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

Summary:
Pull Request resolved: #1556

This is a mitigation for #1549.

Updates `babel-plugin-syntax-hermes-parser` to include a new `parseLangTypes` option, which we will configure in React Native's Babel preset 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 😅.

Reviewed By: pieterv

Differential Revision: D65272155

fbshipit-source-id: b3be10410962ffffd4e0d483d10fe88d865c0783

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

@@ -16,22 +16,44 @@ 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.mockClear();
+  });
+
+  test('should parse Flow files', () => {
+    const code = MODULE_PREAMBLE + 'const a: number = 1;';
+    const output = transformSync(code, {
+      plugins: [hermesParserPlugin],
+    });
+    expect(output.code).toMatchInlineSnapshot(`
+      ""use strict";
+
+      const a = 1;"
+    `);
+    expect(parseSpy).toBeCalledTimes(1);
+  });
+
+  test('should parse files without @flow annotation', () => {
+    const code = NON_FLOW_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 +66,7 @@ describe('babel-plugin-syntax-hermes-parser', () => {
     expect(parseSpy).toBeCalledTimes(0);
   });
 
-  test('test component syntax parsing', () => {
-    const parseSpy = jest.spyOn(HermesParser, 'parse');
+  test('should parse component syntax when enabled', () => {
     const code = MODULE_PREAMBLE + 'component Foo() {}';
     const output = transformSync(code, {
       plugins: [hermesParserPlugin],
@@ -60,4 +81,28 @@ describe('babel-plugin-syntax-hermes-parser', () => {
     `);
     expect(parseSpy).toBeCalledTimes(1);
   });
+
+  describe("with parseLangTypes = 'flow'", () => {
+    test('should parse Flow files', () => {
+      const code = MODULE_PREAMBLE + 'const a: number = 1;';
+      const output = transformSync(code, {
+        plugins: [hermesParserPlugin],
+      });
+      expect(output.code).toMatchInlineSnapshot(`
+      ""use strict";
+
+      const a = 1;"
+    `);
+      expect(parseSpy).toBeCalledTimes(1);
+    });
+
+    test('should skip files without @flow annotation ', () => {
+      const code = NON_FLOW_MODULE_PREAMBLE + 'class Foo {}';
+      const output = transformSync(code, {
+        plugins: [[hermesParserPlugin, {parseLangTypes: 'flow'}]],
+      });
+      expect(output.code).toBe(code);
+      expect(parseSpy).toBeCalledTimes(0);
+    });
+  });
 });

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

@@ -14,12 +14,28 @@ import type {ParserOptions} from 'hermes-parser';
 
 import * as HermesParser from 'hermes-parser';
 
+type Options = {
+  /**
+   * When set to 'flow', will check files for a `@flow` annotation to apply
+   * this plugin, otherwise falling back to Babel's parser.
+   *
+   * This is independent of `parserOpts.flow`, which may customise 'detect'
+   * behaviour within hermes-parser (downstream from this plugin).
+   *
+   * Defaults to 'all'.
+   */
+  parseLangTypes?: 'flow' | 'all',
+};
+
 export default function BabelPluginSyntaxHermesParser(
   // $FlowExpectedError[unclear-type] We don't have types for this.
   api: any,
+  options: Options,
 ): $ReadOnly<{...}> {
   api.assertVersion('^7.0.0 || ^8.0.0-alpha.6');
 
+  const {parseLangTypes = 'all'} = options;
+
   let curParserOpts: ParserOptions = {};
   let curFilename: ?string = null;
 
@@ -42,14 +58,20 @@ export default function BabelPluginSyntaxHermesParser(
       ) {
         return;
       }
-      const opts: ParserOptions = {};
+
+      const parserOpts: ParserOptions = {};
       for (const [key, value] of Object.entries(curParserOpts)) {
         if (HermesParser.ParserOptionsKeys.has(key)) {
           // $FlowExpectedError[incompatible-type]
-          opts[key] = value;
+          parserOpts[key] = value;
         }
       }
-      return HermesParser.parse(code, {...opts, babel: true});
+
+      if (parseLangTypes === 'flow' && !/@flow/.test(code)) {
+        return;
+      }
+
+      return HermesParser.parse(code, {...parserOpts, babel: true});
     },
 
     pre() {