        category: reason,
+        description: `React Compiler only works when your components follow all the rules of React, disabling them may result in unexpected or incorrect behavior. Found suppression \`${suppressionRange.disableComment.value.trim()}\``,
         severity: ErrorSeverity.InvalidReact,
-        loc: suppressionRange.disableComment.loc ?? null,
         suggestions: [
           {
             description: suggestion,
@@ -197,6 +196,10 @@ export function suppressionsToCompilerError(
             op: CompilerSuggestionOperation.Remove,
           },
         ],
+      }).withDetail({
+        kind: 'error',
+        loc: suppressionRange.disableComment.loc ?? null,
+        message: 'Found React rule suppression',
       }),
     );
   }

compiler/packages/babel-plugin-react-compiler/src/Flood/FlowTypes.ts

@@ -0,0 +1,752 @@
+/**
+ * TypeScript definitions for Flow type JSON representations
+ * Based on the output of /data/sandcastle/boxes/fbsource/fbcode/flow/src/typing/convertTypes.ml
+ */
+
+// Base type for all Flow types with a kind field
+export interface BaseFlowType {
+  kind: string;
+}
+
+// Type for representing polarity
+export type Polarity = 'positive' | 'negative' | 'neutral';
+
+// Type for representing a name that might be null
+export type OptionalName = string | null;
+
+// Open type
+export interface OpenType extends BaseFlowType {
+  kind: 'Open';
+}
+
+// Def type
+export interface DefType extends BaseFlowType {
+  kind: 'Def';
+  def: DefT;
+}
+
+// Eval type
+export interface EvalType extends BaseFlowType {
+  kind: 'Eval';
+  type: FlowType;
+  destructor: Destructor;
+}
+
+// Generic type
+export interface GenericType extends BaseFlowType {
+  kind: 'Generic';
+  name: string;
+  bound: FlowType;
+  no_infer: boolean;
+}
+
+// ThisInstance type
+export interface ThisInstanceType extends BaseFlowType {
+  kind: 'ThisInstance';
+  instance: InstanceT;
+  is_this: boolean;
+  name: string;
+}
+
+// ThisTypeApp type
+export interface ThisTypeAppType extends BaseFlowType {
+  kind: 'ThisTypeApp';
+  t1: FlowType;
+  t2: FlowType;
+  t_list?: Array<FlowType>;
+}
+
+// TypeApp type
+export interface TypeAppType extends BaseFlowType {
+  kind: 'TypeApp';
+  type: FlowType;
+  targs: Array<FlowType>;
+  from_value: boolean;
+  use_desc: boolean;
+}
+
+// FunProto type
+export interface FunProtoType extends BaseFlowType {
+  kind: 'FunProto';
+}
+
+// ObjProto type
+export interface ObjProtoType extends BaseFlowType {
+  kind: 'ObjProto';
+}
+
+// NullProto type
+export interface NullProtoType extends BaseFlowType {
+  kind: 'NullProto';
+}
+
+// FunProtoBind type
+export interface FunProtoBindType extends BaseFlowType {
+  kind: 'FunProtoBind';
+}
+
+// Intersection type
+export interface IntersectionType extends BaseFlowType {
+  kind: 'Intersection';
+  members: Array<FlowType>;
+}
+
+// Union type
+export interface UnionType extends BaseFlowType {
+  kind: 'Union';
+  members: Array<FlowType>;
+}
+
+// Maybe type
+export interface MaybeType extends BaseFlowType {
+  kind: 'Maybe';
+  type: FlowType;
+}
+
+// Optional type
+export interface OptionalType extends BaseFlowType {
+  kind: 'Optional';
+  type: FlowType;
+  use_desc: boolean;
+}
+
+// Keys type
+export interface KeysType extends BaseFlowType {
+  kind: 'Keys';
+  type: FlowType;
+}
+
+// Annot type
+export interface AnnotType extends BaseFlowType {
+  kind: 'Annot';
+  type: FlowType;
+  use_desc: boolean;
+}
+
+// Opaque type
+export interface OpaqueType extends BaseFlowType {
+  kind: 'Opaque';
+  opaquetype: {
+    opaque_id: string;
+    underlying_t: FlowType | null;
+    super_t: FlowType | null;
+    opaque_type_args: Array<{
+      name: string;
+      type: FlowType;
+      polarity: Polarity;
+    }>;
+    opaque_name: string;
+  };
+}
+
+// Namespace type
+export interface NamespaceType extends BaseFlowType {
+  kind: 'Namespace';
+  namespace_symbol: {
+    symbol: string;
+  };
+  values_type: FlowType;
+  types_tmap: PropertyMap;
+}
+
+// Any type
+export interface AnyType extends BaseFlowType {
+  kind: 'Any';
+}
+
+// StrUtil type
+export interface StrUtilType extends BaseFlowType {
+  kind: 'StrUtil';
+  op: 'StrPrefix' | 'StrSuffix';
+  prefix?: string;
+  suffix?: string;
+  remainder?: FlowType;
+}
+
+// TypeParam definition
+export interface TypeParam {
+  name: string;
+  bound: FlowType;
+  polarity: Polarity;
+  default: FlowType | null;
+}
+
+// EnumInfo types
+export type EnumInfo = ConcreteEnum | AbstractEnum;
+
+export interface ConcreteEnum {
+  kind: 'ConcreteEnum';
+  enum_name: string;
+  enum_id: string;
+  members: Array<string>;
+  representation_t: FlowType;
+  has_unknown_members: boolean;
+}
+
+export interface AbstractEnum {
+  kind: 'AbstractEnum';
+  representation_t: FlowType;
+}
+
+// CanonicalRendersForm types
+export type CanonicalRendersForm =
+  | InstrinsicRenders
+  | NominalRenders
+  | StructuralRenders
+  | DefaultRenders;
+
+export interface InstrinsicRenders {
+  kind: 'InstrinsicRenders';
+  name: string;
+}
+
+export interface NominalRenders {
+  kind: 'NominalRenders';
+  renders_id: string;
+  renders_name: string;
+  renders_super: FlowType;
+}
+
+export interface StructuralRenders {
+  kind: 'StructuralRenders';
+  renders_variant: 'RendersNormal' | 'RendersMaybe' | 'RendersStar';
+  renders_structural_type: FlowType;
+}
+
+export interface DefaultRenders {
+  kind: 'DefaultRenders';
+}
+
+// InstanceT definition
+export interface InstanceT {
+  inst: InstType;
+  static: FlowType;
+  super: FlowType;
+  implements: Array<FlowType>;
+}
+
+// InstType definition
+export interface InstType {
+  class_name: string | null;
+  class_id: string;
+  type_args: Array<{
+    name: string;
+    type: FlowType;
+    polarity: Polarity;
+  }>;
+  own_props: PropertyMap;
+  proto_props: PropertyMap;
+  call_t: null | {
+    id: number;
+    call: FlowType;
+  };
+}
+
+// DefT types
+export type DefT =
+  | NumGeneralType
+  | StrGeneralType
+  | BoolGeneralType
+  | BigIntGeneralType
+  | EmptyType
+  | MixedType
+  | NullType
+  | VoidType
+  | SymbolType
+  | FunType
+  | ObjType
+  | ArrType
+  | ClassType
+  | InstanceType
+  | SingletonStrType
+  | NumericStrKeyType
+  | SingletonNumType
+  | SingletonBoolType
+  | SingletonBigIntType
+  | TypeType
+  | PolyType
+  | ReactAbstractComponentType
+  | RendersType
+  | EnumValueType
+  | EnumObjectType;
+
+export interface NumGeneralType extends BaseFlowType {
+  kind: 'NumGeneral';
+}
+
+export interface StrGeneralType extends BaseFlowType {
+  kind: 'StrGeneral';
+}
+
+export interface BoolGeneralType extends BaseFlowType {
+  kind: 'BoolGeneral';
+}
+
+export interface BigIntGeneralType extends BaseFlowType {
+  kind: 'BigIntGeneral';
+}
+
+export interface EmptyType extends BaseFlowType {
+  kind: 'Empty';
+}
+
+export interface MixedType extends BaseFlowType {
+  kind: 'Mixed';
+}
+
+export interface NullType extends BaseFlowType {
+  kind: 'Null';
+}
+
+export interface VoidType extends BaseFlowType {
+  kind: 'Void';
+}
+
+export interface SymbolType extends BaseFlowType {
+  kind: 'Symbol';
+}
+
+export interface FunType extends BaseFlowType {
+  kind: 'Fun';
+  static: FlowType;
+  funtype: FunTypeObj;
+}
+
+export interface ObjType extends BaseFlowType {
+  kind: 'Obj';
+  objtype: ObjTypeObj;
+}
+
+export interface ArrType extends BaseFlowType {
+  kind: 'Arr';
+  arrtype: ArrTypeObj;
+}
+
+export interface ClassType extends BaseFlowType {
+  kind: 'Class';
+  type: FlowType;
+}
+
+export interface InstanceType extends BaseFlowType {
+  kind: 'Instance';
+  instance: InstanceT;
+}
+
+export interface SingletonStrType extends BaseFlowType {
+  kind: 'SingletonStr';
+  from_annot: boolean;
+  value: string;
+}
+
+export interface NumericStrKeyType extends BaseFlowType {
+  kind: 'NumericStrKey';
+  number: string;
+  string: string;
+}
+
+export interface SingletonNumType extends BaseFlowType {
+  kind: 'SingletonNum';
+  from_annot: boolean;
+  number: string;
+  string: string;
+}
+
+export interface SingletonBoolType extends BaseFlowType {
+  kind: 'SingletonBool';
+  from_annot: boolean;
+  value: boolean;
+}
+
+export interface SingletonBigIntType extends BaseFlowType {
+  kind: 'SingletonBigInt';
+  from_annot: boolean;
+  value: string;
+}
+
+export interface TypeType extends BaseFlowType {
+  kind: 'Type';
+  type_kind: TypeTKind;
+  type: FlowType;
+}
+
+export type TypeTKind =
+  | 'TypeAliasKind'
+  | 'TypeParamKind'
+  | 'OpaqueKind'
+  | 'ImportTypeofKind'
+  | 'ImportClassKind'
+  | 'ImportEnumKind'
+  | 'InstanceKind'
+  | 'RenderTypeKind';
+
+export interface PolyType extends BaseFlowType {
+  kind: 'Poly';
+  tparams: Array<TypeParam>;
+  t_out: FlowType;
+  id: string;
+}
+
+export interface ReactAbstractComponentType extends BaseFlowType {
+  kind: 'ReactAbstractComponent';
+  config: FlowType;
+  renders: FlowType;
+  instance: ComponentInstance;
+  component_kind: ComponentKind;
+}
+
+export type ComponentInstance =
+  | {kind: 'RefSetterProp'; type: FlowType}
+  | {kind: 'Omitted'};
+
+export type ComponentKind =
+  | {kind: 'Structural'}
+  | {kind: 'Nominal'; id: string; name: string; types: Array<FlowType> | null};
+
+export interface RendersType extends BaseFlowType {
+  kind: 'Renders';
+  form: CanonicalRendersForm;
+}
+
+export interface EnumValueType extends BaseFlowType {
+  kind: 'EnumValue';
+  enum_info: EnumInfo;
+}
+
+export interface EnumObjectType extends BaseFlowType {
+  kind: 'EnumObject';
+  enum_value_t: FlowType;
+  enum_info: EnumInfo;
+}
+
+// ObjKind types
+export type ObjKind =
+  | {kind: 'Exact'}
+  | {kind: 'Inexact'}
+  | {kind: 'Indexed'; dicttype: DictType};
+
+// DictType definition
+export interface DictType {
+  dict_name: string | null;
+  key: FlowType;
+  value: FlowType;
+  dict_polarity: Polarity;
+}
+
+// ArrType types
+export type ArrTypeObj = ArrayAT | TupleAT | ROArrayAT;
+
+export interface ArrayAT {
+  kind: 'ArrayAT';
+  elem_t: FlowType;
+}
+
+export interface TupleAT {
+  kind: 'TupleAT';
+  elem_t: FlowType;
+  elements: Array<TupleElement>;
+  min_arity: number;
+  max_arity: number;
+  inexact: boolean;
+}
+
+export interface ROArrayAT {
+  kind: 'ROArrayAT';
+  elem_t: FlowType;
+}
+
+// TupleElement definition
+export interface TupleElement {
+  name: string | null;
+  t: FlowType;
+  polarity: Polarity;
+  optional: boolean;
+}
+
+// Flags definition
+export interface Flags {
+  obj_kind: ObjKind;
+}
+
+// Property types
+export type Property =
+  | FieldProperty
+  | GetProperty
+  | SetProperty
+  | GetSetProperty
+  | MethodProperty;
+
+export interface FieldProperty {
+  kind: 'Field';
+  type: FlowType;
+  polarity: Polarity;
+}
+
+export interface GetProperty {
+  kind: 'Get';
+  type: FlowType;
+}
+
+export interface SetProperty {
+  kind: 'Set';
+  type: FlowType;
+}
+
+export interface GetSetProperty {
+  kind: 'GetSet';
+  get_type: FlowType;
+  set_type: FlowType;
+}
+
+export interface MethodProperty {
+  kind: 'Method';
+  type: FlowType;
+}
+
+// PropertyMap definition
+export interface PropertyMap {
+  [key: string]: Property; // For other properties in the map
+}
+
+// ObjType definition
+export interface ObjTypeObj {
+  flags: Flags;
+  props: PropertyMap;
+  proto_t: FlowType;
+  call_t: number | null;
+}
+
+// FunType definition
+export interface FunTypeObj {
+  this_t: {
+    type: FlowType;
+    status: ThisStatus;
+  };
+  params: Array<{
+    name: string | null;
+    type: FlowType;
+  }>;
+  rest_param: null | {
+    name: string | null;
+    type: FlowType;
+  };
+  return_t: FlowType;
+  type_guard: null | {
+    inferred: boolean;
+    param_name: string;
+    type_guard: FlowType;
+    one_sided: boolean;
+  };
+  effect: Effect;
+}
+
+// ThisStatus types
+export type ThisStatus =
+  | {kind: 'This_Method'; unbound: boolean}
+  | {kind: 'This_Function'};
+
+// Effect types
+export type Effect =
+  | {kind: 'HookDecl'; id: string}
+  | {kind: 'HookAnnot'}
+  | {kind: 'ArbitraryEffect'}
+  | {kind: 'AnyEffect'};
+
+// Destructor types
+export type Destructor =
+  | NonMaybeTypeDestructor
+  | PropertyTypeDestructor
+  | ElementTypeDestructor
+  | OptionalIndexedAccessNonMaybeTypeDestructor
+  | OptionalIndexedAccessResultTypeDestructor
+  | ExactTypeDestructor
+  | ReadOnlyTypeDestructor
+  | PartialTypeDestructor
+  | RequiredTypeDestructor
+  | SpreadTypeDestructor
+  | SpreadTupleTypeDestructor
+  | RestTypeDestructor
+  | ValuesTypeDestructor
+  | ConditionalTypeDestructor
+  | TypeMapDestructor
+  | ReactElementPropsTypeDestructor
+  | ReactElementConfigTypeDestructor
+  | ReactCheckComponentConfigDestructor
+  | ReactDRODestructor
+  | MakeHooklikeDestructor
+  | MappedTypeDestructor
+  | EnumTypeDestructor;
+
+export interface NonMaybeTypeDestructor {
+  kind: 'NonMaybeType';
+}
+
+export interface PropertyTypeDestructor {
+  kind: 'PropertyType';
+  name: string;
+}
+
+export interface ElementTypeDestructor {
+  kind: 'ElementType';
+  index_type: FlowType;
+}
+
+export interface OptionalIndexedAccessNonMaybeTypeDestructor {
+  kind: 'OptionalIndexedAccessNonMaybeType';
+  index: OptionalIndexedAccessIndex;
+}
+
+export type OptionalIndexedAccessIndex =
+  | {kind: 'StrLitIndex'; name: string}
+  | {kind: 'TypeIndex'; type: FlowType};
+
+export interface OptionalIndexedAccessResultTypeDestructor {
+  kind: 'OptionalIndexedAccessResultType';
+}
+
+export interface ExactTypeDestructor {
+  kind: 'ExactType';
+}
+
+export interface ReadOnlyTypeDestructor {
+  kind: 'ReadOnlyType';
+}
+
+export interface PartialTypeDestructor {
+  kind: 'PartialType';
+}
+
+export interface RequiredTypeDestructor {
+  kind: 'RequiredType';
+}
+
+export interface SpreadTypeDestructor {
+  kind: 'SpreadType';
+  target: SpreadTarget;
+  operands: Array<SpreadOperand>;
+  operand_slice: Slice | null;
+}
+
+export type SpreadTarget =
+  | {kind: 'Value'; make_seal: 'Sealed' | 'Frozen' | 'As_Const'}
+  | {kind: 'Annot'; make_exact: boolean};
+
+export type SpreadOperand = {kind: 'Type'; type: FlowType} | Slice;
+
+export interface Slice {
+  kind: 'Slice';
+  prop_map: PropertyMap;
+  generics: Array<string>;
+  dict: DictType | null;
+  reachable_targs: Array<{
+    type: FlowType;
+    polarity: Polarity;
+  }>;
+}
+
+export interface SpreadTupleTypeDestructor {
+  kind: 'SpreadTupleType';
+  inexact: boolean;
+  resolved_rev: string;
+  unresolved: string;
+}
+
+export interface RestTypeDestructor {
+  kind: 'RestType';
+  merge_mode: RestMergeMode;
+  type: FlowType;
+}
+
+export type RestMergeMode =
+  | {kind: 'SpreadReversal'}
+  | {kind: 'ReactConfigMerge'; polarity: Polarity}
+  | {kind: 'Omit'};
+
+export interface ValuesTypeDestructor {
+  kind: 'ValuesType';
+}
+
+export interface ConditionalTypeDestructor {
+  kind: 'ConditionalType';
+  distributive_tparam_name: string | null;
+  infer_tparams: string;
+  extends_t: FlowType;
+  true_t: FlowType;
+  false_t: FlowType;
+}
+
+export interface TypeMapDestructor {
+  kind: 'ObjectKeyMirror';
+}
+
+export interface ReactElementPropsTypeDestructor {
+  kind: 'ReactElementPropsType';
+}
+
+export interface ReactElementConfigTypeDestructor {
+  kind: 'ReactElementConfigType';
+}
+
+export interface ReactCheckComponentConfigDestructor {
+  kind: 'ReactCheckComponentConfig';
+  props: {
+    [key: string]: Property;
+  };
+}
+
+export interface ReactDRODestructor {
+  kind: 'ReactDRO';
+  dro_type:
+    | 'HookReturn'
+    | 'HookArg'
+    | 'Props'
+    | 'ImmutableAnnot'
+    | 'DebugAnnot';
+}
+
+export interface MakeHooklikeDestructor {
+  kind: 'MakeHooklike';
+}
+
+export interface MappedTypeDestructor {
+  kind: 'MappedType';
+  homomorphic: Homomorphic;
+  distributive_tparam_name: string | null;
+  property_type: FlowType;
+  mapped_type_flags: {
+    variance: Polarity;
+    optional: 'MakeOptional' | 'RemoveOptional' | 'KeepOptionality';
+  };
+}
+
+export type Homomorphic =
+  | {kind: 'Homomorphic'}
+  | {kind: 'Unspecialized'}
+  | {kind: 'SemiHomomorphic'; type: FlowType};
+
+export interface EnumTypeDestructor {
+  kind: 'EnumType';
+}
+
+// Union of all possible Flow types
+export type FlowType =
+  | OpenType
+  | DefType
+  | EvalType
+  | GenericType
+  | ThisInstanceType
+  | ThisTypeAppType
+  | TypeAppType
+  | FunProtoType
+  | ObjProtoType
+  | NullProtoType
+  | FunProtoBindType
+  | IntersectionType
+  | UnionType
+  | MaybeType
+  | OptionalType
+  | KeysType
+  | AnnotType
+  | OpaqueType
+  | NamespaceType
+  | AnyType
+  | StrUtilType;

