[compiler] Script to enable a feature by default and update tests

Similar to ValidateHookUsage, we implement this check in the compiler
for safety but (for now) continue to rely on the existing rule for
actually reporting errors to users.

---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/facebook/react/pull/35192).
* #35201
* #35202
* __->__ #35192

compiler/.claude/settings.local.json

@@ -0,0 +1,9 @@
+{
+  "permissions": {
+    "allow": [
+      "Bash(node scripts/enable-feature-flag.js:*)"
+    ],
+    "deny": [],
+    "ask": []
+  }
+}

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

@@ -1067,7 +1067,15 @@ function getRuleForCategoryImpl(category: ErrorCategory): LintRule {
         name: 'memo-dependencies',
         description:
           'Validates that useMemo() and useCallback() specify comprehensive dependencies without extraneous values. See [`useMemo()` docs](https://react.dev/reference/react/useMemo) for more information.',
-        preset: LintRulePreset.RecommendedLatest,
+        /**
+         * TODO: the "MemoDependencies" rule largely reimplements the "exhaustive-deps" non-compiler rule,
+         * allowing the compiler to ensure it does not regress change behavior due to different dependencies.
+         * We previously relied on the source having ESLint suppressions for any exhaustive-deps violations,
+         * but it's more reliable to verify it within the compiler.
+         *
+         * Long-term we should de-duplicate these implementations.
+         */
+        preset: LintRulePreset.Off,
       };
     }
     case ErrorCategory.IncompatibleLibrary: {

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

@@ -303,9 +303,11 @@ function runWithEnvironment(
   inferReactivePlaces(hir);
   log({kind: 'hir', name: 'InferReactivePlaces', value: hir});
 
-  if (env.config.validateExhaustiveMemoizationDependencies) {
-    // NOTE: this relies on reactivity inference running first
-    validateExhaustiveDependencies(hir).unwrap();
+  if (env.enableValidations) {
+    if (env.config.validateExhaustiveMemoizationDependencies) {
+      // NOTE: this relies on reactivity inference running first
+      validateExhaustiveDependencies(hir).unwrap();
+    }
   }
 
   rewriteInstructionKindsBasedOnReassignment(hir);

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

@@ -284,7 +284,7 @@ export function validateExhaustiveDependencies(
       if (missing.length !== 0) {
         const diagnostic = CompilerDiagnostic.create({
           category: ErrorCategory.MemoDependencies,
-          reason: 'Found non-exhaustive dependencies',
+          reason: 'Found missing memoization dependencies',
           description:
             'Missing dependencies can cause a value not to update when those inputs change, ' +
             'resulting in stale UI',
@@ -309,7 +309,7 @@ export function validateExhaustiveDependencies(
           reason: 'Found unnecessary memoization dependencies',
           description:
             'Unnecessary dependencies can cause a value to update more often than necessary, ' +
-            'which can cause effects to run more than expected',
+            'causing performance regressions and effects to fire more often than expected',
         });
         diagnostic.withDetails({
           kind: 'error',

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.invalid-exhaustive-deps.expect.md

