From 9e3c6665e68c4641b0e5e7bb1f6e3ee3f214828b Mon Sep 17 00:00:00 2001
From: Alex Hunt <huntie@meta.com>
Date: Thu, 31 Oct 2024 07:34:01 -0700
Subject: [PATCH] Skip plugin for non-Flow JS code (#1556)
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Summary:

This is a mitigation for https://github.com/facebook/hermes/issues/1549.

Updates `babel-plugin-syntax-hermes-parser` to abort when the file contents do not include `flow`.

**Context: React Native**

Originally changed in https://github.com/facebook/react-native/pull/46696, as our internal Flow support had diverged from `babel/plugin-syntax-flow` (https://github.com/facebook/react-native/issues/46601).

We effectively have three flavours of JavaScript in support:
- Flow@latest for the `react-native` package, shipped as source — uses `hermes-parser`.
- TypeScript for product code (community template, Expo) — uses `babel/plugin-syntax-typescript`.
- Plain JavaScript/JSX in product code, *which may be extended with additional user Babel plugins and needs lenient parsing* — uses base `babel/parser` (**this change**).

I'd love to simplify this 😅.

Reviewed By: cipolleschi

Differential Revision: D65272155
---
 .../babel-plugin-syntax-hermes-parser-test.js          | 10 +++++++---
 .../js/babel-plugin-syntax-hermes-parser/src/index.js  |  5 +++--
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/tools/hermes-parser/js/babel-plugin-syntax-hermes-parser/__tests__/babel-plugin-syntax-hermes-parser-test.js b/tools/hermes-parser/js/babel-plugin-syntax-hermes-parser/__tests__/babel-plugin-syntax-hermes-parser-test.js
index a03108c2b20..7e2367b1d88 100644
--- a/tools/hermes-parser/js/babel-plugin-syntax-hermes-parser/__tests__/babel-plugin-syntax-hermes-parser-test.js
+++ b/tools/hermes-parser/js/babel-plugin-syntax-hermes-parser/__tests__/babel-plugin-syntax-hermes-parser-test.js
@@ -16,7 +16,7 @@ import {transformSync} from '@babel/core';
 import hermesParserPlugin from '../src';
 import * as HermesParser from 'hermes-parser';
 
-const MODULE_PREAMBLE = '"use strict";\n\n';
+const MODULE_PREAMBLE = '// @flow\n\n"use strict";\n\n';
 
 describe('babel-plugin-syntax-hermes-parser', () => {
   test('test basic parsing', () => {
@@ -37,7 +37,9 @@ describe('babel-plugin-syntax-hermes-parser', () => {
       filename: 'foo.ts',
     });
     expect(output.code).toMatchInlineSnapshot(`
-      ""use strict";
+      "// @flow
+
+      "use strict";
 
       const a = 1;"
     `);
@@ -54,7 +56,9 @@ describe('babel-plugin-syntax-hermes-parser', () => {
       },
     });
     expect(output.code).toMatchInlineSnapshot(`
-      ""use strict";
+      "// @flow
+
+      "use strict";
 
       function Foo() {}"
     `);
diff --git a/tools/hermes-parser/js/babel-plugin-syntax-hermes-parser/src/index.js b/tools/hermes-parser/js/babel-plugin-syntax-hermes-parser/src/index.js
index a04237fb031..ac4730cfd13 100644
--- a/tools/hermes-parser/js/babel-plugin-syntax-hermes-parser/src/index.js
+++ b/tools/hermes-parser/js/babel-plugin-syntax-hermes-parser/src/index.js
@@ -37,8 +37,9 @@ export default function BabelPluginSyntaxHermesParser(
     parserOverride(code: string) {
       const filename = curFilename;
       if (
-        filename != null &&
-        (filename.endsWith('.ts') || filename.endsWith('.tsx'))
+        !/@flow/.test(code) ||
+        (filename != null &&
+          (filename.endsWith('.ts') || filename.endsWith('.tsx')))
       ) {
         return;
       }