compiler/packages/babel-plugin-react-compiler/src/Flood/TypeErrors.ts

@@ -0,0 +1,131 @@
+import {CompilerError, SourceLocation} from '..';
+import {
+  ConcreteType,
+  printConcrete,
+  printType,
+  StructuralValue,
+  Type,
+  VariableId,
+} from './Types';
+
+export function unsupportedLanguageFeature(
+  desc: string,
+  loc: SourceLocation,
+): never {
+  CompilerError.throwInvalidJS({
+    reason: `Typedchecker does not currently support language feature: ${desc}`,
+    loc,
+  });
+}
+
+export type UnificationError =
+  | {
+      kind: 'TypeUnification';
+      left: ConcreteType<Type>;
+      right: ConcreteType<Type>;
+    }
+  | {
+      kind: 'StructuralUnification';
+      left: StructuralValue;
+      right: ConcreteType<Type>;
+    };
+
+function printUnificationError(err: UnificationError): string {
+  if (err.kind === 'TypeUnification') {
+    return `${printConcrete(err.left, printType)} is incompatible with ${printConcrete(err.right, printType)}`;
+  } else {
+    return `structural ${err.left.kind} is incompatible with ${printConcrete(err.right, printType)}`;
+  }
+}
+
+export function raiseUnificationErrors(
+  errs: null | Array<UnificationError>,
+  loc: SourceLocation,
+): void {
+  if (errs != null) {
+    if (errs.length === 0) {
+      CompilerError.invariant(false, {
+        reason: 'Should not have array of zero errors',
+        loc,
+      });
+    } else if (errs.length === 1) {
+      CompilerError.throwInvalidJS({
+        reason: `Unable to unify types because ${printUnificationError(errs[0])}`,
+        loc,
+      });
+    } else {
+      const messages = errs
+        .map(err => `\t* ${printUnificationError(err)}`)
+        .join('\n');
+      CompilerError.throwInvalidJS({
+        reason: `Unable to unify types because:\n${messages}`,
+        loc,
+      });
+    }
+  }
+}
+
+export function unresolvableTypeVariable(
+  id: VariableId,
+  loc: SourceLocation,
+): never {
+  CompilerError.throwInvalidJS({
+    reason: `Unable to resolve free variable ${id} to a concrete type`,
+    loc,
+  });
+}
+
+export function cannotAddVoid(explicit: boolean, loc: SourceLocation): never {
+  if (explicit) {
+    CompilerError.throwInvalidJS({
+      reason: `Undefined is not a valid operand of \`+\``,
+      loc,
+    });
+  } else {
+    CompilerError.throwInvalidJS({
+      reason: `Value may be undefined, which is not a valid operand of \`+\``,
+      loc,
+    });
+  }
+}
+
+export function unsupportedTypeAnnotation(
+  desc: string,
+  loc: SourceLocation,
+): never {
+  CompilerError.throwInvalidJS({
+    reason: `Typedchecker does not currently support type annotation: ${desc}`,
+    loc,
+  });
+}
+
+export function checkTypeArgumentArity(
+  desc: string,
+  expected: number,
+  actual: number,
+  loc: SourceLocation,
+): void {
+  if (expected !== actual) {
+    CompilerError.throwInvalidJS({
+      reason: `Expected ${desc} to have ${expected} type parameters, got ${actual}`,
+      loc,
+    });
+  }
+}
+
+export function notAFunction(desc: string, loc: SourceLocation): void {
+  CompilerError.throwInvalidJS({
+    reason: `Cannot call ${desc} because it is not a function`,
+    loc,
+  });
+}
+
+export function notAPolymorphicFunction(
+  desc: string,
+  loc: SourceLocation,
+): void {
+  CompilerError.throwInvalidJS({
+    reason: `Cannot call ${desc} with type arguments because it is not a polymorphic function`,
+    loc,
+  });
+}

compiler/packages/babel-plugin-react-compiler/src/Flood/TypeUtils.ts

@@ -0,0 +1,312 @@
+import {GeneratedSource} from '../HIR';
+import {assertExhaustive} from '../Utils/utils';
+import {unsupportedLanguageFeature} from './TypeErrors';
+import {
+  ConcreteType,
+  ResolvedType,
+  TypeParameter,
+  TypeParameterId,
+  DEBUG,
+  printConcrete,
+  printType,
+} from './Types';
+
+export function substitute(
+  type: ConcreteType<ResolvedType>,
+  typeParameters: Array<TypeParameter<ResolvedType>>,
+  typeArguments: Array<ResolvedType>,
+): ResolvedType {
+  const substMap = new Map<TypeParameterId, ResolvedType>();
+  for (let i = 0; i < typeParameters.length; i++) {
+    // TODO: Length checks to make sure type params match up with args
+    const typeParameter = typeParameters[i];
+    const typeArgument = typeArguments[i];
+    substMap.set(typeParameter.id, typeArgument);
+  }
+  const substitutionFunction = (t: ResolvedType): ResolvedType => {
+    // TODO: We really want a stateful mapper or visitor here so that we can model nested polymorphic types
+    if (t.type.kind === 'Generic' && substMap.has(t.type.id)) {
+      const substitutedType = substMap.get(t.type.id)!;
+      return substitutedType;
+    }
+
+    return {
+      kind: 'Concrete',
+      type: mapType(substitutionFunction, t.type),
+      platform: t.platform,
+    };
+  };
+
+  const substituted = mapType(substitutionFunction, type);
+
+  if (DEBUG) {
+    let substs = '';
+    for (let i = 0; i < typeParameters.length; i++) {
+      const typeParameter = typeParameters[i];
+      const typeArgument = typeArguments[i];
+      substs += `[${typeParameter.name}${typeParameter.id} := ${printType(typeArgument)}]`;
+    }
+    console.log(
+      `${printConcrete(type, printType)}${substs} = ${printConcrete(substituted, printType)}`,
+    );
+  }
+
+  return {kind: 'Concrete', type: substituted, platform: /* TODO */ 'shared'};
+}
+
+export function mapType<T, U>(
+  f: (t: T) => U,
+  type: ConcreteType<T>,
+): ConcreteType<U> {
+  switch (type.kind) {
+    case 'Mixed':
+    case 'Number':
+    case 'String':
+    case 'Boolean':
+    case 'Void':
+      return type;
+
+    case 'Nullable':
+      return {
+        kind: 'Nullable',
+        type: f(type.type),
+      };
+
+    case 'Array':
+      return {
+        kind: 'Array',
+        element: f(type.element),
+      };
+
+    case 'Set':
+      return {
+        kind: 'Set',
+        element: f(type.element),
+      };
+
+    case 'Map':
+      return {
+        kind: 'Map',
+        key: f(type.key),
+        value: f(type.value),
+      };
+
+    case 'Function':
+      return {
+        kind: 'Function',
+        typeParameters:
+          type.typeParameters?.map(param => ({
+            id: param.id,
+            name: param.name,
+            bound: f(param.bound),
+          })) ?? null,
+        params: type.params.map(f),
+        returnType: f(type.returnType),
+      };
+
+    case 'Component': {
+      return {
+        kind: 'Component',
+        children: type.children != null ? f(type.children) : null,
+        props: new Map([...type.props.entries()].map(([k, v]) => [k, f(v)])),
+      };
+    }
+
+    case 'Generic':
+      return {
+        kind: 'Generic',
+        id: type.id,
+        bound: f(type.bound),
+      };
+
+    case 'Object':
+      return type;
+
+    case 'Tuple':
+      return {
+        kind: 'Tuple',
+        id: type.id,
+        members: type.members.map(f),
+      };
+
+    case 'Structural':
+      return type;
+
+    case 'Enum':
+    case 'Union':
+    case 'Instance':
+      unsupportedLanguageFeature(type.kind, GeneratedSource);
+
+    default:
+      assertExhaustive(type, 'Unknown type kind');
+  }
+}
+
+export function diff<R, T>(
+  a: ConcreteType<T>,
+  b: ConcreteType<T>,
+  onChild: (a: T, b: T) => R,
+  onChildMismatch: (child: R, cur: R) => R,
+  onMismatch: (a: ConcreteType<T>, b: ConcreteType<T>, cur: R) => R,
+  init: R,
+): R {
+  let errors = init;
+
+  // Check if kinds match
+  if (a.kind !== b.kind) {
+    errors = onMismatch(a, b, errors);
+    return errors;
+  }
+
+  // Based on kind, check other properties
+  switch (a.kind) {
+    case 'Mixed':
+    case 'Number':
+    case 'String':
+    case 'Boolean':
+    case 'Void':
+      // Simple types, no further checks needed
+      break;
+
+    case 'Nullable':
+      // Check the nested type
+      errors = onChildMismatch(onChild(a.type, (b as typeof a).type), errors);
+      break;
+
+    case 'Array':
+    case 'Set':
+      // Check the element type
+      errors = onChildMismatch(
+        onChild(a.element, (b as typeof a).element),
+        errors,
+      );
+      break;
+
+    case 'Map':
+      // Check both key and value types
+      errors = onChildMismatch(onChild(a.key, (b as typeof a).key), errors);
+      errors = onChildMismatch(onChild(a.value, (b as typeof a).value), errors);
+      break;
+
+    case 'Function': {
+      const bFunc = b as typeof a;
+
+      // Check type parameters
+      if ((a.typeParameters == null) !== (bFunc.typeParameters == null)) {
+        errors = onMismatch(a, b, errors);
+      }
+
+      if (a.typeParameters != null && bFunc.typeParameters != null) {
+        if (a.typeParameters.length !== bFunc.typeParameters.length) {
+          errors = onMismatch(a, b, errors);
+        }
+
+        // Type parameters are just numbers, so we can compare them directly
+        for (let i = 0; i < a.typeParameters.length; i++) {
+          if (a.typeParameters[i] !== bFunc.typeParameters[i]) {
+            errors = onMismatch(a, b, errors);
+          }
+        }
+      }
+
+      // Check parameters
+      if (a.params.length !== bFunc.params.length) {
+        errors = onMismatch(a, b, errors);
+      }
+
+      for (let i = 0; i < a.params.length; i++) {
+        errors = onChildMismatch(onChild(a.params[i], bFunc.params[i]), errors);
+      }
+
+      // Check return type
+      errors = onChildMismatch(onChild(a.returnType, bFunc.returnType), errors);
+      break;
+    }
+
+    case 'Component': {
+      const bComp = b as typeof a;
+
+      // Check children
+      if (a.children !== bComp.children) {
+        errors = onMismatch(a, b, errors);
+      }
+
+      // Check props
+      if (a.props.size !== bComp.props.size) {
+        errors = onMismatch(a, b, errors);
+      }
+
+      for (const [k, v] of a.props) {
+        const bProp = bComp.props.get(k);
+        if (bProp == null) {
+          errors = onMismatch(a, b, errors);
+        } else {
+          errors = onChildMismatch(onChild(v, bProp), errors);
+        }
+      }
+
+      break;
+    }
+
+    case 'Generic': {
+      // Check that the type parameter IDs match
+      if (a.id !== (b as typeof a).id) {
+        errors = onMismatch(a, b, errors);
+      }
+      break;
+    }
+    case 'Structural': {
+      const bStruct = b as typeof a;
+
+      // Check that the structural IDs match
+      if (a.id !== bStruct.id) {
+        errors = onMismatch(a, b, errors);
+      }
+      break;
+    }
+    case 'Object': {
+      const bNom = b as typeof a;
+
+      // Check that the nominal IDs match
+      if (a.id !== bNom.id) {
+        errors = onMismatch(a, b, errors);
+      }
+      break;
+    }
+
+    case 'Tuple': {
+      const bTuple = b as typeof a;
+
+      // Check that the tuple IDs match
+      if (a.id !== bTuple.id) {
+        errors = onMismatch(a, b, errors);
+      }
+      for (let i = 0; i < a.members.length; i++) {
+        errors = onChildMismatch(
+          onChild(a.members[i], bTuple.members[i]),
+          errors,
+        );
+      }
+
+      break;
+    }
+
+    case 'Enum':
+    case 'Instance':
+    case 'Union': {
+      unsupportedLanguageFeature(a.kind, GeneratedSource);
+    }
+
+    default:
+      assertExhaustive(a, 'Unknown type kind');
+  }
+
+  return errors;
+}
+
+export function filterOptional(t: ResolvedType): ResolvedType {
+  if (t.kind === 'Concrete' && t.type.kind === 'Nullable') {
+    return t.type.type;
+  }
+  return t;
+}

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

@@ -107,10 +107,9 @@ export function lower(
       if (binding.kind !== 'Identifier') {
         builder.errors.pushDiagnostic(
           CompilerDiagnostic.create({
-            category: 'Could not find binding',
-            description: `[BuildHIR] Could not find binding for param \`${param.node.name}\``,
             severity: ErrorSeverity.Invariant,
-            suggestions: null,
+            category: 'Could not find binding',
+            description: `[BuildHIR] Could not find binding for param \`${param.node.name}\`.`,
           }).withDetail({
             kind: 'error',
             loc: param.node.loc ?? null,
@@ -172,10 +171,9 @@ export function lower(
     } else {
       builder.errors.pushDiagnostic(
         CompilerDiagnostic.create({
-          category: `Handle ${param.node.type} parameters`,
-          description: `[BuildHIR] Add support for ${param.node.type} parameters`,
           severity: ErrorSeverity.Todo,
-          suggestions: null,
+          category: `Handle ${param.node.type} parameters`,
+          description: `[BuildHIR] Add support for ${param.node.type} parameters.`,
         }).withDetail({
           kind: 'error',
           loc: param.node.loc ?? null,
@@ -191,6 +189,7 @@ export function lower(
     const fallthrough = builder.reserve('block');
     const terminal: ReturnTerminal = {
       kind: 'return',
+      returnVariant: 'Implicit',
       loc: GeneratedSource,
       value: lowerExpressionToTemporary(builder, body),
       id: makeInstructionId(0),
@@ -205,8 +204,7 @@ export function lower(
       CompilerDiagnostic.create({
         severity: ErrorSeverity.InvalidJS,
         category: `Unexpected function body kind`,
-        description: `Expected function body to be an expression or a block statement, got \`${body.type}\``,
-        suggestions: null,
+        description: `Expected function body to be an expression or a block statement, got \`${body.type}\`.`,
       }).withDetail({
         kind: 'error',
         loc: body.node.loc ?? null,
@@ -222,6 +220,7 @@ export function lower(
   builder.terminate(
     {
       kind: 'return',
+      returnVariant: 'Void',
       loc: GeneratedSource,
       value: lowerValueToTemporary(builder, {
         kind: 'Primitive',
@@ -305,6 +304,7 @@ function lowerStatement(
       }
       const terminal: ReturnTerminal = {
         kind: 'return',
+        returnVariant: 'Explicit',
         loc: stmt.node.loc ?? GeneratedSource,
         value,
         id: makeInstructionId(0),

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

@@ -49,6 +49,7 @@ import {
 } from './ObjectShape';
 import {Scope as BabelScope, NodePath} from '@babel/traverse';
 import {TypeSchema} from './TypeSchema';
+import {FlowTypeEnv} from '../Flood/Types';
 
 export const ReactElementSymbolSchema = z.object({
   elementSymbol: z.union([
@@ -243,6 +244,12 @@ export const EnvironmentConfigSchema = z.object({
    */
   enableUseTypeAnnotations: z.boolean().default(false),
 
+  /**
+   * Allows specifying a function that can populate HIR with type information from
+   * Flow
+   */
+  flowTypeProvider: z.nullable(z.function().args(z.string())).default(null),
+
   /**
    * Enable a new model for mutability and aliasing inference
    */
@@ -323,6 +330,12 @@ export const EnvironmentConfigSchema = z.object({
    */
   validateNoSetStateInEffects: z.boolean().default(false),
 
+  /**
+   * Validates that effects are not used to calculate derived data which could instead be computed
+   * during render.
+   */
+  validateNoDerivedComputationsInEffects: z.boolean().default(false),
+
   /**
    * Validates against creating JSX within a try block and recommends using an error boundary
    * instead.
@@ -608,7 +621,7 @@ export const EnvironmentConfigSchema = z.object({
    *
    * Here the variables `ref` and `myRef` will be typed as Refs.
    */
-  enableTreatRefLikeIdentifiersAsRefs: z.boolean().default(false),
+  enableTreatRefLikeIdentifiersAsRefs: z.boolean().default(true),
 
   /*
    * If specified a value, the compiler lowers any calls to `useContext` to use
@@ -631,6 +644,17 @@ export const EnvironmentConfigSchema = z.object({
    * ```
    */
   lowerContextAccess: ExternalFunctionSchema.nullable().default(null),
+
+  /**
+   * If enabled, will validate useMemos that don't return any values:
+   *
+   * Valid:
+   *   useMemo(() => foo, [foo]);
+   *   useMemo(() => { return foo }, [foo]);
+   * Invalid:
+   *   useMemo(() => { ... }, [...]);
+   */
+  validateNoVoidUseMemo: z.boolean().default(false),
 });
 
 export type EnvironmentConfig = z.infer<typeof EnvironmentConfigSchema>;
@@ -680,6 +704,8 @@ export class Environment {
   #hoistedIdentifiers: Set<t.Identifier>;
   parentFunction: NodePath<t.Function>;
 
+  #flowTypeEnvironment: FlowTypeEnv | null;
+
   constructor(
     scope: BabelScope,
     fnType: ReactFunctionType,
@@ -748,6 +774,26 @@ export class Environment {
     this.parentFunction = parentFunction;
     this.#contextIdentifiers = contextIdentifiers;
     this.#hoistedIdentifiers = new Set();
+
+    if (config.flowTypeProvider != null) {
+      this.#flowTypeEnvironment = new FlowTypeEnv();
+      CompilerError.invariant(code != null, {
+        reason:
+          'Expected Environment to be initialized with source code when a Flow type provider is specified',
+        loc: null,
+      });
+      this.#flowTypeEnvironment.init(this, code);
+    } else {
+      this.#flowTypeEnvironment = null;
+    }
+  }
+
+  get typeContext(): FlowTypeEnv {
+    CompilerError.invariant(this.#flowTypeEnvironment != null, {
+      reason: 'Flow type environment not initialized',
+      loc: null,
+    });
+    return this.#flowTypeEnvironment;
   }
 
   get isInferredMemoEnabled(): boolean {

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

@@ -114,6 +114,99 @@ const TYPED_GLOBALS: Array<[string, BuiltInType]> = [
           returnValueKind: ValueKind.Mutable,
         }),
       ],
+      [
+        'entries',
+        addFunction(DEFAULT_SHAPES, [], {
+          positionalParams: [Effect.Capture],
+          restParam: null,
+          returnType: {kind: 'Object', shapeId: BuiltInArrayId},
+          calleeEffect: Effect.Read,
+          returnValueKind: ValueKind.Mutable,
+          aliasing: {
+            receiver: '@receiver',
+            params: ['@object'],
+            rest: null,
+            returns: '@returns',
+            temporaries: [],
+            effects: [
+              {
+                kind: 'Create',
+                into: '@returns',
+                reason: ValueReason.KnownReturnSignature,
+                value: ValueKind.Mutable,
+              },
+              // Object values are captured into the return
+              {
+                kind: 'Capture',
+                from: '@object',
+                into: '@returns',
+              },
+            ],
+          },
+        }),
+      ],
+      [
+        'keys',
+        addFunction(DEFAULT_SHAPES, [], {
+          positionalParams: [Effect.Read],
+          restParam: null,
+          returnType: {kind: 'Object', shapeId: BuiltInArrayId},
+          calleeEffect: Effect.Read,
+          returnValueKind: ValueKind.Mutable,
+          aliasing: {
+            receiver: '@receiver',
+            params: ['@object'],
+            rest: null,
+            returns: '@returns',
+            temporaries: [],
+            effects: [
+              {
+                kind: 'Create',
+                into: '@returns',
+                reason: ValueReason.KnownReturnSignature,
+                value: ValueKind.Mutable,
+              },
+              // Only keys are captured, and keys are immutable
+              {
+                kind: 'ImmutableCapture',
+                from: '@object',
+                into: '@returns',
+              },
+            ],
+          },
+        }),
+      ],
+      [
+        'values',
+        addFunction(DEFAULT_SHAPES, [], {
+          positionalParams: [Effect.Capture],
+          restParam: null,
+          returnType: {kind: 'Object', shapeId: BuiltInArrayId},
+          calleeEffect: Effect.Read,
+          returnValueKind: ValueKind.Mutable,
+          aliasing: {
+            receiver: '@receiver',
+            params: ['@object'],
+            rest: null,
+            returns: '@returns',
+            temporaries: [],
+            effects: [
+              {
+                kind: 'Create',
+                into: '@returns',
+                reason: ValueReason.KnownReturnSignature,
+                value: ValueKind.Mutable,
+              },
+              // Object values are captured into the return
+              {
+                kind: 'Capture',
+                from: '@object',
+                into: '@returns',
+              },
+            ],
+          },
+        }),
+      ],
     ]),
   ],
   [