@@ -53,7 +53,7 @@ function Component({x, y, z}) {
 ```
 Found 4 errors:
 
-Error: Found non-exhaustive dependencies
+Error: Found missing memoization dependencies
 
 Missing dependencies can cause a value not to update when those inputs change, resulting in stale UI.
 
@@ -66,7 +66,7 @@ error.invalid-exhaustive-deps.ts:7:11
    9 |   }, [x?.y.z?.a.b]);
   10 |   const b = useMemo(() => {
 
-Error: Found non-exhaustive dependencies
+Error: Found missing memoization dependencies
 
 Missing dependencies can cause a value not to update when those inputs change, resulting in stale UI.
 
@@ -81,7 +81,7 @@ error.invalid-exhaustive-deps.ts:15:11
 
 Error: Found unnecessary memoization dependencies
 
-Unnecessary dependencies can cause a value to update more often than necessary, which can cause effects to run more than expected.
+Unnecessary dependencies can cause a value to update more often than necessary, causing performance regressions and effects to fire more often than expected.
 
 error.invalid-exhaustive-deps.ts:31:5
   29 |     return [];
@@ -92,7 +92,7 @@ error.invalid-exhaustive-deps.ts:31:5
   33 |   const ref2 = useRef(null);
   34 |   const ref = z ? ref1 : ref2;
 
-Error: Found non-exhaustive dependencies
+Error: Found missing memoization dependencies
 
 Missing dependencies can cause a value not to update when those inputs change, resulting in stale UI.
 

compiler/scripts/enable-feature-flag.js

@@ -0,0 +1,347 @@
+#!/usr/bin/env node
+/**
+ * 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.
+ */
+
+'use strict';
+
+const fs = require('fs');
+const path = require('path');
+const {execSync} = require('child_process');
+const yargs = require('yargs/yargs');
+const {hideBin} = require('yargs/helpers');
+
+// Constants
+const COMPILER_ROOT = path.resolve(__dirname, '..');
+const ENVIRONMENT_TS_PATH = path.join(
+  COMPILER_ROOT,
+  'packages/babel-plugin-react-compiler/src/HIR/Environment.ts'
+);
+const FIXTURES_PATH = path.join(
+  COMPILER_ROOT,
+  'packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler'
+);
+const FIXTURE_EXTENSIONS = ['.js', '.jsx', '.ts', '.tsx'];
+
+/**
+ * Parse command line arguments
+ */
+function parseArgs() {
+  const argv = yargs(hideBin(process.argv))
+    .usage('Usage: $0 <flag-name>')
+    .command('$0 <flag-name>', 'Enable a feature flag by default', yargs => {
+      yargs.positional('flag-name', {
+        describe: 'Name of the feature flag to enable',
+        type: 'string',
+      });
+    })
+    .example(
+      '$0 validateExhaustiveMemoizationDependencies',
+      'Enable the validateExhaustiveMemoizationDependencies flag'
+    )
+    .help('h')
+    .alias('h', 'help')
+    .strict()
+    .parseSync();
+
+  return argv['flag-name'];
+}
+
+/**
+ * Enable a feature flag in Environment.ts by changing default(false) to default(true)
+ */
+function enableFlagInEnvironment(flagName) {
+  console.log(`\nEnabling flag "${flagName}" in Environment.ts...`);
+
+  const content = fs.readFileSync(ENVIRONMENT_TS_PATH, 'utf8');
+
+  // Check if the flag exists with default(false)
+  const flagPatternFalse = new RegExp(
+    `(${escapeRegex(flagName)}:\\s*z\\.boolean\\(\\)\\.default\\()false(\\))`,
+    'g'
+  );
+
+  if (!flagPatternFalse.test(content)) {
+    // Check if flag exists at all
+    const flagExistsPattern = new RegExp(
+      `${escapeRegex(flagName)}:\\s*z\\.boolean\\(\\)`,
+      'g'
+    );
+    if (flagExistsPattern.test(content)) {
+      // Check if it's already true
+      const flagPatternTrue = new RegExp(
+        `${escapeRegex(flagName)}:\\s*z\\.boolean\\(\\)\\.default\\(true\\)`,
+        'g'
+      );
+      if (flagPatternTrue.test(content)) {
+        console.error(`Error: Flag "${flagName}" already has default(true)`);
+        process.exit(1);
+      }
+      console.error(
+        `Error: Flag "${flagName}" exists but doesn't have default(false)`
+      );
+      process.exit(1);
+    }
+    console.error(`Error: Flag "${flagName}" not found in Environment.ts`);
+    process.exit(1);
+  }
+
+  // Perform the replacement
+  const newContent = content.replace(flagPatternFalse, '$1true$2');
+
+  // Verify the replacement worked
+  if (content === newContent) {
+    console.error(`Error: Failed to replace flag "${flagName}"`);
+    process.exit(1);
+  }
+
+  fs.writeFileSync(ENVIRONMENT_TS_PATH, newContent, 'utf8');
+  console.log(`Successfully enabled "${flagName}" in Environment.ts`);
+}
+
+/**
+ * Helper to escape regex special characters
+ */
+function escapeRegex(string) {
+  return string.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
+}
+
+/**
+ * Run yarn snap and capture output
+ */
+function runTests() {
+  console.log('\nRunning test suite (yarn snap)...');
+
+  try {
+    const output = execSync('yarn snap', {
+      cwd: COMPILER_ROOT,
+      encoding: 'utf8',
+      stdio: 'pipe',
+      maxBuffer: 10 * 1024 * 1024, // 10MB buffer
+    });
+    return {success: true, output};
+  } catch (error) {
+    // yarn snap exits with code 1 when tests fail, which throws an error
+    return {success: false, output: error.stdout || error.message};
+  }
+}
+
+/**
+ * Parse failing test names from test output
+ */
+function parseFailingTests(output) {
+  const failingTests = [];
+
+  // Look for lines that contain "FAIL:" followed by the test name
+  // Format: "FAIL: test-name" or with ANSI codes
+  const lines = output.split('\n');
