Report an actionable error when '@react-native/codegen' is not built

packages/babel-plugin-codegen/__tests__/codegen-load-test.js

@@ -0,0 +1,76 @@
+/**
+ * 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.
+ *
+ * @flow strict-local
+ * @format
+ */
+
+class ModuleNotFoundError extends Error {
+  code: string = 'MODULE_NOT_FOUND';
+}
+
+function missingLibError(): Error {
+  return new ModuleNotFoundError(
+    "Cannot find module '@react-native/codegen/lib/parsers/flow/parser'",
+  );
+}
+
+// Failure thrown by the virtual `lib/` mock; tests override it per case.
+let mockLibLoadError: Error = missingLibError();
+
+// `lib/` is gitignored build output and may not exist on disk, so it is
+// mocked virtually for every test. This stays a top-level `jest.mock`
+// because only it accepts the options argument under this repo's Flow libdef.
+jest.mock(
+  '@react-native/codegen/lib/parsers/flow/parser',
+  () => {
+    throw mockLibLoadError;
+  },
+  {virtual: true},
+);
+
+describe('codegen loading', () => {
+  beforeEach(() => {
+    mockLibLoadError = missingLibError();
+  });
+
+  it('throws an actionable error when @react-native/codegen is not built', () => {
+    jest.isolateModules(() => {
+      jest.doMock('@react-native/codegen/src/parsers/flow/parser', () => {
+        throw new SyntaxError("Unexpected token '{'");
+      });
+      expect(() => {
+        // $FlowFixMe[untyped-import] `../index.js` is `@noflow`.
+        require('../index.js');
+      }).toThrow(/never built.*yarn workspace @react-native\/codegen build/s);
+    });
+  });
+
+  it('throws an actionable error when @react-native/codegen is not installed', () => {
+    jest.isolateModules(() => {
+      jest.doMock('@react-native/codegen/src/parsers/flow/parser', () => {
+        throw new ModuleNotFoundError('Cannot find module');
+      });
+      expect(() => {
+        // $FlowFixMe[untyped-import] `../index.js` is `@noflow`.
+        require('../index.js');
+      }).toThrow(/could not be found/);
+    });
+  });
+
+  it('throws an actionable error when the built lib is broken', () => {
+    jest.isolateModules(() => {
+      mockLibLoadError = new SyntaxError("Unexpected token '{'");
+      jest.doMock('@react-native/codegen/src/parsers/flow/parser', () => {
+        throw new ModuleNotFoundError('Cannot find module');
+      });
+      expect(() => {
+        // $FlowFixMe[untyped-import] `../index.js` is `@noflow`.
+        require('../index.js');
+      }).toThrow(/failed to load.*Rebuild/s);
+    });
+  });
+});

packages/babel-plugin-codegen/index.js

@@ -15,22 +15,49 @@ let FlowParser, TypeScriptParser, RNCodegen;
 const {cheap: traverseCheap} = require('@babel/traverse').default;
 const {basename} = require('node:path');
 
+function getCodegenLoadError(srcError, libError) {
+  if (libError != null && libError.code !== 'MODULE_NOT_FOUND') {
+    return (
+      'Unable to load @react-native/codegen: its built `lib/` output failed ' +
+      'to load, so NativeComponent specs cannot be transformed. Rebuild it, ' +
+      'e.g. `yarn workspace @react-native/codegen build`, then retry.'
+    );
+  }
+  if (srcError != null && srcError.code !== 'MODULE_NOT_FOUND') {
+    return (
+      'Unable to load @react-native/codegen: the package is checked out ' +
+      'from source but was never built (its `lib/` output is missing), so ' +
+      'NativeComponent specs cannot be transformed. Build it first, e.g. ' +
+      '`yarn workspace @react-native/codegen build`, then retry.'
+    );
+  }
+  return (
+    'Unable to load @react-native/codegen: the package could not be found, ' +
+    'so NativeComponent specs cannot be transformed. Install dependencies ' +
+    'first, then retry.'
+  );
+}
+
 try {
   FlowParser =
     require('@react-native/codegen/src/parsers/flow/parser').FlowParser;
   TypeScriptParser =
     require('@react-native/codegen/src/parsers/typescript/parser').TypeScriptParser;
   RNCodegen = require('@react-native/codegen/src/generators/RNCodegen');
-} catch (e) {
-  // Fallback to lib when source doesn't exit (e.g. when installed as a dev dependency)
-  FlowParser =
-    // $FlowFixMe[cannot-resolve-module]
-    require('@react-native/codegen/lib/parsers/flow/parser').FlowParser;
-  TypeScriptParser =
+} catch (srcError) {
+  // Fallback to lib when source isn't usable (e.g. when installed as a dev dependency)
+  try {
+    FlowParser =
+      // $FlowFixMe[cannot-resolve-module]
+      require('@react-native/codegen/lib/parsers/flow/parser').FlowParser;
+    TypeScriptParser =
+      // $FlowFixMe[cannot-resolve-module]
+      require('@react-native/codegen/lib/parsers/typescript/parser').TypeScriptParser;
     // $FlowFixMe[cannot-resolve-module]
-    require('@react-native/codegen/lib/parsers/typescript/parser').TypeScriptParser;
-  // $FlowFixMe[cannot-resolve-module]
-  RNCodegen = require('@react-native/codegen/lib/generators/RNCodegen');
+    RNCodegen = require('@react-native/codegen/lib/generators/RNCodegen');
+  } catch (libError) {
+    throw new Error(getCodegenLoadError(srcError, libError), {cause: libError});
+  }
 }
 
 const flowParser = new FlowParser();