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

@@ -14,6 +14,7 @@ import type {HookKind} from './ObjectShape';
 import {Type, makeType} from './Types';
 import {z} from 'zod';
 import type {AliasingEffect} from '../Inference/AliasingEffects';
+import {isReservedWord} from '../Utils/Keyword';
 
 /*
  * *******************************************************************************************
@@ -446,8 +447,20 @@ export type ThrowTerminal = {
 };
 export type Case = {test: Place | null; block: BlockId};
 
+export type ReturnVariant = 'Void' | 'Implicit' | 'Explicit';
 export type ReturnTerminal = {
   kind: 'return';
+  /**
+   * Void:
+   *   () => { ... }
+   *   function() { ... }
+   * Implicit (ArrowFunctionExpression only):
+   *   () => foo
+   * Explicit:
+   *   () => { return ... }
+   *   function () { return ... }
+   */
+  returnVariant: ReturnVariant;
   loc: SourceLocation;
   value: Place;
   id: InstructionId;
@@ -1308,12 +1321,21 @@ export function forkTemporaryIdentifier(
  * original source code.
  */
 export function makeIdentifierName(name: string): ValidatedIdentifier {
-  CompilerError.invariant(t.isValidIdentifier(name), {
-    reason: `Expected a valid identifier name`,
-    loc: GeneratedSource,
-    description: `\`${name}\` is not a valid JavaScript identifier`,
-    suggestions: null,
-  });
+  if (isReservedWord(name)) {
+    CompilerError.throwInvalidJS({
+      reason: 'Expected a non-reserved identifier name',
+      loc: GeneratedSource,
+      description: `\`${name}\` is a reserved word in JavaScript and cannot be used as an identifier name`,
+      suggestions: null,
+    });
+  } else {
+    CompilerError.invariant(t.isValidIdentifier(name), {
+      reason: `Expected a valid identifier name`,
+      loc: GeneratedSource,
+      description: `\`${name}\` is not a valid JavaScript identifier`,
+      suggestions: null,
+    });
+  }
   return {
     kind: 'named',
     value: name as ValidIdentifierName,

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

@@ -142,6 +142,7 @@ function parseAliasingSignatureConfig(
   const effects = typeConfig.effects.map(
     (effect: AliasingEffectConfig): AliasingEffect => {
       switch (effect.kind) {
+        case 'ImmutableCapture':
         case 'CreateFrom':
         case 'Capture':
         case 'Alias':
@@ -1211,6 +1212,8 @@ addObject(BUILTIN_SHAPES, BuiltInRefValueId, [
   ['*', {kind: 'Object', shapeId: BuiltInRefValueId}],
 ]);
 
+addObject(BUILTIN_SHAPES, ReanimatedSharedValueId, []);
+
 addFunction(
   BUILTIN_SHAPES,
   [],

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

@@ -215,7 +215,7 @@ export function printTerminal(terminal: Terminal): Array<string> | string {
       break;
     }
     case 'return': {
-      value = `[${terminal.id}] Return${
+      value = `[${terminal.id}] Return ${terminal.returnVariant}${
         terminal.value != null ? ' ' + printPlace(terminal.value) : ''
       }`;
       if (terminal.effects != null) {
@@ -943,7 +943,10 @@ export function printAliasingEffect(effect: AliasingEffect): string {
       return `Assign ${printPlaceForAliasEffect(effect.into)} = ${printPlaceForAliasEffect(effect.from)}`;
     }
     case 'Alias': {
-      return `Alias ${printPlaceForAliasEffect(effect.into)} = ${printPlaceForAliasEffect(effect.from)}`;
+      return `Alias ${printPlaceForAliasEffect(effect.into)} <- ${printPlaceForAliasEffect(effect.from)}`;
+    }
+    case 'MaybeAlias': {
+      return `MaybeAlias ${printPlaceForAliasEffect(effect.into)} <- ${printPlaceForAliasEffect(effect.from)}`;
     }
     case 'Capture': {
       return `Capture ${printPlaceForAliasEffect(effect.into)} <- ${printPlaceForAliasEffect(effect.from)}`;

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

@@ -111,6 +111,19 @@ export const AliasEffectSchema: z.ZodType<AliasEffectConfig> = z.object({
   into: LifetimeIdSchema,
 });
 
+export type ImmutableCaptureEffectConfig = {
+  kind: 'ImmutableCapture';
+  from: string;
+  into: string;
+};
+
+export const ImmutableCaptureEffectSchema: z.ZodType<ImmutableCaptureEffectConfig> =
+  z.object({
+    kind: z.literal('ImmutableCapture'),
+    from: LifetimeIdSchema,
+    into: LifetimeIdSchema,
+  });
+
 export type CaptureEffectConfig = {
   kind: 'Capture';
   from: string;
@@ -187,6 +200,7 @@ export type AliasingEffectConfig =
   | AssignEffectConfig
   | AliasEffectConfig
   | CaptureEffectConfig
+  | ImmutableCaptureEffectConfig
   | ImpureEffectConfig
   | MutateEffectConfig
   | MutateTransitiveConditionallyConfig
@@ -199,6 +213,7 @@ export const AliasingEffectSchema: z.ZodType<AliasingEffectConfig> = z.union([
   AssignEffectSchema,
   AliasEffectSchema,
   CaptureEffectSchema,
+  ImmutableCaptureEffectSchema,
   ImpureEffectSchema,
   MutateEffectSchema,
   MutateTransitiveConditionallySchema,

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

@@ -777,6 +777,7 @@ export function mapTerminalSuccessors(
     case 'return': {
       return {
         kind: 'return',
+        returnVariant: terminal.returnVariant,
         loc: terminal.loc,
         value: terminal.value,
         id: makeInstructionId(0),

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

@@ -90,6 +90,23 @@ export type AliasingEffect =
    * c could be mutating a.
    */
   | {kind: 'Alias'; from: Place; into: Place}
+
+  /**
+   * Indicates the potential for information flow from `from` to `into`. This is used for a specific
+   * case: functions with unknown signatures. If the compiler sees a call such as `foo(x)`, it has to
+   * consider several possibilities (which may depend on the arguments):
+   * - foo(x) returns a new mutable value that does not capture any information from x.
+   * - foo(x) returns a new mutable value that *does* capture information from x.
+   * - foo(x) returns x itself, ie foo is the identity function
+   *
+   * The same is true of functions that take multiple arguments: `cond(a, b, c)` could conditionally
+   * return b or c depending on the value of a.
+   *
+   * To represent this case, MaybeAlias represents the fact that an aliasing relationship could exist.
+   * Any mutations that flow through this relationship automatically become conditional.
+   */
+  | {kind: 'MaybeAlias'; from: Place; into: Place}
+
   /**
    * Records direct assignment: `into = from`.
    */
@@ -183,7 +200,8 @@ export function hashEffect(effect: AliasingEffect): string {
     case 'ImmutableCapture':
     case 'Assign':
     case 'Alias':
-    case 'Capture': {
+    case 'Capture':
+    case 'MaybeAlias': {
       return [
         effect.kind,
         effect.from.identifier.id,

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

@@ -85,7 +85,8 @@ function lowerWithMutationAliasing(fn: HIRFunction): void {
       case 'Assign':
       case 'Alias':
       case 'Capture':
-      case 'CreateFrom': {
+      case 'CreateFrom':
+      case 'MaybeAlias': {
         capturedOrMutated.add(effect.from.identifier.id);
         break;
       }

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

@@ -5,7 +5,12 @@
  * LICENSE file in the root directory of this source tree.
  */
 
-import {CompilerError, SourceLocation} from '..';
+import {
+  CompilerDiagnostic,
+  CompilerError,
+  ErrorSeverity,
+  SourceLocation,
+} from '..';
 import {
   CallExpression,
   Effect,
@@ -30,6 +35,7 @@ import {
   makeInstructionId,
 } from '../HIR';
 import {createTemporaryPlace, markInstructionIds} from '../HIR/HIRBuilder';
+import {Result} from '../Utils/Result';
 
 type ManualMemoCallee = {
   kind: 'useMemo' | 'useCallback';
@@ -283,26 +289,43 @@ function extractManualMemoizationArgs(
   instr: TInstruction<CallExpression> | TInstruction<MethodCall>,
   kind: 'useCallback' | 'useMemo',
   sidemap: IdentifierSidemap,
+  errors: CompilerError,
 ): {
-  fnPlace: Place;
+  fnPlace: Place | null;
   depsList: Array<ManualMemoDependency> | null;
 } {
   const [fnPlace, depsListPlace] = instr.value.args as Array<
     Place | SpreadPattern | undefined
   >;
   if (fnPlace == null) {
-    CompilerError.throwInvalidReact({
-      reason: `Expected a callback function to be passed to ${kind}`,
-      loc: instr.value.loc,
-      suggestions: null,
-    });
+    errors.pushDiagnostic(
+      CompilerDiagnostic.create({
+        severity: ErrorSeverity.InvalidReact,
+        category: `Expected a callback function to be passed to ${kind}`,
+        description: `Expected a callback function to be passed to ${kind}`,
+        suggestions: null,
+      }).withDetail({
+        kind: 'error',
+        loc: instr.value.loc,
+        message: `Expected a callback function to be passed to ${kind}`,
+      }),
+    );
+    return {fnPlace: null, depsList: null};
   }
   if (fnPlace.kind === 'Spread' || depsListPlace?.kind === 'Spread') {
-    CompilerError.throwInvalidReact({
-      reason: `Unexpected spread argument to ${kind}`,
-      loc: instr.value.loc,
-      suggestions: null,
-    });
+    errors.pushDiagnostic(
+      CompilerDiagnostic.create({
+        severity: ErrorSeverity.InvalidReact,
+        category: `Unexpected spread argument to ${kind}`,
+        description: `Unexpected spread argument to ${kind}`,
+        suggestions: null,
+      }).withDetail({
+        kind: 'error',
+        loc: instr.value.loc,
+        message: `Unexpected spread argument to ${kind}`,
+      }),
+    );
+    return {fnPlace: null, depsList: null};
   }
   let depsList: Array<ManualMemoDependency> | null = null;
   if (depsListPlace != null) {
@@ -310,23 +333,40 @@ function extractManualMemoizationArgs(
       depsListPlace.identifier.id,
     );
     if (maybeDepsList == null) {
-      CompilerError.throwInvalidReact({
-        reason: `Expected the dependency list for ${kind} to be an array literal`,
-        suggestions: null,
-        loc: depsListPlace.loc,
-      });
+      errors.pushDiagnostic(
+        CompilerDiagnostic.create({
+          severity: ErrorSeverity.InvalidReact,
+          category: `Expected the dependency list for ${kind} to be an array literal`,
+          description: `Expected the dependency list for ${kind} to be an array literal`,
+          suggestions: null,
+        }).withDetail({
+          kind: 'error',
+          loc: depsListPlace.loc,
+          message: `Expected the dependency list for ${kind} to be an array literal`,
+        }),
+      );
+      return {fnPlace, depsList: null};
     }
-    depsList = maybeDepsList.map(dep => {
+    depsList = [];
+    for (const dep of maybeDepsList) {
       const maybeDep = sidemap.maybeDeps.get(dep.identifier.id);
       if (maybeDep == null) {
-        CompilerError.throwInvalidReact({
-          reason: `Expected the dependency list to be an array of simple expressions (e.g. \`x\`, \`x.y.z\`, \`x?.y?.z\`)`,
-          suggestions: null,
-          loc: dep.loc,
-        });
+        errors.pushDiagnostic(
+          CompilerDiagnostic.create({
+            severity: ErrorSeverity.InvalidReact,
+            category: `Expected the dependency list to be an array of simple expressions (e.g. \`x\`, \`x.y.z\`, \`x?.y?.z\`)`,
+            description: `Expected the dependency list to be an array of simple expressions (e.g. \`x\`, \`x.y.z\`, \`x?.y?.z\`)`,
+            suggestions: null,
+          }).withDetail({
+            kind: 'error',
+            loc: dep.loc,
+            message: `Expected the dependency list to be an array of simple expressions (e.g. \`x\`, \`x.y.z\`, \`x?.y?.z\`)`,
+          }),
+        );
+      } else {
+        depsList.push(maybeDep);
       }
-      return maybeDep;
-    });
+    }
   }
   return {
     fnPlace,
@@ -341,8 +381,14 @@ function extractManualMemoizationArgs(
  * rely on type inference to find useMemo/useCallback invocations, and instead does basic tracking
  * of globals and property loads to find both direct calls as well as usage via the React namespace,
  * eg `React.useMemo()`.
+ *
+ * This pass also validates that useMemo callbacks return a value (not void), ensuring that useMemo
+ * is only used for memoizing values and not for running arbitrary side effects.
  */
-export function dropManualMemoization(func: HIRFunction): void {
+export function dropManualMemoization(
+  func: HIRFunction,
+): Result<void, CompilerError> {
+  const errors = new CompilerError();
   const isValidationEnabled =
     func.env.config.validatePreserveExistingMemoizationGuarantees ||
     func.env.config.validateNoSetStateInRender ||
@@ -389,7 +435,47 @@ export function dropManualMemoization(func: HIRFunction): void {
             instr as TInstruction<CallExpression> | TInstruction<MethodCall>,
             manualMemo.kind,
             sidemap,
+            errors,
           );
+
+          if (fnPlace == null) {
+            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({
+                    severity: ErrorSeverity.InvalidReact,
+                    category: '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,
+                  }).withDetail({
+                    kind: 'error',
+                    loc: instr.value.loc,
+                    message: 'useMemo() callbacks must return a value',
+                  }),
+                );
+              }
+            }
+          }
+
           instr.value = getManualMemoizationReplacement(
             fnPlace,
             instr.value.loc,
@@ -410,11 +496,19 @@ export function dropManualMemoization(func: HIRFunction): void {
              * is rare and likely sketchy.
              */
             if (!sidemap.functions.has(fnPlace.identifier.id)) {
-              CompilerError.throwInvalidReact({
-                reason: `Expected the first argument to be an inline function expression`,
-                suggestions: [],
-                loc: fnPlace.loc,
-              });
+              errors.pushDiagnostic(
+                CompilerDiagnostic.create({
+                  severity: ErrorSeverity.InvalidReact,
+                  category: `Expected the first argument to be an inline function expression`,
+                  description: `Expected the first argument to be an inline function expression`,
+                  suggestions: [],
+                }).withDetail({
+                  kind: 'error',
+                  loc: fnPlace.loc,
+                  message: `Expected the first argument to be an inline function expression`,
+                }),
+              );
+              continue;
             }
             const memoDecl: Place =
               manualMemo.kind === 'useMemo'
@@ -486,6 +580,8 @@ export function dropManualMemoization(func: HIRFunction): void {
       markInstructionIds(func.body);
     }
   }
+
+  return errors.asResult();
 }
 
 function findOptionalPlaces(fn: HIRFunction): Set<IdentifierId> {
@@ -530,3 +626,17 @@ 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/Inference/InferMutationAliasingEffects.ts

@@ -447,23 +447,22 @@ function applySignature(
               reason: value.reason,
               context: new Set(),
             });
-            const message =
+            const variable =
               effect.value.identifier.name !== null &&
               effect.value.identifier.name.kind === 'named'
-                ? `\`${effect.value.identifier.name.value}\` cannot be modified`
-                : 'This value cannot be modified';
+                ? `\`${effect.value.identifier.name.value}\``
+                : 'value';
             effects.push({
               kind: 'MutateFrozen',
               place: effect.value,
               error: CompilerDiagnostic.create({
                 severity: ErrorSeverity.InvalidReact,
                 category: 'This value cannot be modified',
-                description: reason,
-                suggestions: null,
+                description: `${reason}.`,
               }).withDetail({
                 kind: 'error',
                 loc: effect.value.loc,
-                message,
+                message: `${variable} cannot be modified`,
               }),
             });
           }
@@ -692,6 +691,7 @@ function applyEffect(
       }
       break;
     }
+    case 'MaybeAlias':
     case 'Alias':
     case 'Capture': {
       CompilerError.invariant(
@@ -956,7 +956,7 @@ function applyEffect(
             context,
             state,
             // OK: recording information flow
-            {kind: 'Alias', from: operand, into: effect.into},
+            {kind: 'MaybeAlias', from: operand, into: effect.into},
             initialized,
             effects,
           );
@@ -1018,30 +1018,30 @@ function applyEffect(
             effect.value.identifier.declarationId,
           )
         ) {
-          const description =
+          const variable =
             effect.value.identifier.name !== null &&
             effect.value.identifier.name.kind === 'named'
-              ? `Variable \`${effect.value.identifier.name.value}\``
-              : 'This variable';
+              ? `\`${effect.value.identifier.name.value}\``
+              : null;
           const hoistedAccess = context.hoistedContextDeclarations.get(
             effect.value.identifier.declarationId,
           );
           const diagnostic = CompilerDiagnostic.create({
             severity: ErrorSeverity.InvalidReact,
             category: 'Cannot access variable before it is declared',
-            description: `${description} is accessed before it is declared, which prevents the earlier access from updating when this value changes over time`,
+            description: `${variable ?? 'This variable'} is accessed before it is declared, which prevents the earlier access from updating when this value changes over time.`,
           });
           if (hoistedAccess != null && hoistedAccess.loc != effect.value.loc) {
             diagnostic.withDetail({
               kind: 'error',
               loc: hoistedAccess.loc,
-              message: 'Variable accessed before it is declared',
+              message: `${variable ?? 'variable'} accessed before it is declared`,
             });
           }
           diagnostic.withDetail({
             kind: 'error',
             loc: effect.value.loc,
-            message: 'The variable is declared here',
+            message: `${variable ?? 'variable'} is declared here`,
           });
 
           applyEffect(
@@ -1061,11 +1061,11 @@ function applyEffect(
             reason: value.reason,
             context: new Set(),
           });
-          const message =
+          const variable =
             effect.value.identifier.name !== null &&
             effect.value.identifier.name.kind === 'named'
-              ? `\`${effect.value.identifier.name.value}\` cannot be modified`
-              : 'This value cannot be modified';
+              ? `\`${effect.value.identifier.name.value}\``
+              : 'value';
           applyEffect(
             context,
             state,
@@ -1078,11 +1078,11 @@ function applyEffect(
               error: CompilerDiagnostic.create({
                 severity: ErrorSeverity.InvalidReact,
                 category: 'This value cannot be modified',
-                description: reason,
+                description: `${reason}.`,
               }).withDetail({
                 kind: 'error',
                 loc: effect.value.loc,
-                message,
+                message: `${variable} cannot be modified`,
               }),
             },
             initialized,
@@ -1324,7 +1324,7 @@ class InferenceState {
             return 'mutate-global';
           }
           case ValueKind.MaybeFrozen: {
-            return 'none';
+            return 'mutate-frozen';
           }
           default: {
             assertExhaustive(kind, `Unexpected kind ${kind}`);
@@ -2002,6 +2002,7 @@ function computeSignatureForInstruction(
       break;
     }
     case 'StoreGlobal': {
+      const variable = `\`${value.name}\``;
       effects.push({
         kind: 'MutateGlobal',
         place: value.value,
@@ -2009,13 +2010,11 @@ function computeSignatureForInstruction(
           severity: ErrorSeverity.InvalidReact,
           category:
             'Cannot reassign variables declared outside of the component/hook',
-          description:
-            'Reassigning a variable declared outside of the component/hook is a form of side effect, which can cause unpredictable behavior depending on when the component happens to re-render. If this variable is used in rendering, use useState instead. Otherwise, consider updating it in an effect (https://react.dev/reference/rules/components-and-hooks-must-be-pure#side-effects-must-run-outside-of-render)',
-          suggestions: null,
+          description: `Variable ${variable} is declared outside of the component/hook. Reassigning this value during render is a form of side effect, which can cause unpredictable behavior depending on when the component happens to re-render. If this variable is used in rendering, use useState instead. Otherwise, consider updating it in an effect. (https://react.dev/reference/rules/components-and-hooks-must-be-pure#side-effects-must-run-outside-of-render)`,
         }).withDetail({
           kind: 'error',
           loc: instr.loc,
-          message: 'Cannot reassign variable',
+          message: `${variable} cannot be reassigned`,
         }),
       });
       effects.push({kind: 'Assign', from: value.value, into: lvalue});
@@ -2114,7 +2113,6 @@ function computeEffectsForLegacySignature(
             ? `\`${signature.canonicalName}\` is an impure function. `
             : '') +
           'Calling an impure function can produce unstable results that update unpredictably when the component happens to re-render. (https://react.dev/reference/rules/components-and-hooks-must-be-pure#components-and-hooks-must-be-idempotent)',
-        suggestions: null,
       }).withDetail({
         kind: 'error',
         loc,
@@ -2379,6 +2377,7 @@ function computeEffectsForSignature(
   // Apply substitutions
   for (const effect of signature.effects) {
     switch (effect.kind) {
+      case 'MaybeAlias':
       case 'Assign':
       case 'ImmutableCapture':
       case 'Alias':