+  for (const line of lines) {
+    // Remove ANSI codes for easier parsing
+    const cleanLine = line.replace(/\x1b\[[0-9;]*m/g, '');
+
+    // Match "FAIL: test-name"
+    const match = cleanLine.match(/^FAIL:\s*(.+)$/);
+    if (match) {
+      failingTests.push(match[1].trim());
+    }
+  }
+
+  return failingTests;
+}
+
+/**
+ * Find the fixture file for a given test name
+ */
+function findFixtureFile(testName) {
+  const basePath = path.join(FIXTURES_PATH, testName);
+
+  for (const ext of FIXTURE_EXTENSIONS) {
+    const filePath = basePath + ext;
+    if (fs.existsSync(filePath)) {
+      return filePath;
+    }
+  }
+
+  return null;
+}
+
+/**
+ * Add pragma to disable the feature flag in a fixture file
+ */
+function addPragmaToFixture(filePath, flagName) {
+  const content = fs.readFileSync(filePath, 'utf8');
+  const lines = content.split('\n');
+
+  if (lines.length === 0) {
+    console.warn(`Warning: Empty file ${filePath}`);
+    return false;
+  }
+
+  const firstLine = lines[0];
+  const pragma = `@${flagName}:false`;
+
+  // Check if pragma already exists
+  if (firstLine.includes(pragma)) {
+    return false; // Already has the pragma
+  }
+
+  // Check if first line is a single-line comment
+  if (firstLine.trim().startsWith('//')) {
+    // Append pragma to existing comment
+    lines[0] = firstLine + ' ' + pragma;
+  } else if (firstLine.trim().startsWith('/*')) {
+    // Multi-line comment - insert new line before it
+    lines.unshift('// ' + pragma);
+  } else {
+    // No comment - insert new comment as first line
+    lines.unshift('// ' + pragma);
+  }
+
+  fs.writeFileSync(filePath, lines.join('\n'), 'utf8');
+  return true;
+}
+
+/**
+ * Update snapshot files
+ */
+function updateSnapshots() {
+  console.log('\nUpdating snapshots (yarn snap -u)...');
+
+  try {
+    execSync('yarn snap -u', {
+      cwd: COMPILER_ROOT,
+      encoding: 'utf8',
+      stdio: 'pipe',
+      maxBuffer: 10 * 1024 * 1024,
+    });
+    console.log('Snapshots updated successfully');
+    return true;
+  } catch (error) {
+    console.error('Error updating snapshots:', error.message);
+    return false;
+  }
+}
+
+/**
+ * Verify all tests pass
+ */
+function verifyAllTestsPass() {
+  console.log('\nRunning final verification (yarn snap)...');
+
+  const {success, output} = runTests();
+
+  // Parse summary line: "N Tests, N Passed, N Failed"
+  const summaryMatch = output.match(
+    /(\d+)\s+Tests,\s+(\d+)\s+Passed,\s+(\d+)\s+Failed/
+  );
+
+  if (summaryMatch) {
+    const [, total, passed, failed] = summaryMatch;
+    console.log(
+      `\nTest Results: ${total} Tests, ${passed} Passed, ${failed} Failed`
+    );
+
+    if (failed === '0') {
+      console.log('All tests passed!');
+      return true;
+    } else {
+      console.error(`${failed} tests still failing`);
+      const failingTests = parseFailingTests(output);
+      if (failingTests.length > 0) {
+        console.error('\nFailing tests:');
+        failingTests.forEach(test => console.error(`  - ${test}`));
+      }
+      return false;
+    }
+  }
+
+  return success;
+}
+
+/**
+ * Main function
+ */
+async function main() {
+  const flagName = parseArgs();
+
+  console.log(`\nEnabling flag: '${flagName}'`);
+
+  try {
+    // Step 1: Enable flag in Environment.ts
+    enableFlagInEnvironment(flagName);
+
+    // Step 2: Run tests to find failures
+    const {output} = runTests();
+    const failingTests = parseFailingTests(output);
+
+    console.log(`\nFound ${failingTests.length} failing tests`);
+
+    if (failingTests.length === 0) {
+      console.log('No failing tests! Feature flag enabled successfully.');
+      process.exit(0);
+    }
+
+    // Step 3: Add pragma to each failing fixture
+    console.log(`\nAdding '@${flagName}:false' pragma to failing fixtures...`);
+
+    const notFound = [];
+    let notFoundCount = 0;
+
+    for (const testName of failingTests) {
+      const fixturePath = findFixtureFile(testName);
+
+      if (!fixturePath) {
+        console.warn(`Could not find fixture file for: ${testName}`);
+        notFound.push(fixturePath);
+        continue;
+      }
+
+      const updated = addPragmaToFixture(fixturePath, flagName);
+      if (updated) {
+        updatedCount++;
+        console.log(`  Updated: ${testName}`);
+      }
+    }
+
+    console.log(
+      `\nSummary: Updated ${updatedCount} fixtures, ${notFoundCount} not found`
+    );
+
+    if (notFoundCount.length !== 0) {
+      console.error(
+        '\nFailed to update snapshots, could not find:\n' + notFound.join('\n')
+      );
+      process.exit(1);
+    }
+
+    // Step 4: Update snapshots
+    if (!updateSnapshots()) {
+      console.error('\nFailed to update snapshots');
+      process.exit(1);
+    }
+
+    // Step 5: Verify all tests pass
+    if (!verifyAllTestsPass()) {
+      console.error('\nVerification failed: Some tests are still failing');
+      process.exit(1);
+    }
+
+    console.log('\nSuccess! Feature flag enabled and all tests passing.');
+    console.log(`\nSummary:`);
+    console.log(`  - Enabled "${flagName}" in Environment.ts`);
+    console.log(`  - Updated ${updatedCount} fixture files with pragma`);
+    console.log(`  - All tests passing`);
+
+    process.exit(0);
+  } catch (error) {
+    console.error('\nFatal error:', error.message);
+    console.error(error.stack);
+    process.exit(1);
+  }
+}
+
+// Run the script
+main();