Skip plugin for non-Flow JS code (#1556)

Summary:

This is a mitigation for #1549.

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

**Context: React Native**

Originally changed in facebook/react-native#46696, as our internal Flow support had diverged from `babel/plugin-syntax-flow` (facebook/react-native#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 😅.

Differential Revision: D65272155

tools/hermes-parser/js/babel-plugin-syntax-hermes-parser/__tests__/babel-plugin-syntax-hermes-parser-test.js

@@ -16,22 +16,31 @@ 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';
+const NON_FLOW_MODULE_PREAMBLE = '"use strict";\n\n';
 
 describe('babel-plugin-syntax-hermes-parser', () => {
-  test('test basic parsing', () => {
-    const parseSpy = jest.spyOn(HermesParser, 'parse');
-    const code = MODULE_PREAMBLE + 'const a = 1;';
+  const parseSpy = jest.spyOn(HermesParser, 'parse');
+
+  afterEach(() => {
+    parseSpy.mockReset();
+  });
+
+  test('should parse Flow files', () => {
+    const code = MODULE_PREAMBLE + 'const a: number = 1;';
     const output = transformSync(code, {
       plugins: [hermesParserPlugin],
     });
-    expect(output.code).toBe(code);
+    expect(output.code).toMatchInlineSnapshot(`
+      ""use strict";
+
+      const a = 1;"
+    `);
     expect(parseSpy).toBeCalledTimes(1);
   });
 
-  test('test skip TS', () => {
-    const parseSpy = jest.spyOn(HermesParser, 'parse');
-    const code = MODULE_PREAMBLE + 'const a: string = 1;';
+  test('should skip TypeScript files', () => {
+    const code = NON_FLOW_MODULE_PREAMBLE + 'const a: number = 1;';
     const output = transformSync(code, {
       plugins: [hermesParserPlugin],
       filename: 'foo.ts',
@@ -44,8 +53,32 @@ describe('babel-plugin-syntax-hermes-parser', () => {
     expect(parseSpy).toBeCalledTimes(0);
   });
 
-  test('test component syntax parsing', () => {
-    const parseSpy = jest.spyOn(HermesParser, 'parse');
+  test('should skip JS files without @flow annotation', () => {
+    const code = NON_FLOW_MODULE_PREAMBLE + 'class Foo {}';
+    const output = transformSync(code, {
+      plugins: [hermesParserPlugin],
+    });
+    expect(output.code).toBe(code);
+    expect(parseSpy).toBeCalledTimes(0);
+  });
+
+  test("should parse all JS files when parserOpts.flow = 'all'", () => {
+    const code = NON_FLOW_MODULE_PREAMBLE + 'const a: number = 1;';
+    const output = transformSync(code, {
+      plugins: [hermesParserPlugin],
+      parserOpts: {
+        flow: 'all',
+      },
+    });
+    expect(output.code).toMatchInlineSnapshot(`
+      ""use strict";
+
+      const a = 1;"
+    `);
+    expect(parseSpy).toBeCalledTimes(1);
+  });
+
+  test('should parse component syntax when enabled', () => {
     const code = MODULE_PREAMBLE + 'component Foo() {}';
     const output = transformSync(code, {
       plugins: [hermesParserPlugin],

tools/hermes-parser/js/babel-plugin-syntax-hermes-parser/src/index.js

@@ -42,13 +42,19 @@ export default function BabelPluginSyntaxHermesParser(
       ) {
         return;
       }
+
       const opts: ParserOptions = {};
       for (const [key, value] of Object.entries(curParserOpts)) {
         if (HermesParser.ParserOptionsKeys.has(key)) {
           // $FlowExpectedError[incompatible-type]
           opts[key] = value;
         }
       }
+
+      if (opts.flow !== 'all' && !/@flow/.test(code)) {
+        return;
+      }
+
       return HermesParser.parse(code, {...opts, babel: true});
     },