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

@@ -160,6 +160,8 @@ export function inferMutationAliasingRanges(
           state.assign(index++, effect.from, effect.into);
         } else if (effect.kind === 'Alias') {
           state.assign(index++, effect.from, effect.into);
+        } else if (effect.kind === 'MaybeAlias') {
+          state.maybeAlias(index++, effect.from, effect.into);
         } else if (effect.kind === 'Capture') {
           state.capture(index++, effect.from, effect.into);
         } else if (
@@ -247,6 +249,7 @@ export function inferMutationAliasingRanges(
   }
   for (const param of [...fn.context, ...fn.params]) {
     const place = param.kind === 'Identifier' ? param : param.place;
+
     const node = state.nodes.get(place.identifier);
     if (node == null) {
       continue;
@@ -346,7 +349,8 @@ export function inferMutationAliasingRanges(
           case 'Assign':
           case 'Alias':
           case 'Capture':
-          case 'CreateFrom': {
+          case 'CreateFrom':
+          case 'MaybeAlias': {
             const isMutatedOrReassigned =
               effect.into.identifier.mutableRange.end > instr.id;
             if (isMutatedOrReassigned) {
@@ -567,7 +571,12 @@ type Node = {
   createdFrom: Map<Identifier, number>;
   captures: Map<Identifier, number>;
   aliases: Map<Identifier, number>;
-  edges: Array<{index: number; node: Identifier; kind: 'capture' | 'alias'}>;
+  maybeAliases: Map<Identifier, number>;
+  edges: Array<{
+    index: number;
+    node: Identifier;
+    kind: 'capture' | 'alias' | 'maybeAlias';
+  }>;
   transitive: {kind: MutationKind; loc: SourceLocation} | null;
   local: {kind: MutationKind; loc: SourceLocation} | null;
   lastMutated: number;
@@ -585,6 +594,7 @@ class AliasingState {
       createdFrom: new Map(),
       captures: new Map(),
       aliases: new Map(),
+      maybeAliases: new Map(),
       edges: [],
       transitive: null,
       local: null,
@@ -630,6 +640,18 @@ class AliasingState {
     }
   }
 
+  maybeAlias(index: number, from: Place, into: Place): void {
+    const fromNode = this.nodes.get(from.identifier);
+    const toNode = this.nodes.get(into.identifier);
+    if (fromNode == null || toNode == null) {
+      return;
+    }
+    fromNode.edges.push({index, node: into.identifier, kind: 'maybeAlias'});
+    if (!toNode.maybeAliases.has(from.identifier)) {
+      toNode.maybeAliases.set(from.identifier, index);
+    }
+  }
+
   render(index: number, start: Identifier, errors: CompilerError): void {
     const seen = new Set<Identifier>();
     const queue: Array<Identifier> = [start];
@@ -673,22 +695,24 @@ class AliasingState {
     // Null is used for simulated mutations
     end: InstructionId | null,
     transitive: boolean,
-    kind: MutationKind,
+    startKind: MutationKind,
     loc: SourceLocation,
     errors: CompilerError,
   ): void {
-    const seen = new Set<Identifier>();
+    const seen = new Map<Identifier, MutationKind>();
     const queue: Array<{
       place: Identifier;
       transitive: boolean;
       direction: 'backwards' | 'forwards';
-    }> = [{place: start, transitive, direction: 'backwards'}];
+      kind: MutationKind;
+    }> = [{place: start, transitive, direction: 'backwards', kind: startKind}];
     while (queue.length !== 0) {
-      const {place: current, transitive, direction} = queue.pop()!;
-      if (seen.has(current)) {
+      const {place: current, transitive, direction, kind} = queue.pop()!;
+      const previousKind = seen.get(current);
+      if (previousKind != null && previousKind >= kind) {
         continue;
       }
-      seen.add(current);
+      seen.set(current, kind);
       const node = this.nodes.get(current);
       if (node == null) {
         continue;
@@ -724,13 +748,18 @@ class AliasingState {
         if (edge.index >= index) {
           break;
         }
-        queue.push({place: edge.node, transitive, direction: 'forwards'});
+        queue.push({place: edge.node, transitive, direction: 'forwards', kind});
       }
       for (const [alias, when] of node.createdFrom) {
         if (when >= index) {
           continue;
         }
-        queue.push({place: alias, transitive: true, direction: 'backwards'});
+        queue.push({
+          place: alias,
+          transitive: true,
+          direction: 'backwards',
+          kind,
+        });
       }
       if (direction === 'backwards' || node.value.kind !== 'Phi') {
         /**
@@ -747,7 +776,25 @@ class AliasingState {
           if (when >= index) {
             continue;
           }
-          queue.push({place: alias, transitive, direction: 'backwards'});
+          queue.push({place: alias, transitive, direction: 'backwards', kind});
+        }
+        /**
+         * MaybeAlias indicates potential data flow from unknown function calls,
+         * so we downgrade mutations through these aliases to consider them
+         * conditional. This means we'll consider them for mutation *range*
+         * purposes but not report validation errors for mutations, since
+         * we aren't sure that the `from` value could actually be aliased.
+         */
+        for (const [alias, when] of node.maybeAliases) {
+          if (when >= index) {
+            continue;
+          }
+          queue.push({
+            place: alias,
+            transitive,
+            direction: 'backwards',
+            kind: MutationKind.Conditional,
+          });
         }
       }
       /**
@@ -758,7 +805,12 @@ class AliasingState {
           if (when >= index) {
             continue;
           }
-          queue.push({place: capture, transitive, direction: 'backwards'});
+          queue.push({
+            place: capture,
+            transitive,
+            direction: 'backwards',
+            kind,
+          });
         }
       }
     }

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

@@ -21,7 +21,6 @@ import {
   isStableType,
   isStableTypeContainer,
   isUseOperator,
-  isUseRefType,
 } from '../HIR';
 import {PostDominator} from '../HIR/Dominator';
 import {
@@ -70,13 +69,6 @@ class StableSidemap {
               isStable: false,
             });
           }
-        } else if (
-          this.env.config.enableTreatRefLikeIdentifiersAsRefs &&
-          isUseRefType(lvalue.identifier)
-        ) {
-          this.map.set(lvalue.identifier.id, {
-            isStable: true,
-          });
         }
         break;
       }

compiler/packages/babel-plugin-react-compiler/src/Inference/MUTABILITY_ALIASING_MODEL.md

@@ -153,6 +153,10 @@ This is somewhat the inverse of `Capture`. The `CreateFrom` effect describes tha
 
 Describes immutable data flow from one value to another. This is not currently used for anything, but is intended to eventually power a more sophisticated escape analysis.
 
+### MaybeAlias
+
+Describes potential data flow that the compiler knows may occur behind a function call, but cannot be sure about. For example, `foo(x)` _may_ be the identity function and return `x`, or `cond(a, b, c)` may conditionally return `b` or `c` depending on the value of `a`, but those functions could just as easily return new mutable values and not capture any information from their arguments. MaybeAlias represents that we have to consider the potential for data flow when deciding mutable ranges, but should be conservative about reporting errors. For example, `foo(someFrozenValue).property = true` should not error since we don't know for certain that foo returns its input.
+
 ### State-Changing Effects
 
 The following effects describe state changes to specific values, not data flow. In many cases, JavaScript semantics will involve a combination of both data-flow effects *and* state-change effects. For example, `object.property = value` has data flow (`Capture object <- value`) and mutation (`Mutate object`).
@@ -347,6 +351,17 @@ a.b = b; // capture
 mutate(a); // can transitively mutate b
 ```
 
+### MaybeAlias makes mutation conditional
+
+Because we don't know for certain that the aliasing occurs, we consider the mutation conditional against the source.
+
+```
+MaybeAlias a <- b
+Mutate a
+=>
+MutateConditional b
+```
+
 ### Freeze Does Not Freeze the Value
 
 Freeze does not freeze the value itself:

compiler/packages/babel-plugin-react-compiler/src/Optimization/LowerContextAccess.ts

@@ -237,6 +237,7 @@ function emitSelectorFn(env: Environment, keys: Array<string>): Instruction {
     terminal: {
       id: makeInstructionId(0),
       kind: 'return',
+      returnVariant: 'Explicit',
       loc: GeneratedSource,
       value: arrayInstr.lvalue,
       effects: null,

compiler/packages/babel-plugin-react-compiler/src/Optimization/OutlineJsx.ts

@@ -352,6 +352,7 @@ function emitOutlinedFn(
     terminal: {
       id: makeInstructionId(0),
       kind: 'return',
+      returnVariant: 'Explicit',
       loc: GeneratedSource,
       value: instructions.at(-1)!.lvalue,
       effects: null,

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

@@ -119,6 +119,7 @@ class FindLastUsageVisitor extends ReactiveFunctionVisitor<void> {
 
 class Transform extends ReactiveFunctionTransform<ReactiveScopeDependencies | null> {
   lastUsage: Map<DeclarationId, InstructionId>;
+  temporaries: Map<DeclarationId, DeclarationId> = new Map();
 
   constructor(lastUsage: Map<DeclarationId, InstructionId>) {
     super();
@@ -215,6 +216,12 @@ class Transform extends ReactiveFunctionTransform<ReactiveScopeDependencies | nu
                 current.lvalues.add(
                   instr.instruction.lvalue.identifier.declarationId,
                 );
+                if (instr.instruction.value.kind === 'LoadLocal') {
+                  this.temporaries.set(
+                    instr.instruction.lvalue.identifier.declarationId,
+                    instr.instruction.value.place.identifier.declarationId,
+                  );
+                }
               }
               break;
             }
@@ -236,6 +243,13 @@ class Transform extends ReactiveFunctionTransform<ReactiveScopeDependencies | nu
                   )) {
                     current.lvalues.add(lvalue.identifier.declarationId);
                   }
+                  this.temporaries.set(
+                    instr.instruction.value.lvalue.place.identifier
+                      .declarationId,
+                    this.temporaries.get(
+                      instr.instruction.value.value.identifier.declarationId,
+                    ) ?? instr.instruction.value.value.identifier.declarationId,
+                  );
                 } else {
                   log(
                     `Reset scope @${current.block.scope.id} from StoreLocal in [${instr.instruction.id}]`,
@@ -260,7 +274,7 @@ class Transform extends ReactiveFunctionTransform<ReactiveScopeDependencies | nu
         case 'scope': {
           if (
             current !== null &&
-            canMergeScopes(current.block, instr) &&
+            canMergeScopes(current.block, instr, this.temporaries) &&
             areLValuesLastUsedByScope(
               instr.scope,
               current.lvalues,
@@ -426,6 +440,7 @@ function areLValuesLastUsedByScope(
 function canMergeScopes(
   current: ReactiveScopeBlock,
   next: ReactiveScopeBlock,
+  temporaries: Map<DeclarationId, DeclarationId>,
 ): boolean {
   // Don't merge scopes with reassignments
   if (
@@ -465,11 +480,14 @@ function canMergeScopes(
     (next.scope.dependencies.size !== 0 &&
       [...next.scope.dependencies].every(
         dep =>
+          dep.path.length === 0 &&
           isAlwaysInvalidatingType(dep.identifier.type) &&
           Iterable_some(
             current.scope.declarations.values(),
             decl =>
-              decl.identifier.declarationId === dep.identifier.declarationId,
+              decl.identifier.declarationId === dep.identifier.declarationId ||
+              decl.identifier.declarationId ===
+                temporaries.get(dep.identifier.declarationId),
           ),
       ))
   ) {
@@ -477,12 +495,16 @@ function canMergeScopes(
     return true;
   }
   log(`  cannot merge scopes:`);
-  log(`  ${printReactiveScopeSummary(current.scope)}`);
-  log(`  ${printReactiveScopeSummary(next.scope)}`);
+  log(
+    `  ${printReactiveScopeSummary(current.scope)} ${[...current.scope.declarations.values()].map(decl => decl.identifier.declarationId)}`,
+  );
+  log(
+    `  ${printReactiveScopeSummary(next.scope)} ${[...next.scope.dependencies].map(dep => `${dep.identifier.declarationId} ${temporaries.get(dep.identifier.declarationId) ?? dep.identifier.declarationId}`)}`,
+  );
   return false;
 }
 
-function isAlwaysInvalidatingType(type: Type): boolean {
+export function isAlwaysInvalidatingType(type: Type): boolean {
   switch (type.kind) {
     case 'Object': {
       switch (type.shapeId) {

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

@@ -14,6 +14,7 @@ import {
   Identifier,
   IdentifierId,
   Instruction,
+  InstructionKind,
   makePropertyLiteral,
   makeType,
   PropType,
@@ -194,12 +195,29 @@ function* generateInstructionTypes(
       break;
     }
 
-    // We intentionally do not infer types for context variables
+    // We intentionally do not infer types for most context variables
     case 'DeclareContext':
-    case 'StoreContext':
     case 'LoadContext': {
       break;
     }
+    case 'StoreContext': {
+      /**
+       * The caveat is StoreContext const, where we know the value is
+       * assigned once such that everywhere the value is accessed, it
+       * must have the same type from the rvalue.
+       *
+       * A concrete example where this is useful is `const ref = useRef()`
+       * where the ref is referenced before its declaration in a function
+       * expression, causing it to be converted to a const context variable.
+       */
+      if (value.lvalue.kind === InstructionKind.Const) {
+        yield equation(
+          value.lvalue.place.identifier.type,
+          value.value.identifier.type,
+        );
+      }
+      break;
+    }
 
     case 'StoreLocal': {
       if (env.config.enableUseTypeAnnotations) {
@@ -433,6 +451,18 @@ function* generateInstructionTypes(
 
     case 'JsxExpression':
     case 'JsxFragment': {
+      if (env.config.enableTreatRefLikeIdentifiersAsRefs) {
+        if (value.kind === 'JsxExpression') {
+          for (const prop of value.props) {
+            if (prop.kind === 'JsxAttribute' && prop.name === 'ref') {
+              yield equation(prop.place.identifier.type, {
+                kind: 'Object',
+                shapeId: BuiltInUseRefId,
+              });
+            }
+          }
+        }
+      }
       yield equation(left, {kind: 'Object', shapeId: BuiltInJsxId});
       break;
     }
@@ -448,7 +478,36 @@ function* generateInstructionTypes(
       yield equation(left, returnType);
       break;
     }
-    case 'PropertyStore':
+    case 'PropertyStore': {
+      /**
+       * Infer types based on assignments to known object properties
+       * This is important for refs, where assignment to `<maybeRef>.current`
+       * can help us infer that an object itself is a ref
+       */
+      yield equation(
+        /**
+         * Our property type declarations are best-effort and we haven't tested
+         * using them to drive inference of rvalues from lvalues. We want to emit
+         * a Property type in order to infer refs from `.current` accesses, but
+         * stay conservative by not otherwise inferring anything about rvalues.
+         * So we use a dummy type here.
+         *
+         * TODO: consider using the rvalue type here
+         */
+        makeType(),
+        // unify() only handles properties in the second position
+        {
+          kind: 'Property',
+          objectType: value.object.identifier.type,
+          objectName: getName(names, value.object.identifier.id),
+          propertyName: {
+            kind: 'literal',
+            value: value.property,
+          },
+        },
+      );
+      break;
+    }
     case 'DeclareLocal':
     case 'RegExpLiteral':
     case 'MetaProperty':

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

@@ -1,110 +0,0 @@
-/**
- * Copyright (c) Meta Platforms, Inc. and affiliates.
- *
- * This source code is licensed under the MIT license found in the
- * LICENSE file in the root directory of this source tree.
- */
-
-import {HIRFunction, IdentifierId, Type, typeEquals} from '../HIR';
-
-/**
- * Temporary workaround for InferTypes not propagating the types of phis.
- * Previously, LeaveSSA would replace all the identifiers for each phi (operands and
- * the phi itself) with a single "canonical" identifier, generally chosen as the first
- * operand to flow into the phi. In case of a phi whose operand was a phi, this could
- * sometimes be an operand from the earlier phi.
- *
- * As a result, even though InferTypes did not propagate types for phis, LeaveSSA
- * could end up replacing the phi Identifier with another identifer from an operand,
- * which _did_ have a type inferred.
- *
- * This didn't affect the initial construction of mutable ranges because InferMutableRanges
- * runs before LeaveSSA - thus, the types propagated by LeaveSSA only affected later optimizations,
- * notably MergeScopesThatInvalidateTogether which uses type to determine if a scope's output
- * will always invalidate with its input.
- *
- * The long-term correct approach is to update InferTypes to infer the types of phis,
- * but this is complicated because InferMutableRanges inadvertently depends on phis
- * never having a known type, such that a Store effect cannot occur on a phi value.
- * Once we fix InferTypes to infer phi types, then we'll also have to update InferMutableRanges
- * to handle this case.
- *
- * As a temporary workaround, this pass propagates the type of phis and can be called
- * safely *after* InferMutableRanges. Unlike LeaveSSA, this pass only propagates the
- * type if all operands have the same type, it's its more correct.
- */
-export function propagatePhiTypes(fn: HIRFunction): void {
-  /**
-   * We track which SSA ids have had their types propagated to handle nested ternaries,
-   * see the StoreLocal handling below
-   */
-  const propagated = new Set<IdentifierId>();
-  for (const [, block] of fn.body.blocks) {
-    for (const phi of block.phis) {
-      /*
-       * We replicate the previous LeaveSSA behavior and only propagate types for
-       * unnamed variables. LeaveSSA would have chosen one of the operands as the
-       * canonical id and taken its type as the type of all identifiers. We're
-       * more conservative and only propagate if the types are the same and the
-       * phi didn't have a type inferred.
-       *
-       * Note that this can change output slightly in cases such as
-       * `cond ? <div /> : null`.
-       *
-       * Previously the first operand's type (BuiltInJsx) would have been propagated,
-       * and this expression may have been merged with subsequent reactive scopes
-       * since it appears (based on that type) to always invalidate.
-       *
-       * But the correct type is `BuiltInJsx | null`, which we can't express and
-       * so leave as a generic `Type`, which does not always invalidate and therefore
-       * does not merge with subsequent scopes.
-       *
-       * We also don't propagate scopes for named variables, to preserve compatibility
-       * with previous LeaveSSA behavior.
-       */
-      if (
-        phi.place.identifier.type.kind !== 'Type' ||
-        phi.place.identifier.name !== null
-      ) {
-        continue;
-      }
-      let type: Type | null = null;
-      for (const [, operand] of phi.operands) {
-        if (type === null) {
-          type = operand.identifier.type;
-        } else if (!typeEquals(type, operand.identifier.type)) {
-          type = null;
-          break;
-        }
-      }
-      if (type !== null) {
-        phi.place.identifier.type = type;
-        propagated.add(phi.place.identifier.id);
-      }
-    }
-    for (const instr of block.instructions) {
-      const {value} = instr;
-      switch (value.kind) {
-        case 'StoreLocal': {
-          /**
-           * Nested ternaries can lower to a form with an intermediate StoreLocal where
-           * the value.lvalue is the temporary of the outer ternary, and the value.value
-           * is the result of the inner ternary.
-           *
-           * This is a common pattern in practice and easy enough to support. Again, the
-           * long-term approach is to update InferTypes and InferMutableRanges.
-           */
-          const lvalue = value.lvalue.place;
-          if (
-            propagated.has(value.value.identifier.id) &&
-            lvalue.identifier.type.kind === 'Type' &&
-            lvalue.identifier.name === null
-          ) {
-            lvalue.identifier.type = value.value.identifier.type;
-            propagated.add(lvalue.identifier.id);
-          }
-        }
-      }
-    }
-  }
-}

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

@@ -78,6 +78,10 @@ export default class DisjointSet<T> {
     return root;
   }
 
+  has(item: T): boolean {
+    return this.#entries.has(item);
+  }
+
   /*
    * Forces the set into canonical form, ie with all items pointing directly to
    * their root, and returns a Map representing the mapping of items to their roots.

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

@@ -0,0 +1,87 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+/**
+ * https://tc39.es/ecma262/multipage/ecmascript-language-lexical-grammar.html#sec-keywords-and-reserved-words
+ */
+
+/**
+ * Note: `await` and `yield` are contextually allowed as identifiers.
+ *   await: reserved inside async functions and modules
+ *   yield: reserved inside generator functions
+ *
+ * Note: `async` is not reserved.
+ */
+const RESERVED_WORDS = new Set([
+  'break',
+  'case',
+  'catch',
+  'class',
+  'const',
+  'continue',
+  'debugger',
+  'default',
+  'delete',
+  'do',
+  'else',
+  'enum',
+  'export',
+  'extends',
+  'false',
+  'finally',
+  'for',
+  'function',
+  'if',
+  'import',
+  'in',
+  'instanceof',
+  'new',
+  'null',
+  'return',
+  'super',
+  'switch',
+  'this',
+  'throw',
+  'true',
+  'try',
+  'typeof',
+  'var',
+  'void',
+  'while',
+  'with',
+]);
+
+/**
+ * Reserved when a module has a 'use strict' directive.
+ */
+const STRICT_MODE_RESERVED_WORDS = new Set([
+  'let',
+  'static',
+  'implements',
+  'interface',
+  'package',
+  'private',
+  'protected',
+  'public',
+]);
+/**
+ * The names arguments and eval are not keywords, but they are subject to some restrictions in
+ * strict mode code.
+ */
+const STRICT_MODE_RESTRICTED_WORDS = new Set(['eval', 'arguments']);
+
+/**
+ * Conservative check for whether an identifer name is reserved or not. We assume that code is
+ * written with strict mode.
+ */
+export function isReservedWord(identifierName: string): boolean {
+  return (
+    RESERVED_WORDS.has(identifierName) ||
+    STRICT_MODE_RESERVED_WORDS.has(identifierName) ||
+    STRICT_MODE_RESTRICTED_WORDS.has(identifierName)
+  );
+}

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

@@ -113,8 +113,13 @@ function* splitPragma(
  */
 function parseConfigPragmaEnvironmentForTest(
   pragma: string,
+  defaultConfig: PartialEnvironmentConfig,
 ): EnvironmentConfig {
-  const maybeConfig: Partial<Record<keyof EnvironmentConfig, unknown>> = {};
+  // throw early if the defaults are invalid
+  EnvironmentConfigSchema.parse(defaultConfig);
+
+  const maybeConfig: Partial<Record<keyof EnvironmentConfig, unknown>> =
+    defaultConfig;
 
   for (const {key, value: val} of splitPragma(pragma)) {
     if (!hasOwnProperty(EnvironmentConfigSchema.shape, key)) {
@@ -174,9 +179,13 @@ export function parseConfigPragmaForTests(
   pragma: string,
   defaults: {
     compilationMode: CompilationMode;
+    environment?: PartialEnvironmentConfig;
   },
 ): PluginOptions {
-  const environment = parseConfigPragmaEnvironmentForTest(pragma);
+  const environment = parseConfigPragmaEnvironmentForTest(
+    pragma,
+    defaults.environment ?? {},
+  );
   const options: Record<keyof PluginOptions, unknown> = {
     ...defaultOptions,
     panicThreshold: 'all_errors',

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

@@ -33,12 +33,12 @@ export function assertExhaustive(_: never, errorMsg: string): never {
 // Modifies @param array in place, retaining only the items where the predicate returns true.
 export function retainWhere<T>(
   array: Array<T>,
-  predicate: (item: T) => boolean,
+  predicate: (item: T, index: number) => boolean,
 ): void {
   let writeIndex = 0;
   for (let readIndex = 0; readIndex < array.length; readIndex++) {
     const item = array[readIndex];
-    if (predicate(item) === true) {
+    if (predicate(item, readIndex) === true) {
       array[writeIndex++] = item;
     }
   }

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

@@ -29,15 +29,20 @@ export function validateLocalsNotReassignedAfterRender(fn: HIRFunction): void {
   );
   if (reassignment !== null) {
     const errors = new CompilerError();
+    const variable =
+      reassignment.identifier.name != null &&
+      reassignment.identifier.name.kind === 'named'
+        ? `\`${reassignment.identifier.name.value}\``
+        : 'variable';
     errors.pushDiagnostic(
       CompilerDiagnostic.create({
         severity: ErrorSeverity.InvalidReact,
-        category: 'Cannot reassign a variable after render completes',
-        description: `Reassigning ${reassignment.identifier.name != null && reassignment.identifier.name.kind === 'named' ? `variable \`${reassignment.identifier.name.value}\`` : 'a variable'} after render has completed can cause inconsistent behavior on subsequent renders. Consider using state instead`,
+        category: 'Cannot reassign variable after render completes',
+        description: `Reassigning ${variable} after render has completed can cause inconsistent behavior on subsequent renders. Consider using state instead.`,
       }).withDetail({
         kind: 'error',
         loc: reassignment.loc,
-        message: 'Cannot reassign variable after render completes',
+        message: `Cannot reassign ${variable} after render completes`,
       }),
     );
     throw errors;
@@ -78,16 +83,25 @@ function getContextReassignment(
           // if the function or its depends reassign, propagate that fact on the lvalue
           if (reassignment !== null) {
             if (isAsync || value.loweredFunc.func.async) {
-              CompilerError.throwInvalidReact({
-                reason:
-                  'Reassigning a variable in an async function can cause inconsistent behavior on subsequent renders. Consider using state instead',
-                description:
-                  reassignment.identifier.name !== null &&
-                  reassignment.identifier.name.kind === 'named'
-                    ? `Variable \`${reassignment.identifier.name.value}\` cannot be reassigned after render`
-                    : '',
-                loc: reassignment.loc,
-              });
+              const errors = new CompilerError();
+              const variable =
+                reassignment.identifier.name !== null &&
+                reassignment.identifier.name.kind === 'named'
+                  ? `\`${reassignment.identifier.name.value}\``
+                  : 'variable';
+              errors.pushDiagnostic(
+                CompilerDiagnostic.create({
+                  severity: ErrorSeverity.InvalidReact,
+                  category: 'Cannot reassign variable in async function',
+                  description:
+                    'Reassigning a variable in an async function can cause inconsistent behavior on subsequent renders. Consider using state instead',
+                }).withDetail({
+                  kind: 'error',
+                  loc: reassignment.loc,
+                  message: `Cannot reassign ${variable}`,
+                }),
+              );
+              throw errors;
             }
             reassigningFunctions.set(lvalue.identifier.id, reassignment);
           }

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

@@ -0,0 +1,388 @@
+/**
+ * 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 {CompilerError, ErrorSeverity, SourceLocation} from '..';
+import {
+  ArrayExpression,
+  BlockId,
+  FunctionExpression,
+  HIRFunction,
+  IdentifierId,
+  Place,
+  isSetStateType,
+  isUseEffectHookType,
+} from '../HIR';
+import {printInstruction, printPlace} from '../HIR/PrintHIR';
+import {
+  eachInstructionValueOperand,
+  eachTerminalOperand,
+} from '../HIR/visitors';
+
+type SetStateCall = {
+  loc: SourceLocation;
+  propsSource: Place | null; // null means state-derived, non-null means props-derived
+};
+
+/**
+ * Validates that useEffect is not used for derived computations which could/should
+ * be performed in render.
+ *
+ * See https://react.dev/learn/you-might-not-need-an-effect#updating-state-based-on-props-or-state
+ *
+ * Example:
+ *
+ * ```
+ * // 🔴 Avoid: redundant state and unnecessary Effect
+ * const [fullName, setFullName] = useState('');
+ * useEffect(() => {
+ *   setFullName(firstName + ' ' + lastName);
+ * }, [firstName, lastName]);
+ * ```
+ *
+ * Instead use:
+ *
+ * ```
+ * // ✅ Good: calculated during rendering
+ * const fullName = firstName + ' ' + lastName;
+ * ```
+ */
+export function validateNoDerivedComputationsInEffects(fn: HIRFunction): void {
+  const candidateDependencies: Map<IdentifierId, ArrayExpression> = new Map();
+  const functions: Map<IdentifierId, FunctionExpression> = new Map();
+  const locals: Map<IdentifierId, IdentifierId> = new Map();
+  const derivedFromProps: Map<IdentifierId, Place> = new Map();
+
+  const errors = new CompilerError();
+
+  if (fn.fnType === 'Hook') {
+    for (const param of fn.params) {
+      if (param.kind === 'Identifier') {
+        derivedFromProps.set(param.identifier.id, param);
+      }
+    }
+  } else if (fn.fnType === 'Component') {
+    const props = fn.params[0];
+    if (props != null && props.kind === 'Identifier') {
+      derivedFromProps.set(props.identifier.id, props);
+    }
+  }
+
+  for (const block of fn.body.blocks.values()) {
+    for (const instr of block.instructions) {
+      const {lvalue, value} = instr;
+
+      // Track props derivation through instruction effects
+      if (instr.effects != null) {
+        for (const effect of instr.effects) {
+          switch (effect.kind) {
+            case 'Assign':
+            case 'Alias':
+            case 'MaybeAlias':
+            case 'Capture': {
+              const source = derivedFromProps.get(effect.from.identifier.id);
+              if (source != null) {
+                derivedFromProps.set(effect.into.identifier.id, source);
+              }
+              break;
+            }
+          }
+        }
+      }
+
+      /**
+       * TODO: figure out why property access off of props does not create an Assign or Alias/Maybe
+       * Alias
+       *
+       * import {useEffect, useState} from 'react'
+       *
+       *        function Component(props) {
+       *          const [displayValue, setDisplayValue] = useState('');
+       *
+       *          useEffect(() => {
+       *            const computed = props.prefix + props.value + props.suffix;
+       *                             ^^^^^^^^^^^^   ^^^^^^^^^^^   ^^^^^^^^^^^^
+       *                             we want to track that these are from props
+       *            setDisplayValue(computed);
+       *          }, [props.prefix, props.value, props.suffix]);
+       *
+       *          return <div>{displayValue}</div>;
+       *        }
+       */
+      if (value.kind === 'FunctionExpression') {
+        for (const [, block] of value.loweredFunc.func.body.blocks) {
+          for (const instr of block.instructions) {
+            if (instr.effects != null) {
+              console.group(printInstruction(instr));
+              for (const effect of instr.effects) {
+                console.log(effect);
+                switch (effect.kind) {
+                  case 'Assign':
+                  case 'Alias':
+                  case 'MaybeAlias':
+                  case 'Capture': {
+                    const source = derivedFromProps.get(
+                      effect.from.identifier.id,
+                    );
+                    if (source != null) {
+                      derivedFromProps.set(effect.into.identifier.id, source);
+                    }
+                    break;
+                  }
+                }
+              }
+            }
+            console.groupEnd();
+          }
+        }
+      }
+
+      for (const [, place] of derivedFromProps) {
+        console.log(printPlace(place));
+      }
+
+      if (value.kind === 'LoadLocal') {
+        locals.set(lvalue.identifier.id, value.place.identifier.id);
+      } else if (value.kind === 'ArrayExpression') {
+        candidateDependencies.set(lvalue.identifier.id, value);
+      } else if (value.kind === 'FunctionExpression') {
+        functions.set(lvalue.identifier.id, value);
+      } else if (
+        value.kind === 'CallExpression' ||
+        value.kind === 'MethodCall'
+      ) {
+        const callee =
+          value.kind === 'CallExpression' ? value.callee : value.property;
+        if (
+          isUseEffectHookType(callee.identifier) &&
+          value.args.length === 2 &&
+          value.args[0].kind === 'Identifier' &&
+          value.args[1].kind === 'Identifier'
+        ) {
+          const effectFunction = functions.get(value.args[0].identifier.id);
+          const deps = candidateDependencies.get(value.args[1].identifier.id);
+          if (
+            effectFunction != null &&
+            deps != null &&
+            deps.elements.length !== 0 &&
+            deps.elements.every(element => element.kind === 'Identifier')
+          ) {
+            const dependencies: Array<IdentifierId> = deps.elements.map(dep => {
+              CompilerError.invariant(dep.kind === 'Identifier', {
+                reason: `Dependency is checked as a place above`,
+                loc: value.loc,
+              });
+              return locals.get(dep.identifier.id) ?? dep.identifier.id;
+            });
+            validateEffect(
+              effectFunction.loweredFunc.func,
+              dependencies,
+              derivedFromProps,
+              errors,
+            );
+          }
+        }
+      }
+    }
+  }
+  if (errors.hasErrors()) {
+    throw errors;
+  }
+}
+
+function validateEffect(
+  effectFunction: HIRFunction,
+  effectDeps: Array<IdentifierId>,
+  derivedFromProps: Map<IdentifierId, Place>,
+  errors: CompilerError,
+): void {
+  for (const operand of effectFunction.context) {
+    if (isSetStateType(operand.identifier)) {
+      continue;
+    } else if (effectDeps.find(dep => dep === operand.identifier.id) != null) {
+      continue;
+    } else if (derivedFromProps.has(operand.identifier.id)) {
+      continue;
+    } else {
+      // Captured something other than the effect dep or setState
+      console.log('early return 1');
+      return;
+    }
+  }
+  for (const dep of effectDeps) {
+    console.log({dep});
+    if (
+      effectFunction.context.find(operand => operand.identifier.id === dep) ==
+        null ||
+      derivedFromProps.has(dep) === false
+    ) {
+      console.log('early return 2');
+      // effect dep wasn't actually used in the function
+      return;
+    }
+  }
+
+  const seenBlocks: Set<BlockId> = new Set();
+  const values: Map<IdentifierId, Array<IdentifierId>> = new Map();
+  const effectDerivedFromProps: Map<IdentifierId, Place> = new Map();
+
+  for (const dep of effectDeps) {
+    console.log({dep});
+    values.set(dep, [dep]);
+    const propsSource = derivedFromProps.get(dep);
+    if (propsSource != null) {
+      effectDerivedFromProps.set(dep, propsSource);
+    }
+  }
+
+  const setStateCalls: Array<SetStateCall> = [];
+  for (const block of effectFunction.body.blocks.values()) {
+    for (const pred of block.preds) {
+      if (!seenBlocks.has(pred)) {
+        // skip if block has a back edge
+        return;
+      }
+    }
+    for (const phi of block.phis) {
+      const aggregateDeps: Set<IdentifierId> = new Set();
+      let propsSource: Place | null = null;
+
+      for (const operand of phi.operands.values()) {
+        const deps = values.get(operand.identifier.id);
+        if (deps != null) {
+          for (const dep of deps) {
+            aggregateDeps.add(dep);
+          }
+        }
+        const source = effectDerivedFromProps.get(operand.identifier.id);
+        if (source != null) {
+          propsSource = source;
+        }
+      }
+
+      if (aggregateDeps.size !== 0) {
+        values.set(phi.place.identifier.id, Array.from(aggregateDeps));
+      }
+      if (propsSource != null) {
+        effectDerivedFromProps.set(phi.place.identifier.id, propsSource);
+      }
+    }
+    for (const instr of block.instructions) {
+      switch (instr.value.kind) {
+        case 'Primitive':
+        case 'JSXText':
+        case 'LoadGlobal': {
+          break;
+        }
+        case 'LoadLocal': {
+          const deps = values.get(instr.value.place.identifier.id);
+          if (deps != null) {
+            values.set(instr.lvalue.identifier.id, deps);
+          }
+          break;
+        }
+        case 'ComputedLoad':
+        case 'PropertyLoad':
+        case 'BinaryExpression':
+        case 'TemplateLiteral':
+        case 'CallExpression':
+        case 'MethodCall': {
+          const aggregateDeps: Set<IdentifierId> = new Set();
+          for (const operand of eachInstructionValueOperand(instr.value)) {
+            const deps = values.get(operand.identifier.id);
+            if (deps != null) {
+              for (const dep of deps) {
+                aggregateDeps.add(dep);
+              }
+            }
+          }
+          if (aggregateDeps.size !== 0) {
+            values.set(instr.lvalue.identifier.id, Array.from(aggregateDeps));
+          }
+
+          if (
+            instr.value.kind === 'CallExpression' &&
+            isSetStateType(instr.value.callee.identifier) &&
+            instr.value.args.length === 1 &&
+            instr.value.args[0].kind === 'Identifier'
+          ) {
+            const deps = values.get(instr.value.args[0].identifier.id);
+            if (deps != null && new Set(deps).size === effectDeps.length) {
+              const propsSource = effectDerivedFromProps.get(
+                instr.value.args[0].identifier.id,
+              );
+
+              setStateCalls.push({
+                loc: instr.value.callee.loc,
+                propsSource: propsSource ?? null,
+              });
+            } else {
+              // doesn't depend on all deps
+              return;
+            }
+          }
+          break;
+        }
+        default: {
+          return;
+        }
+      }
+
+      // Track props derivation through instruction effects
+      if (instr.effects != null) {
+        for (const effect of instr.effects) {
+          switch (effect.kind) {
+            case 'Assign':
+            case 'Alias':
+            case 'MaybeAlias':
+            case 'Capture': {
+              const source = effectDerivedFromProps.get(
+                effect.from.identifier.id,
+              );
+              if (source != null) {
+                effectDerivedFromProps.set(effect.into.identifier.id, source);
+              }
+              break;
+            }
+          }
+        }
+      }
+    }
+    for (const operand of eachTerminalOperand(block.terminal)) {
+      if (values.has(operand.identifier.id)) {
+        //
+        return;
+      }
+    }
+    seenBlocks.add(block.id);
+  }
+
+  for (const call of setStateCalls) {
+    if (call.propsSource != null) {
+      const propName = call.propsSource.identifier.name?.value;
+      const propInfo = propName != null ? ` (from prop '${propName}')` : '';
+
+      errors.push({
+        reason: `Consider lifting state up to the parent component to make this a controlled component. (https://react.dev/learn/you-might-not-need-an-effect#adjusting-some-state-when-a-prop-changes)`,
+        description: `You are using props${propInfo} to update local state in an effect.`,
+        severity: ErrorSeverity.InvalidReact,
+        loc: call.loc,
+        suggestions: null,
+      });
+    } else {
+      errors.push({
+        reason:
+          'You may not need this effect. Values derived from state should be calculated during render, not in an effect. (https://react.dev/learn/you-might-not-need-an-effect#updating-state-based-on-props-or-state)',
+        description:
+          'This effect updates state based on other state values. ' +
+          'Consider calculating this value directly during render',
+        severity: ErrorSeverity.InvalidReact,
+        loc: call.loc,
+        suggestions: null,
+      });
+    }
+  }
+}

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

@@ -57,22 +57,28 @@ export function validateNoFreezingKnownMutableFunctions(
     if (operand.effect === Effect.Freeze) {
       const effect = contextMutationEffects.get(operand.identifier.id);
       if (effect != null) {
+        const place = [...effect.places][0];
+        const variable =
+          place != null &&
+          place.identifier.name != null &&
+          place.identifier.name.kind === 'named'
+            ? `\`${place.identifier.name.value}\``
+            : 'a local variable';
         errors.pushDiagnostic(
           CompilerDiagnostic.create({
             severity: ErrorSeverity.InvalidReact,
             category: 'Cannot modify local variables after render completes',
-            description: `This argument is a function which may reassign or mutate local variables after render, which can cause inconsistent behavior on subsequent renders. Consider using state instead`,
+            description: `This argument is a function which may reassign or mutate ${variable} after render, which can cause inconsistent behavior on subsequent renders. Consider using state instead.`,
           })
             .withDetail({
               kind: 'error',
               loc: operand.loc,
-              message:
-                'This function may (indirectly) reassign or modify local variables after render',
+              message: `This function may (indirectly) reassign or modify ${variable} after render`,
             })
             .withDetail({
               kind: 'error',
               loc: effect.loc,
-              message: 'This modifies a local variable',
+              message: `This modifies ${variable}`,
             }),
         );
       }

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

@@ -5,7 +5,11 @@
  * LICENSE file in the root directory of this source tree.
  */
 
-import {CompilerError, ErrorSeverity} from '../CompilerError';
+import {
+  CompilerDiagnostic,
+  CompilerError,
+  ErrorSeverity,
+} from '../CompilerError';
 import {
   BlockId,
   HIRFunction,
@@ -23,6 +27,7 @@ import {
   eachTerminalOperand,
 } from '../HIR/visitors';
 import {Err, Ok, Result} from '../Utils/Result';
+import {retainWhere} from '../Utils/utils';
 
 /**
  * Validates that a function does not access a ref value during render. This includes a partial check
@@ -75,8 +80,18 @@ type RefAccessRefType =
 
 type RefFnType = {readRefEffect: boolean; returnType: RefAccessType};
 
-class Env extends Map<IdentifierId, RefAccessType> {
+class Env {
   #changed = false;
+  #data: Map<IdentifierId, RefAccessType> = new Map();
+  #temporaries: Map<IdentifierId, Place> = new Map();
+
+  lookup(place: Place): Place {
+    return this.#temporaries.get(place.identifier.id) ?? place;
+  }
+
+  define(place: Place, value: Place): void {
+    this.#temporaries.set(place.identifier.id, value);
+  }
 
   resetChanged(): void {
     this.#changed = false;
@@ -86,8 +101,14 @@ class Env extends Map<IdentifierId, RefAccessType> {
     return this.#changed;
   }
 
-  override set(key: IdentifierId, value: RefAccessType): this {
-    const cur = this.get(key);
+  get(key: IdentifierId): RefAccessType | undefined {
+    const operandId = this.#temporaries.get(key)?.identifier.id ?? key;
+    return this.#data.get(operandId);
+  }
+
+  set(key: IdentifierId, value: RefAccessType): this {
+    const operandId = this.#temporaries.get(key)?.identifier.id ?? key;
+    const cur = this.#data.get(operandId);
     const widenedValue = joinRefAccessTypes(value, cur ?? {kind: 'None'});
     if (
       !(cur == null && widenedValue.kind === 'None') &&
@@ -95,7 +116,8 @@ class Env extends Map<IdentifierId, RefAccessType> {
     ) {
       this.#changed = true;
     }
-    return super.set(key, widenedValue);
+    this.#data.set(operandId, widenedValue);
+    return this;
   }
 }
 
@@ -103,9 +125,48 @@ export function validateNoRefAccessInRender(
   fn: HIRFunction,
 ): Result<void, CompilerError> {
   const env = new Env();
+  collectTemporariesSidemap(fn, env);
   return validateNoRefAccessInRenderImpl(fn, env).map(_ => undefined);
 }
 
+function collectTemporariesSidemap(fn: HIRFunction, env: Env): void {
+  for (const block of fn.body.blocks.values()) {
+    for (const instr of block.instructions) {
+      const {lvalue, value} = instr;
+      switch (value.kind) {
+        case 'LoadLocal': {
+          const temp = env.lookup(value.place);
+          if (temp != null) {
+            env.define(lvalue, temp);
+          }
+          break;
+        }
+        case 'StoreLocal': {
+          const temp = env.lookup(value.value);
+          if (temp != null) {
+            env.define(lvalue, temp);
+            env.define(value.lvalue.place, temp);
+          }
+          break;
+        }
+        case 'PropertyLoad': {
+          if (
+            isUseRefType(value.object.identifier) &&
+            value.property === 'current'
+          ) {
+            continue;
+          }
+          const temp = env.lookup(value.object);
+          if (temp != null) {
+            env.define(lvalue, temp);
+          }
+          break;
+        }
+      }
+    }
+  }
+}
+
 function refTypeOfType(place: Place): RefAccessType {
   if (isRefValueType(place.identifier)) {
     return {kind: 'RefValue'};
@@ -258,12 +319,27 @@ function validateNoRefAccessInRenderImpl(
     env.set(place.identifier.id, type);
   }
 
+  const interpolatedAsJsx = new Set<IdentifierId>();
+  for (const block of fn.body.blocks.values()) {
+    for (const instr of block.instructions) {
+      const {value} = instr;
+      if (value.kind === 'JsxExpression' || value.kind === 'JsxFragment') {
+        if (value.children != null) {
+          for (const child of value.children) {
+            interpolatedAsJsx.add(child.identifier.id);
+          }
+        }
+      }
+    }
+  }
+
   for (let i = 0; (i == 0 || env.hasChanged()) && i < 10; i++) {
     env.resetChanged();
     returnValues = [];
-    const safeBlocks = new Map<BlockId, RefId>();
+    const safeBlocks: Array<{block: BlockId; ref: RefId}> = [];
     const errors = new CompilerError();
     for (const [, block] of fn.body.blocks) {
+      retainWhere(safeBlocks, entry => entry.block !== block.id);
       for (const phi of block.phis) {
         env.set(
           phi.place.identifier.id,
@@ -385,28 +461,79 @@ function validateNoRefAccessInRenderImpl(
             const hookKind = getHookKindForType(fn.env, callee.identifier.type);
             let returnType: RefAccessType = {kind: 'None'};
             const fnType = env.get(callee.identifier.id);
+            let didError = false;
             if (fnType?.kind === 'Structure' && fnType.fn !== null) {
               returnType = fnType.fn.returnType;
               if (fnType.fn.readRefEffect) {
-                errors.push({
-                  severity: ErrorSeverity.InvalidReact,
-                  reason:
-                    'This function accesses a ref value (the `current` property), which may not be accessed during render. (https://react.dev/reference/react/useRef)',
-                  loc: callee.loc,
-                  description:
-                    callee.identifier.name !== null &&
-                    callee.identifier.name.kind === 'named'
-                      ? `Function \`${callee.identifier.name.value}\` accesses a ref`
-                      : null,
-                  suggestions: null,
-                });
+                didError = true;
+                errors.pushDiagnostic(
+                  CompilerDiagnostic.create({
+                    severity: ErrorSeverity.InvalidReact,
+                    category: 'Cannot access refs during render',
+                    description: ERROR_DESCRIPTION,
+                  }).withDetail({
+                    kind: 'error',
+                    loc: callee.loc,
+                    message: `This function accesses a ref value`,
+                  }),
+                );
               }
             }
-            for (const operand of eachInstructionValueOperand(instr.value)) {
-              if (hookKind != null) {
-                validateNoDirectRefValueAccess(errors, operand, env);
-              } else {
-                validateNoRefAccess(errors, env, operand, operand.loc);
+            /*
+             * If we already reported an error on this instruction, don't report
+             * duplicate errors
+             */
+            if (!didError) {
+              const isRefLValue = isUseRefType(instr.lvalue.identifier);
+              for (const operand of eachInstructionValueOperand(instr.value)) {
+                /**
+                 * By default we check that function call operands are not refs,
+                 * ref values, or functions that can access refs.
+                 */
+                if (
+                  isRefLValue ||
+                  (hookKind != null &&
+                    hookKind !== 'useState' &&
+                    hookKind !== 'useReducer')
+                ) {
+                  /**
+                   * Special cases:
+                   *
+                   * 1. the lvalue is a ref
+                   * In general passing a ref to a function may access that ref
+                   * value during render, so we disallow it.
+                   *
+                   * The main exception is the "mergeRefs" pattern, ie a function
+                   * that accepts multiple refs as arguments (or an array of refs)
+                   * and returns a new, aggregated ref. If the lvalue is a ref,
+                   * we assume that the user is doing this pattern and allow passing
+                   * refs.
+                   *
+                   * Eg `const mergedRef = mergeRefs(ref1, ref2)`
+                   *
+                   * 2. calling hooks
+                   *
+                   * Hooks are independently checked to ensure they don't access refs
+                   * during render.
+                   */
+                  validateNoDirectRefValueAccess(errors, operand, env);
+                } else if (interpolatedAsJsx.has(instr.lvalue.identifier.id)) {
+                  /**
+                   * Special case: the lvalue is passed as a jsx child
+                   *
+                   * For example `<Foo>{renderHelper(ref)}</Foo>`. Here we have more
+                   * context and infer that the ref is being passed to a component-like
+                   * render function which attempts to obey the rules.
+                   */
+                  validateNoRefValueAccess(errors, env, operand);
+                } else {
+                  validateNoRefPassedToFunction(
+                    errors,
+                    env,
+                    operand,
+                    operand.loc,
+                  );
+                }
               }
             }
             env.set(instr.lvalue.identifier.id, returnType);
@@ -439,23 +566,39 @@ function validateNoRefAccessInRenderImpl(
           case 'PropertyStore':
           case 'ComputedDelete':
           case 'ComputedStore': {
-            const safe = safeBlocks.get(block.id);
             const target = env.get(instr.value.object.identifier.id);
+            let safe: (typeof safeBlocks)['0'] | null | undefined = null;
             if (
               instr.value.kind === 'PropertyStore' &&
-              safe != null &&
-              target?.kind === 'Ref' &&
-              target.refId === safe
+              target != null &&
+              target.kind === 'Ref'
             ) {
-              safeBlocks.delete(block.id);
-            } else {
-              validateNoRefAccess(errors, env, instr.value.object, instr.loc);
+              safe = safeBlocks.find(entry => entry.ref === target.refId);
             }
-            for (const operand of eachInstructionValueOperand(instr.value)) {
-              if (operand === instr.value.object) {
-                continue;
+            if (safe != null) {
+              retainWhere(safeBlocks, entry => entry !== safe);
+            } else {
+              validateNoRefUpdate(errors, env, instr.value.object, instr.loc);
+            }
+            if (
+              instr.value.kind === 'ComputedDelete' ||
+              instr.value.kind === 'ComputedStore'
+            ) {
+              validateNoRefValueAccess(errors, env, instr.value.property);
+            }
+            if (
+              instr.value.kind === 'ComputedStore' ||
+              instr.value.kind === 'PropertyStore'
+            ) {
+              validateNoDirectRefValueAccess(errors, instr.value.value, env);
+              const type = env.get(instr.value.value.identifier.id);
+              if (type != null && type.kind === 'Structure') {
+                let objectType: RefAccessType = type;
+                if (target != null) {
+                  objectType = joinRefAccessTypes(objectType, target);
+                }
+                env.set(instr.value.object.identifier.id, objectType);
               }
-              validateNoRefValueAccess(errors, env, operand);
             }
             break;
           }
@@ -535,8 +678,11 @@ function validateNoRefAccessInRenderImpl(
 
       if (block.terminal.kind === 'if') {
         const test = env.get(block.terminal.test.identifier.id);
-        if (test?.kind === 'Guard') {
-          safeBlocks.set(block.terminal.consequent, test.refId);
+        if (
+          test?.kind === 'Guard' &&
+          safeBlocks.find(entry => entry.ref === test.refId) == null
+        ) {
+          safeBlocks.push({block: block.terminal.fallthrough, ref: test.refId});
         }
       }
 
@@ -583,18 +729,17 @@ function destructure(
 
 function guardCheck(errors: CompilerError, operand: Place, env: Env): void {
   if (env.get(operand.identifier.id)?.kind === 'Guard') {
-    errors.push({
-      severity: ErrorSeverity.InvalidReact,
-      reason:
-        'Ref values (the `current` property) may not be accessed during render. (https://react.dev/reference/react/useRef)',
-      loc: operand.loc,
-      description:
-        operand.identifier.name !== null &&
-        operand.identifier.name.kind === 'named'
-          ? `Cannot access ref value \`${operand.identifier.name.value}\``
-          : null,
-      suggestions: null,
-    });
+    errors.pushDiagnostic(
+      CompilerDiagnostic.create({
+        severity: ErrorSeverity.InvalidReact,
+        category: 'Cannot access refs during render',
+        description: ERROR_DESCRIPTION,
+      }).withDetail({
+        kind: 'error',
+        loc: operand.loc,
+        message: `Cannot access ref value during render`,
+      }),
+    );
   }
 }
 
@@ -608,22 +753,21 @@ function validateNoRefValueAccess(
     type?.kind === 'RefValue' ||
     (type?.kind === 'Structure' && type.fn?.readRefEffect)
   ) {
-    errors.push({
-      severity: ErrorSeverity.InvalidReact,
-      reason:
-        'Ref values (the `current` property) may not be accessed during render. (https://react.dev/reference/react/useRef)',
-      loc: (type.kind === 'RefValue' && type.loc) || operand.loc,
-      description:
-        operand.identifier.name !== null &&
-        operand.identifier.name.kind === 'named'
-          ? `Cannot access ref value \`${operand.identifier.name.value}\``
-          : null,
-      suggestions: null,
-    });
+    errors.pushDiagnostic(
+      CompilerDiagnostic.create({
+        severity: ErrorSeverity.InvalidReact,
+        category: 'Cannot access refs during render',
+        description: ERROR_DESCRIPTION,
+      }).withDetail({
+        kind: 'error',
+        loc: (type.kind === 'RefValue' && type.loc) || operand.loc,
+        message: `Cannot access ref value during render`,
+      }),
+    );
   }
 }
 
-function validateNoRefAccess(
+function validateNoRefPassedToFunction(
   errors: CompilerError,
   env: Env,
   operand: Place,
@@ -635,18 +779,39 @@ function validateNoRefAccess(
     type?.kind === 'RefValue' ||
     (type?.kind === 'Structure' && type.fn?.readRefEffect)
   ) {
-    errors.push({
-      severity: ErrorSeverity.InvalidReact,
-      reason:
-        'Ref values (the `current` property) may not be accessed during render. (https://react.dev/reference/react/useRef)',
-      loc: (type.kind === 'RefValue' && type.loc) || loc,
-      description:
-        operand.identifier.name !== null &&
-        operand.identifier.name.kind === 'named'
-          ? `Cannot access ref value \`${operand.identifier.name.value}\``
-          : null,
-      suggestions: null,
-    });
+    errors.pushDiagnostic(
+      CompilerDiagnostic.create({
+        severity: ErrorSeverity.InvalidReact,
+        category: 'Cannot access refs during render',
+        description: ERROR_DESCRIPTION,
+      }).withDetail({
+        kind: 'error',
+        loc: (type.kind === 'RefValue' && type.loc) || loc,
+        message: `Passing a ref to a function may read its value during render`,
+      }),
+    );
+  }
+}
+
+function validateNoRefUpdate(
+  errors: CompilerError,
+  env: Env,
+  operand: Place,
+  loc: SourceLocation,
+): void {
+  const type = destructure(env.get(operand.identifier.id));
+  if (type?.kind === 'Ref' || type?.kind === 'RefValue') {
+    errors.pushDiagnostic(
+      CompilerDiagnostic.create({
+        severity: ErrorSeverity.InvalidReact,
+        category: 'Cannot access refs during render',
+        description: ERROR_DESCRIPTION,
+      }).withDetail({
+        kind: 'error',
+        loc: (type.kind === 'RefValue' && type.loc) || loc,
+        message: `Cannot update ref during render`,
+      }),
+    );
   }
 }
 
@@ -657,17 +822,22 @@ function validateNoDirectRefValueAccess(
 ): void {
   const type = destructure(env.get(operand.identifier.id));
   if (type?.kind === 'RefValue') {
-    errors.push({
-      severity: ErrorSeverity.InvalidReact,
-      reason:
-        'Ref values (the `current` property) may not be accessed during render. (https://react.dev/reference/react/useRef)',
-      loc: type.loc ?? operand.loc,
-      description:
-        operand.identifier.name !== null &&
-        operand.identifier.name.kind === 'named'
-          ? `Cannot access ref value \`${operand.identifier.name.value}\``
-          : null,
-      suggestions: null,
-    });
+    errors.pushDiagnostic(
+      CompilerDiagnostic.create({
+        severity: ErrorSeverity.InvalidReact,
+        category: 'Cannot access refs during render',
+        description: ERROR_DESCRIPTION,
+      }).withDetail({
+        kind: 'error',
+        loc: type.loc ?? operand.loc,
+        message: `Cannot access ref value during render`,
+      }),
+    );
   }
 }
+
+const ERROR_DESCRIPTION =
+  'React refs are values that are not needed for rendering. Refs should only be accessed ' +
+  'outside of render, such as in event handlers or effects. ' +
+  'Accessing a ref value (the `current` property) during render can cause your component ' +
+  'not to update as expected (https://react.dev/reference/react/useRef)';

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

@@ -97,16 +97,21 @@ export function validateNoSetStateInEffects(
                 errors.pushDiagnostic(
                   CompilerDiagnostic.create({
                     category:
-                      'Calling setState within an effect can trigger cascading renders',
+                      'Calling setState synchronously within an effect can trigger cascading renders',
                     description:
-                      'Calling setState directly within a useEffect causes cascading renders that can hurt performance, and is not recommended. Consider alternatives to useEffect. (https://react.dev/learn/you-might-not-need-an-effect)',
+                      'Effects are intended to synchronize state between React and external systems such as manually updating the DOM, state management libraries, or other platform APIs. ' +
+                      'In general, the body of an effect should do one or both of the following:\n' +
+                      '* Update external systems with the latest state from React.\n' +
+                      '* Subscribe for updates from some external system, calling setState in a callback function when external state changes.\n\n' +
+                      'Calling setState synchronously within an effect body causes cascading renders that can hurt performance, and is not recommended. ' +
+                      '(https://react.dev/learn/you-might-not-need-an-effect)',
                     severity: ErrorSeverity.InvalidReact,
                     suggestions: null,
                   }).withDetail({
                     kind: 'error',
                     loc: setState.loc,
                     message:
-                      'Avoid calling setState() in the top-level of an effect',
+                      'Avoid calling setState() directly within an effect',
                   }),
                 );
               }

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

@@ -5,7 +5,11 @@
  * LICENSE file in the root directory of this source tree.
  */
 
-import {CompilerError, ErrorSeverity} from '../CompilerError';
+import {
+  CompilerDiagnostic,
+  CompilerError,
+  ErrorSeverity,
+} from '../CompilerError';
 import {HIRFunction, IdentifierId, SourceLocation} from '../HIR';
 import {Result} from '../Utils/Result';
 
@@ -59,20 +63,23 @@ export function validateStaticComponents(
               value.tag.identifier.id,
             );
             if (location != null) {
-              error.push({
-                reason: `Components created during render will reset their state each time they are created. Declare components outside of render. `,
-                severity: ErrorSeverity.InvalidReact,
-                loc: value.tag.loc,
-                description: null,
-                suggestions: null,
-              });
-              error.push({
-                reason: `The component may be created during render`,
-                severity: ErrorSeverity.InvalidReact,
-                loc: location,
-                description: null,
-                suggestions: null,
-              });
+              error.pushDiagnostic(
+                CompilerDiagnostic.create({
+                  severity: ErrorSeverity.InvalidReact,
+                  category: 'Cannot create components during render',
+                  description: `Components created during render will reset their state each time they are created. Declare components outside of render. `,
+                })
+                  .withDetail({
+                    kind: 'error',
+                    loc: value.tag.loc,
+                    message: 'This component is created during render',
+                  })
+                  .withDetail({
+                    kind: 'error',
+                    loc: location,
+                    message: 'The component is created during render here',
+                  }),
+              );
             }
           }
         }

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

@@ -82,7 +82,7 @@ export function validateUseMemo(fn: HIRFunction): Result<void, CompilerError> {
               }).withDetail({
                 kind: 'error',
                 loc,
-                message: '',
+                message: 'Callbacks with parameters are not supported',
               }),
             );
           }
@@ -92,9 +92,9 @@ export function validateUseMemo(fn: HIRFunction): Result<void, CompilerError> {
               CompilerDiagnostic.create({
                 severity: ErrorSeverity.InvalidReact,
                 category:
-                  'useMemo callbacks may not be async or generator functions',
+                  'useMemo() callbacks may not be async or generator functions',
                 description:
-                  'useMemo() callbacks are called once and must synchronously return a value',
+                  'useMemo() callbacks are called once and must synchronously return a value.',
                 suggestions: null,
               }).withDetail({
                 kind: 'error',

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

@@ -58,7 +58,8 @@ it('logs failed compilation', () => {
 
   expect(event.detail.severity).toEqual('InvalidReact');
   //@ts-ignore
-  const {start, end, identifierName} = event.detail.loc as t.SourceLocation;
+  const {start, end, identifierName} =
+    event.detail.primaryLocation() as t.SourceLocation;
   expect(start).toEqual({column: 28, index: 28, line: 1});
   expect(end).toEqual({column: 33, index: 33, line: 1});
   expect(identifierName).toEqual('props');

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/allow-assigning-ref-accessing-function-to-object-property-if-not-mutated.expect.md

@@ -0,0 +1,52 @@
+
+## Input
+
+```javascript
+import {useRef} from 'react';
+import {Stringify} from 'shared-runtime';
+
+function Component(props) {
+  const ref = useRef(props.value);
+  const object = {};
+  object.foo = () => ref.current;
+  return <Stringify object={object} shouldInvokeFns={true} />;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{value: 42}],
+};
+
+```
+
+## Code
+
+```javascript
+import { c as _c } from "react/compiler-runtime";
+import { useRef } from "react";
+import { Stringify } from "shared-runtime";
+
+function Component(props) {
+  const $ = _c(1);
+  const ref = useRef(props.value);
+  let t0;
+  if ($[0] === Symbol.for("react.memo_cache_sentinel")) {
+    const object = {};
+    object.foo = () => ref.current;
+    t0 = <Stringify object={object} shouldInvokeFns={true} />;
+    $[0] = t0;
+  } else {
+    t0 = $[0];
+  }
+  return t0;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ value: 42 }],
+};
+
+```
+      
+### Eval output
+(kind: ok) <div>{"object":{"foo":{"kind":"Function","result":42}},"shouldInvokeFns":true}</div>

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/allow-assigning-ref-accessing-function-to-object-property-if-not-mutated.js

@@ -0,0 +1,14 @@
+import {useRef} from 'react';
+import {Stringify} from 'shared-runtime';
+
+function Component(props) {
+  const ref = useRef(props.value);
+  const object = {};
+  object.foo = () => ref.current;
+  return <Stringify object={object} shouldInvokeFns={true} />;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{value: 42}],
+};

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/allow-merge-refs-pattern.expect.md

@@ -0,0 +1,44 @@
+
+## Input
+
+```javascript
+// @enableTreatRefLikeIdentifiersAsRefs @validateRefAccessDuringRender
+
+import {useRef} from 'react';
+
+function Component() {
+  const ref = useRef(null);
+  const ref2 = useRef(null);
+  const mergedRef = mergeRefs([ref], ref2);
+
+  return <Stringify ref={mergedRef} />;
+}
+
+```
+
+## Code
+
+```javascript
+import { c as _c } from "react/compiler-runtime"; // @enableTreatRefLikeIdentifiersAsRefs @validateRefAccessDuringRender
+
+import { useRef } from "react";
+
+function Component() {
+  const $ = _c(1);
+  const ref = useRef(null);
+  const ref2 = useRef(null);
+  const mergedRef = mergeRefs([ref], ref2);
+  let t0;
+  if ($[0] === Symbol.for("react.memo_cache_sentinel")) {
+    t0 = <Stringify ref={mergedRef} />;
+    $[0] = t0;
+  } else {
+    t0 = $[0];
+  }
+  return t0;
+}
+
+```
+      
+### Eval output
+(kind: exception) Fixture not implemented

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/allow-merge-refs-pattern.js

@@ -0,0 +1,11 @@
+// @enableTreatRefLikeIdentifiersAsRefs @validateRefAccessDuringRender
+
+import {useRef} from 'react';
+
+function Component() {
+  const ref = useRef(null);
+  const ref2 = useRef(null);
+  const mergedRef = mergeRefs([ref], ref2);
+
+  return <Stringify ref={mergedRef} />;
+}

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/allow-passing-ref-to-render-helper-props-object.expect.md

@@ -0,0 +1,45 @@
+
+## Input
+
+```javascript
+// @enableTreatRefLikeIdentifiersAsRefs @validateRefAccessDuringRender
+
+import {useRef} from 'react';
+
+function Component(props) {
+  const ref = useRef(null);
+
+  return <Foo>{props.render({ref})}</Foo>;
+}
+
+```
+
+## Code
+
+```javascript
+import { c as _c } from "react/compiler-runtime"; // @enableTreatRefLikeIdentifiersAsRefs @validateRefAccessDuringRender
+
+import { useRef } from "react";
+
+function Component(props) {
+  const $ = _c(3);
+  const ref = useRef(null);
+
+  const T0 = Foo;
+  const t0 = props.render({ ref });
+  let t1;
+  if ($[0] !== T0 || $[1] !== t0) {
+    t1 = <T0>{t0}</T0>;
+    $[0] = T0;
+    $[1] = t0;
+    $[2] = t1;
+  } else {
+    t1 = $[2];
+  }
+  return t1;
+}
+
+```
+      
+### Eval output
+(kind: exception) Fixture not implemented

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/allow-passing-ref-to-render-helper-props-object.js

@@ -0,0 +1,9 @@
+// @enableTreatRefLikeIdentifiersAsRefs @validateRefAccessDuringRender
+
+import {useRef} from 'react';
+
+function Component(props) {
+  const ref = useRef(null);
+
+  return <Foo>{props.render({ref})}</Foo>;
+}

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/allow-passing-ref-to-render-helper.expect.md

@@ -0,0 +1,49 @@
+
+## Input
+
+```javascript
+// @enableTreatRefLikeIdentifiersAsRefs @validateRefAccessDuringRender
+
+import {useRef} from 'react';
+
+function Component(props) {
+  const ref = useRef(null);
+
+  return <Foo>{props.render(ref)}</Foo>;
+}
+
+```
+
+## Code
+
+```javascript
+import { c as _c } from "react/compiler-runtime"; // @enableTreatRefLikeIdentifiersAsRefs @validateRefAccessDuringRender
+
+import { useRef } from "react";
+
+function Component(props) {
+  const $ = _c(4);
+  const ref = useRef(null);
+  let t0;
+  if ($[0] !== props.render) {
+    t0 = props.render(ref);
+    $[0] = props.render;
+    $[1] = t0;
+  } else {
+    t0 = $[1];
+  }
+  let t1;
+  if ($[2] !== t0) {
+    t1 = <Foo>{t0}</Foo>;
+    $[2] = t0;
+    $[3] = t1;
+  } else {
+    t1 = $[3];
+  }
+  return t1;
+}
+
+```
+      
+### Eval output
+(kind: exception) Fixture not implemented

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/allow-passing-ref-to-render-helper.js

@@ -0,0 +1,9 @@
+// @enableTreatRefLikeIdentifiersAsRefs @validateRefAccessDuringRender
+
+import {useRef} from 'react';
+
+function Component(props) {
+  const ref = useRef(null);
+
+  return <Foo>{props.render(ref)}</Foo>;
+}

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/allow-ref-access-in-effect-indirect.expect.md

@@ -27,6 +27,7 @@ function Component() {
 }
 
 function Child({ref}) {
+  'use no memo';
   // This violates the rules of React, so we access the ref in a child
   // component
   return ref.current;
@@ -100,8 +101,10 @@ function Component() {
   return t6;
 }
 
-function Child(t0) {
-  const { ref } = t0;
+function Child({ ref }) {
+  "use no memo";
+  // This violates the rules of React, so we access the ref in a child
+  // component
   return ref.current;
 }
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/allow-ref-access-in-effect-indirect.js

@@ -23,6 +23,7 @@ function Component() {
 }
 
 function Child({ref}) {
+  'use no memo';
   // This violates the rules of React, so we access the ref in a child
   // component
   return ref.current;

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/allow-ref-access-in-effect.expect.md

@@ -23,6 +23,7 @@ function Component() {
 }
 
 function Child({ref}) {
+  'use no memo';
   // This violates the rules of React, so we access the ref in a child
   // component
   return ref.current;
@@ -86,8 +87,10 @@ function Component() {
   return t5;
 }
 
-function Child(t0) {
-  const { ref } = t0;
+function Child({ ref }) {
+  "use no memo";
+  // This violates the rules of React, so we access the ref in a child
+  // component
   return ref.current;
 }
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/allow-ref-access-in-effect.js

@@ -19,6 +19,7 @@ function Component() {
 }
 
 function Child({ref}) {
+  'use no memo';
   // This violates the rules of React, so we access the ref in a child
   // component
   return ref.current;

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/allow-ref-access-in-unused-callback-nested.expect.md

@@ -25,6 +25,7 @@ function Component() {
 }
 
 function Child({ref}) {
+  'use no memo';
   // This violates the rules of React, so we access the ref in a child
   // component
   return ref.current;
@@ -83,8 +84,10 @@ function Component() {
 }
 function _temp() {}
 
-function Child(t0) {
-  const { ref } = t0;
+function Child({ ref }) {
+  "use no memo";
+  // This violates the rules of React, so we access the ref in a child
+  // component
   return ref.current;
 }
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/allow-ref-access-in-unused-callback-nested.js

@@ -21,6 +21,7 @@ function Component() {
 }
 
 function Child({ref}) {
+  'use no memo';
   // This violates the rules of React, so we access the ref in a child
   // component
   return ref.current;

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/allow-ref-lazy-initialization-with-logical.expect.md

@@ -0,0 +1,68 @@
+
+## Input
+
+```javascript
+// @validateRefAccessDuringRender
+
+import {useRef} from 'react';
+
+function Component(props) {
+  const ref = useRef(null);
+  if (ref.current == null) {
+    // the logical means the ref write is in a different block
+    // from the if consequent. this tests that the "safe" blocks
+    // extend up to the if's fallthrough
+    ref.current = props.unknownKey ?? props.value;
+  }
+  return <Child ref={ref} />;
+}
+
+function Child({ref}) {
+  'use no memo';
+  return ref.current;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{value: 42}],
+};
+
+```
+
+## Code
+
+```javascript
+import { c as _c } from "react/compiler-runtime"; // @validateRefAccessDuringRender
+
+import { useRef } from "react";
+
+function Component(props) {
+  const $ = _c(1);
+  const ref = useRef(null);
+  if (ref.current == null) {
+    ref.current = props.unknownKey ?? props.value;
+  }
+  let t0;
+  if ($[0] === Symbol.for("react.memo_cache_sentinel")) {
+    t0 = <Child ref={ref} />;
+    $[0] = t0;
+  } else {
+    t0 = $[0];
+  }
+  return t0;
+}
+
+function Child({ ref }) {
+  "use no memo";
+  return ref.current;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ value: 42 }],
+};
+
+```
+      
+### Eval output
+(kind: ok) 42

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/allow-ref-lazy-initialization-with-logical.js

@@ -0,0 +1,24 @@
+// @validateRefAccessDuringRender
+
+import {useRef} from 'react';
+
+function Component(props) {
+  const ref = useRef(null);
+  if (ref.current == null) {
+    // the logical means the ref write is in a different block
+    // from the if consequent. this tests that the "safe" blocks
+    // extend up to the if's fallthrough
+    ref.current = props.unknownKey ?? props.value;
+  }
+  return <Child ref={ref} />;
+}
+
+function Child({ref}) {
+  'use no memo';
+  return ref.current;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{value: 42}],
+};

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

@@ -19,7 +19,7 @@ function Component(props) {
 ```javascript
 import { c as _c } from "react/compiler-runtime";
 function Component(props) {
-  const $ = _c(7);
+  const $ = _c(5);
   let t0;
   if ($[0] !== props.x) {
     t0 = foo(props.x);
@@ -31,26 +31,19 @@ function Component(props) {
   const x = t0;
   let t1;
   if ($[2] !== props || $[3] !== x) {
-    t1 = function () {
+    const fn = function () {
       const arr = [...bar(props)];
       return arr.at(x);
     };
+
+    t1 = fn();
     $[2] = props;
     $[3] = x;
     $[4] = t1;
   } else {
     t1 = $[4];
   }
-  const fn = t1;
-  let t2;
-  if ($[5] !== fn) {
-    t2 = fn();
-    $[5] = fn;
-    $[6] = t2;
-  } else {
-    t2 = $[6];
-  }
-  const fnResult = t2;
+  const fnResult = t1;
   return fnResult;
 }
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/array-map-captures-receiver-noAlias.expect.md

@@ -23,34 +23,18 @@ export const FIXTURE_ENTRYPOINT = {
 ```javascript
 import { c as _c } from "react/compiler-runtime";
 function Component(props) {
-  const $ = _c(6);
+  const $ = _c(2);
   let t0;
   if ($[0] !== props.a) {
-    t0 = { a: props.a };
+    const item = { a: props.a };
+    const items = [item];
+    t0 = items.map(_temp);
     $[0] = props.a;
     $[1] = t0;
   } else {
     t0 = $[1];
   }
-  const item = t0;
-  let t1;
-  if ($[2] !== item) {
-    t1 = [item];
-    $[2] = item;
-    $[3] = t1;
-  } else {
-    t1 = $[3];
-  }
-  const items = t1;
-  let t2;
-  if ($[4] !== items) {
-    t2 = items.map(_temp);
-    $[4] = items;
-    $[5] = t2;
-  } else {
-    t2 = $[5];
-  }
-  const mapped = t2;
+  const mapped = t0;
   return mapped;
 }
 function _temp(item_0) {

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/array-map-noAlias-escaping-function.expect.md

@@ -21,26 +21,18 @@ export const FIXTURE_ENTRYPOINT = {
 ```javascript
 import { c as _c } from "react/compiler-runtime";
 function Component(props) {
-  const $ = _c(4);
+  const $ = _c(2);
   const f = _temp;
   let t0;
   if ($[0] !== props.items) {
-    t0 = [...props.items].map(f);
+    const x = [...props.items].map(f);
+    t0 = [x, f];
     $[0] = props.items;
     $[1] = t0;
   } else {
     t0 = $[1];
   }
-  const x = t0;
-  let t1;
-  if ($[2] !== x) {
-    t1 = [x, f];
-    $[2] = x;
-    $[3] = t1;
-  } else {
-    t1 = $[3];
-  }
-  return t1;
+  return t0;
 }
 function _temp(item) {
   return item;

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/capturing-arrow-function-1.expect.md

@@ -23,27 +23,19 @@ export const FIXTURE_ENTRYPOINT = {
 ```javascript
 import { c as _c } from "react/compiler-runtime";
 function component(a) {
-  const $ = _c(4);
+  const $ = _c(2);
   let t0;
   if ($[0] !== a) {
-    t0 = { a };
+    const z = { a };
+    t0 = () => {
+      console.log(z);
+    };
     $[0] = a;
     $[1] = t0;
   } else {
     t0 = $[1];
   }
-  const z = t0;
-  let t1;
-  if ($[2] !== z) {
-    t1 = () => {
-      console.log(z);
-    };
-    $[2] = z;
-    $[3] = t1;
-  } else {
-    t1 = $[3];
-  }
-  const x = t1;
+  const x = t0;
   return x;
 }
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/capturing-function-1.expect.md

@@ -23,27 +23,19 @@ export const FIXTURE_ENTRYPOINT = {
 ```javascript
 import { c as _c } from "react/compiler-runtime";
 function component(a) {
-  const $ = _c(4);
+  const $ = _c(2);
   let t0;
   if ($[0] !== a) {
-    t0 = { a };
+    const z = { a };
+    t0 = function () {
+      console.log(z);
+    };
     $[0] = a;
     $[1] = t0;
   } else {
     t0 = $[1];
   }
-  const z = t0;
-  let t1;
-  if ($[2] !== z) {
-    t1 = function () {
-      console.log(z);
-    };
-    $[2] = z;
-    $[3] = t1;
-  } else {
-    t1 = $[3];
-  }
-  const x = t1;
+  const x = t0;
   return x;
 }
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/capturing-function-runs-inference.expect.md

@@ -22,35 +22,19 @@ export const FIXTURE_ENTRYPOINT = {
 import { c as _c } from "react/compiler-runtime";
 import { Stringify } from "shared-runtime";
 function Component(t0) {
-  const $ = _c(6);
+  const $ = _c(2);
   const { a } = t0;
   let t1;
   if ($[0] !== a) {
-    t1 = { a };
+    const z = { a };
+    const p = () => <Stringify>{z}</Stringify>;
+    t1 = p();
     $[0] = a;
     $[1] = t1;
   } else {
     t1 = $[1];
   }
-  const z = t1;
-  let t2;
-  if ($[2] !== z) {
-    t2 = () => <Stringify>{z}</Stringify>;
-    $[2] = z;
-    $[3] = t2;
-  } else {
-    t2 = $[3];
-  }
-  const p = t2;
-  let t3;
-  if ($[4] !== p) {
-    t3 = p();
-    $[4] = p;
-    $[5] = t3;
-  } else {
-    t3 = $[5];
-  }
-  return t3;
+  return t1;
 }
 
 export const FIXTURE_ENTRYPOINT = {

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/capturing-variable-in-nested-block.expect.md

@@ -25,27 +25,19 @@ export const FIXTURE_ENTRYPOINT = {
 ```javascript
 import { c as _c } from "react/compiler-runtime";
 function component(a) {
-  const $ = _c(4);
+  const $ = _c(2);
   let t0;
   if ($[0] !== a) {
-    t0 = { a };
+    const z = { a };
+    t0 = function () {
+      console.log(z);
+    };
     $[0] = a;
     $[1] = t0;
   } else {
     t0 = $[1];
   }
-  const z = t0;
-  let t1;
-  if ($[2] !== z) {
-    t1 = function () {
-      console.log(z);
-    };
-    $[2] = z;
-    $[3] = t1;
-  } else {
-    t1 = $[3];
-  }
-  const x = t1;
+  const x = t0;
   return x;
 }
 

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/capturing-variable-in-nested-function.expect.md

@@ -25,29 +25,21 @@ export const FIXTURE_ENTRYPOINT = {
 ```javascript
 import { c as _c } from "react/compiler-runtime";
 function component(a) {
-  const $ = _c(4);
+  const $ = _c(2);
   let t0;
   if ($[0] !== a) {
-    t0 = { a };
+    const z = { a };
+    t0 = function () {
+      (function () {
+        console.log(z);
+      })();
+    };
     $[0] = a;
     $[1] = t0;
   } else {
     t0 = $[1];
   }
-  const z = t0;
-  let t1;
-  if ($[2] !== z) {
-    t1 = function () {
-      (function () {
-        console.log(z);
-      })();
-    };
-    $[2] = z;
-    $[3] = t1;
-  } else {
-    t1 = $[3];
-  }
-  const x = t1;
+  const x = t0;
   return x;
 }
 

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

@@ -0,0 +1,32 @@
+
+## Input
+
+```javascript
+import {useRef} from 'react';
+
+function useThing(fn) {
+  const fnRef = useRef(fn);
+  const ref = useRef(null);
+
+  if (ref.current === null) {
+    ref.current = function (this: unknown, ...args) {
+      return fnRef.current.call(this, ...args);
+    };
+  }
+  return ref.current;
+}
+
+```
+
+
+## Error
+
+```
+Found 1 error:
+
+Error: Expected a non-reserved identifier name
+
+`this` is a reserved word in JavaScript and cannot be used as an identifier name.
+```
+          
+      

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

@@ -0,0 +1,13 @@
+import {useRef} from 'react';
+
+function useThing(fn) {
+  const fnRef = useRef(fn);
+  const ref = useRef(null);
+
+  if (ref.current === null) {
+    ref.current = function (this: unknown, ...args) {
+      return fnRef.current.call(this, ...args);
+    };
+  }
+  return ref.current;
+}

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error._todo.computed-lval-in-destructure.expect.md

@@ -16,6 +16,7 @@ function Component(props) {
 
 ```
 Found 1 error:
+
 Todo: (BuildHIR::lowerAssignment) Handle computed properties in ObjectPattern
 
 error._todo.computed-lval-in-destructure.ts:3:9
@@ -26,8 +27,6 @@ error._todo.computed-lval-in-destructure.ts:3:9
   4 |
   5 |   return x;
   6 | }
-
-
 ```
           
       

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.assign-global-in-component-tag-function.expect.md

@@ -16,15 +16,16 @@ function Component() {
 
 ```
 Found 1 error:
+
 Error: Cannot reassign variables declared outside of the component/hook
 
-Reassigning a variable declared outside of the component/hook is a form of side effect, which can cause unpredictable behavior depending on when the component happens to re-render. If this variable is used in rendering, use useState instead. Otherwise, consider updating it in an effect (https://react.dev/reference/rules/components-and-hooks-must-be-pure#side-effects-must-run-outside-of-render)
+Variable `someGlobal` is declared outside of the component/hook. Reassigning this value during render is a form of side effect, which can cause unpredictable behavior depending on when the component happens to re-render. If this variable is used in rendering, use useState instead. Otherwise, consider updating it in an effect. (https://react.dev/reference/rules/components-and-hooks-must-be-pure#side-effects-must-run-outside-of-render)
 
 error.assign-global-in-component-tag-function.ts:3:4
   1 | function Component() {
   2 |   const Foo = () => {
 > 3 |     someGlobal = true;
-    |     ^^^^^^^^^^ Cannot reassign variable
+    |     ^^^^^^^^^^ `someGlobal` cannot be reassigned
   4 |   };
   5 |   return <Foo />;
   6 | }

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.assign-global-in-jsx-children.expect.md

@@ -19,15 +19,16 @@ function Component() {
 
 ```
 Found 1 error:
+
 Error: Cannot reassign variables declared outside of the component/hook
 
-Reassigning a variable declared outside of the component/hook is a form of side effect, which can cause unpredictable behavior depending on when the component happens to re-render. If this variable is used in rendering, use useState instead. Otherwise, consider updating it in an effect (https://react.dev/reference/rules/components-and-hooks-must-be-pure#side-effects-must-run-outside-of-render)
+Variable `someGlobal` is declared outside of the component/hook. Reassigning this value during render is a form of side effect, which can cause unpredictable behavior depending on when the component happens to re-render. If this variable is used in rendering, use useState instead. Otherwise, consider updating it in an effect. (https://react.dev/reference/rules/components-and-hooks-must-be-pure#side-effects-must-run-outside-of-render)
 
 error.assign-global-in-jsx-children.ts:3:4
   1 | function Component() {
   2 |   const foo = () => {
 > 3 |     someGlobal = true;
-    |     ^^^^^^^^^^ Cannot reassign variable
+    |     ^^^^^^^^^^ `someGlobal` cannot be reassigned
   4 |   };
   5 |   // Children are generally access/called during render, so
   6 |   // modifying a global in a children function is almost

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.assign-global-in-jsx-spread-attribute.expect.md

@@ -17,6 +17,7 @@ function Component() {
 
 ```
 Found 1 error:
+
 Error: Unexpected reassignment of a variable which was defined outside of the component. Components and hooks should be pure and side-effect free, but variable reassignment is a form of side-effect. If this variable is used in rendering, use useState instead. (https://react.dev/reference/rules/components-and-hooks-must-be-pure#side-effects-must-run-outside-of-render)
 
 error.assign-global-in-jsx-spread-attribute.ts:4:4
@@ -27,8 +28,6 @@ error.assign-global-in-jsx-spread-attribute.ts:4:4
   5 |   };
   6 |   return <div {...foo} />;
   7 | }
-
-
 ```
           
       

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.bailout-on-flow-suppression.expect.md

@@ -17,20 +17,19 @@ function Foo(props) {
 
 ```
 Found 1 error:
-Error: React Compiler has skipped optimizing this component because one or more React rule violations were reported by Flow. React Compiler only works when your components follow all the rules of React, disabling them may result in unexpected or incorrect behavior
 
-$FlowFixMe[react-rule-hook].
+Error: React Compiler has skipped optimizing this component because one or more React rule violations were reported by Flow
+
+React Compiler only works when your components follow all the rules of React, disabling them may result in unexpected or incorrect behavior. Found suppression `$FlowFixMe[react-rule-hook]`
 
 error.bailout-on-flow-suppression.ts:4:2
   2 |
   3 | function Foo(props) {
 > 4 |   // $FlowFixMe[react-rule-hook]
-    |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ React Compiler has skipped optimizing this component because one or more React rule violations were reported by Flow. React Compiler only works when your components follow all the rules of React, disabling them may result in unexpected or incorrect behavior
+    |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Found React rule suppression
   5 |   useX();
   6 |   return null;
   7 | }
-
-
 ```
           
       

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.bailout-on-suppression-of-custom-rule.expect.md

@@ -20,34 +20,32 @@ function lowercasecomponent() {
 
 ```
 Found 2 errors:
-Error: React Compiler has skipped optimizing this component because one or more React ESLint rules were disabled. React Compiler only works when your components follow all the rules of React, disabling them may result in unexpected or incorrect behavior
 
-eslint-disable my-app/react-rule.
+Error: React Compiler has skipped optimizing this component because one or more React ESLint rules were disabled
+
+React Compiler only works when your components follow all the rules of React, disabling them may result in unexpected or incorrect behavior. Found suppression `eslint-disable my-app/react-rule`
 
 error.bailout-on-suppression-of-custom-rule.ts:3:0
   1 | // @eslintSuppressionRules:["my-app","react-rule"]
   2 |
 > 3 | /* eslint-disable my-app/react-rule */
-    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ React Compiler has skipped optimizing this component because one or more React ESLint rules were disabled. React Compiler only works when your components follow all the rules of React, disabling them may result in unexpected or incorrect behavior
+    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Found React rule suppression
   4 | function lowercasecomponent() {
   5 |   'use forget';
   6 |   const x = [];
 
+Error: React Compiler has skipped optimizing this component because one or more React ESLint rules were disabled
 
-Error: React Compiler has skipped optimizing this component because one or more React ESLint rules were disabled. React Compiler only works when your components follow all the rules of React, disabling them may result in unexpected or incorrect behavior
-
-eslint-disable-next-line my-app/react-rule.
+React Compiler only works when your components follow all the rules of React, disabling them may result in unexpected or incorrect behavior. Found suppression `eslint-disable-next-line my-app/react-rule`
 
 error.bailout-on-suppression-of-custom-rule.ts:7:2
    5 |   'use forget';
    6 |   const x = [];
 >  7 |   // eslint-disable-next-line my-app/react-rule
-     |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ React Compiler has skipped optimizing this component because one or more React ESLint rules were disabled. React Compiler only works when your components follow all the rules of React, disabling them may result in unexpected or incorrect behavior
+     |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Found React rule suppression
    8 |   return <div>{x}</div>;
    9 | }
   10 | /* eslint-enable my-app/react-rule */
-
-
 ```
           
       

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.bug-infer-mutation-aliasing-effects.expect.md

@@ -0,0 +1,46 @@
+
+## Input
+
+```javascript
+import {useCallback, useRef} from 'react';
+
+export default function useThunkDispatch(state, dispatch, extraArg) {
+  const stateRef = useRef(state);
+  stateRef.current = state;
+
+  return useCallback(
+    function thunk(action) {
+      if (typeof action === 'function') {
+        return action(thunk, () => stateRef.current, extraArg);
+      } else {
+        dispatch(action);
+        return undefined;
+      }
+    },
+    [dispatch, extraArg]
+  );
+}
+
+```
+
+
+## Error
+
+```
+Found 1 error:
+
+Invariant: [InferMutationAliasingEffects] Expected value kind to be initialized
+
+<unknown> thunk$14.
+
+error.bug-infer-mutation-aliasing-effects.ts:10:22
+   8 |     function thunk(action) {
+   9 |       if (typeof action === 'function') {
+> 10 |         return action(thunk, () => stateRef.current, extraArg);
+     |                       ^^^^^ [InferMutationAliasingEffects] Expected value kind to be initialized
+  11 |       } else {
+  12 |         dispatch(action);
+  13 |         return undefined;
+```
+          
+      

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.bug-infer-mutation-aliasing-effects.js

@@ -0,0 +1,18 @@
+import {useCallback, useRef} from 'react';
+
+export default function useThunkDispatch(state, dispatch, extraArg) {
+  const stateRef = useRef(state);
+  stateRef.current = state;
+
+  return useCallback(
+    function thunk(action) {
+      if (typeof action === 'function') {
+        return action(thunk, () => stateRef.current, extraArg);
+      } else {
+        dispatch(action);
+        return undefined;
+      }
+    },
+    [dispatch, extraArg]
+  );
+}

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.bug-invariant-codegen-methodcall.expect.md

@@ -0,0 +1,31 @@
+
+## Input
+
+```javascript
+const YearsAndMonthsSince = () => {
+  const diff = foo();
+  const months = Math.floor(diff.bar());
+  return <>{months}</>;
+};
+
+```
+
+
+## Error
+
+```
+Found 1 error:
+
+Invariant: [Codegen] Internal error: MethodCall::property must be an unpromoted + unmemoized MemberExpression. Got a `Identifier`
+
+error.bug-invariant-codegen-methodcall.ts:3:17
+  1 | const YearsAndMonthsSince = () => {
+  2 |   const diff = foo();
+> 3 |   const months = Math.floor(diff.bar());
+    |                  ^^^^^^^^^^ [Codegen] Internal error: MethodCall::property must be an unpromoted + unmemoized MemberExpression. Got a `Identifier`
+  4 |   return <>{months}</>;
+  5 | };
+  6 |
+```
+          
+      

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.bug-invariant-codegen-methodcall.js

@@ -0,0 +1,5 @@
+const YearsAndMonthsSince = () => {
+  const diff = foo();
+  const months = Math.floor(diff.bar());
+  return <>{months}</>;
+};

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.bug-invariant-couldnt-find-binding-for-decl.expect.md

@@ -0,0 +1,37 @@
+
+## Input
+
+```javascript
+import {useEffect} from 'react';
+
+export function Foo() {
+  useEffect(() => {
+    try {
+      // do something
+    } catch ({status}) {
+      // do something
+    }
+  }, []);
+}
+
+```
+
+
+## Error
+
+```
+Found 1 error:
+
+Invariant: (BuildHIR::lowerAssignment) Could not find binding for declaration.
+
+error.bug-invariant-couldnt-find-binding-for-decl.ts:7:14
+   5 |     try {
+   6 |       // do something
+>  7 |     } catch ({status}) {
+     |               ^^^^^^ (BuildHIR::lowerAssignment) Could not find binding for declaration.
+   8 |       // do something
+   9 |     }
+  10 |   }, []);
+```
+          
+      

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.bug-invariant-couldnt-find-binding-for-decl.js

@@ -0,0 +1,11 @@
+import {useEffect} from 'react';
+
+export function Foo() {
+  useEffect(() => {
+    try {
+      // do something
+    } catch ({status}) {
+      // do something
+    }
+  }, []);
+}

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

@@ -0,0 +1,32 @@
+
+## Input
+
+```javascript
+import {useMemo} from 'react';
+
+export default function useFoo(text) {
+  return useMemo(() => {
+    try {
+      let formattedText = '';
+      try {
+        formattedText = format(text);
+      } catch {
+        console.log('error');
+      }
+      return formattedText || '';
+    } catch (e) {}
+  }, [text]);
+}
+
+```
+
+
+## Error
+
+```
+Found 1 error:
+
+Invariant: Expected a break target
+```
+          
+      

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.bug-invariant-expected-break-target.js

@@ -0,0 +1,15 @@
+import {useMemo} from 'react';
+
+export default function useFoo(text) {
+  return useMemo(() => {
+    try {
+      let formattedText = '';
+      try {
+        formattedText = format(text);
+      } catch {
+        console.log('error');
+      }
+      return formattedText || '';
+    } catch (e) {}
+  }, [text]);
+}

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

@@ -0,0 +1,44 @@
+
+## Input
+
+```javascript
+import {useMemo} from 'react';
+import {useFoo, formatB, Baz} from './lib';
+
+export const Example = ({data}) => {
+  let a;
+  let b;
+
+  if (data) {
+    ({a, b} = data);
+  }
+
+  const foo = useFoo(a);
+  const bar = useMemo(() => formatB(b), [b]);
+
+  return <Baz foo={foo} bar={bar} />;
+};
+
+```
+
+
+## Error
+
+```
+Found 1 error:
+
+Invariant: Expected consistent kind for destructuring
+
+Other places were `Reassign` but 'mutate? #t8$46[7:9]{reactive}' is const.
+
+error.bug-invariant-expected-consistent-destructuring.ts:9:9
+   7 |
+   8 |   if (data) {
+>  9 |     ({a, b} = data);
+     |          ^ Expected consistent kind for destructuring
+  10 |   }
+  11 |
+  12 |   const foo = useFoo(a);
+```
+          
+      

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

@@ -0,0 +1,16 @@
+import {useMemo} from 'react';
+import {useFoo, formatB, Baz} from './lib';
+
+export const Example = ({data}) => {
+  let a;
+  let b;
+
+  if (data) {
+    ({a, b} = data);
+  }
+
+  const foo = useFoo(a);
+  const bar = useMemo(() => formatB(b), [b]);
+
+  return <Baz foo={foo} bar={bar} />;
+};

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.bug-invariant-local-or-context-references.expect.md

@@ -0,0 +1,46 @@
+
+## Input
+
+```javascript
+import {useState} from 'react';
+import {bar} from './bar';
+
+export const useFoot = () => {
+  const [, setState] = useState(null);
+  try {
+    const {data} = bar();
+    setState({
+      data,
+      error: null,
+    });
+  } catch (err) {
+    setState(_prevState => ({
+      loading: false,
+      error: err,
+    }));
+  }
+};
+
+```
+
+
+## Error
+
+```
+Found 1 error:
+
+Invariant: Expected all references to a variable to be consistently local or context references
+
+Identifier <unknown> err$7 is referenced as a context variable, but was previously referenced as a [object Object] variable.
+
+error.bug-invariant-local-or-context-references.ts:15:13
+  13 |     setState(_prevState => ({
+  14 |       loading: false,
+> 15 |       error: err,
+     |              ^^^ Expected all references to a variable to be consistently local or context references
+  16 |     }));
+  17 |   }
+  18 | };
+```
+          
+      

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.bug-invariant-local-or-context-references.js

@@ -0,0 +1,18 @@
+import {useState} from 'react';
+import {bar} from './bar';
+
+export const useFoot = () => {
+  const [, setState] = useState(null);
+  try {
+    const {data} = bar();
+    setState({
+      data,
+      error: null,
+    });
+  } catch (err) {
+    setState(_prevState => ({
+      loading: false,
+      error: err,
+    }));
+  }
+};

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.bug-invariant-unexpected-terminal-in-optional.expect.md

@@ -0,0 +1,34 @@
+
+## Input
+
+```javascript
+const Foo = ({json}) => {
+  try {
+    const foo = JSON.parse(json)?.foo;
+    return <span>{foo}</span>;
+  } catch {
+    return null;
+  }
+};
+
+```
+
+
+## Error
+
+```
+Found 1 error:
+
+Invariant: Unexpected terminal in optional
+
+error.bug-invariant-unexpected-terminal-in-optional.ts:3:16
+  1 | const Foo = ({json}) => {
+  2 |   try {
+> 3 |     const foo = JSON.parse(json)?.foo;
+    |                 ^^^^ Unexpected terminal in optional
+  4 |     return <span>{foo}</span>;
+  5 |   } catch {
+  6 |     return null;
+```
+          
+      

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.bug-invariant-unexpected-terminal-in-optional.js

@@ -0,0 +1,8 @@
+const Foo = ({json}) => {
+  try {
+    const foo = JSON.parse(json)?.foo;
+    return <span>{foo}</span>;
+  } catch {
+    return null;
+  }
+};

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.bug-invariant-unnamed-temporary.expect.md

@@ -0,0 +1,30 @@
+
+## Input
+
+```javascript
+import Bar from './Bar';
+
+export function Foo() {
+  return (
+    <Bar
+      renderer={(...props) => {
+        return <span {...props}>{displayValue}</span>;
+      }}
+    />
+  );
+}
+
+```
+
+
+## Error
+
+```
+Found 1 error:
+
+Invariant: Expected temporaries to be promoted to named identifiers in an earlier pass
+
+identifier 15 is unnamed.
+```
+          
+      

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.bug-invariant-unnamed-temporary.js

@@ -0,0 +1,11 @@
+import Bar from './Bar';
+
+export function Foo() {
+  return (
+    <Bar
+      renderer={(...props) => {
+        return <span {...props}>{displayValue}</span>;
+      }}
+    />
+  );
+}

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.bug-old-inference-false-positive-ref-validation-in-use-effect.expect.md

@@ -37,9 +37,10 @@ function Component() {
 
 ```
 Found 1 error:
+
 Error: Cannot modify local variables after render completes
 
-This argument is a function which may reassign or mutate local variables after render, which can cause inconsistent behavior on subsequent renders. Consider using state instead
+This argument is a function which may reassign or mutate a local variable after render, which can cause inconsistent behavior on subsequent renders. Consider using state instead.
 
 error.bug-old-inference-false-positive-ref-validation-in-use-effect.ts:20:12
   18 |   );
@@ -53,7 +54,7 @@ error.bug-old-inference-false-positive-ref-validation-in-use-effect.ts:20:12
 > 23 |     }
      | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 > 24 |   }, [update]);
-     | ^^^^ This function may (indirectly) reassign or modify local variables after render
+     | ^^^^ This function may (indirectly) reassign or modify a local variable after render
   25 |
   26 |   return 'ok';
   27 | }

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.call-args-destructuring-asignment-complex.expect.md

@@ -15,6 +15,7 @@ function Component(props) {
 
 ```
 Found 1 error:
+
 Invariant: Const declaration cannot be referenced as an expression
 
 error.call-args-destructuring-asignment-complex.ts:3:9
@@ -25,8 +26,6 @@ error.call-args-destructuring-asignment-complex.ts:3:9
   4 |   return x;
   5 | }
   6 |
-
-
 ```
           
       

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.capitalized-function-call-aliased.expect.md

@@ -15,6 +15,7 @@ function Foo() {
 
 ```
 Found 1 error:
+
 Error: Capitalized functions are reserved for components, which must be invoked with JSX. If this is a component, render it with JSX. Otherwise, ensure that it has no hook calls and rename it to begin with a lowercase letter. Alternatively, if you know for a fact that this function is not a component, you can allowlist it via the compiler config
 
 Bar may be a component..
@@ -26,8 +27,6 @@ error.capitalized-function-call-aliased.ts:4:2
     |   ^^^ Capitalized functions are reserved for components, which must be invoked with JSX. If this is a component, render it with JSX. Otherwise, ensure that it has no hook calls and rename it to begin with a lowercase letter. Alternatively, if you know for a fact that this function is not a component, you can allowlist it via the compiler config
   5 | }
   6 |
-
-
 ```
           
       

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.capitalized-function-call.expect.md

@@ -16,6 +16,7 @@ function Component() {
 
 ```
 Found 1 error:
+
 Error: Capitalized functions are reserved for components, which must be invoked with JSX. If this is a component, render it with JSX. Otherwise, ensure that it has no hook calls and rename it to begin with a lowercase letter. Alternatively, if you know for a fact that this function is not a component, you can allowlist it via the compiler config
 
 SomeFunc may be a component..
@@ -28,8 +29,6 @@ error.capitalized-function-call.ts:3:12
   4 |
   5 |   return x;
   6 | }
-
-
 ```