test: add e2e test to validate generateTrace behavior with TS 4.1.x

Also update README.md to better reflect the current state of this
feature.

Author: mkantor

README.md

@@ -347,12 +347,11 @@ yarn add --dev @types/webpack
 
 ## Profiling types resolution
 
-Starting from TypeScript 4.1.0, you can profile long type checks by
+When using TypeScript 4.1.x in `build` mode you can profile long type checks by
 setting "generateTrace" compiler option. This is an instruction from [microsoft/TypeScript#40063](https://github.com/microsoft/TypeScript/pull/40063):
 
-1. Set "generateTrace": "{folderName}" in your `tsconfig.json`
-2. Look in the resulting folder. If you used build mode, there will be a `legend.json` telling you what went where.
-   Otherwise, there will be `trace.json` file and `types.json` files.
+1. Set "generateTrace": "{folderName}" in your `tsconfig.json` (under `compilerOptions`)
+2. Look in the resulting folder. There will be a `legend.json` telling you what went where.
 3. Navigate to [edge://tracing](edge://tracing) or [chrome://tracing](chrome://tracing) and load `trace.json`
 4. Expand Process 1 with the little triangle in the left sidebar
 5. Click on different blocks to see their payloads in the bottom pane

test/e2e/type-script-tracing.spec.ts

@@ -0,0 +1,38 @@
+import path from 'path';
+
+import { extractWebpackErrors } from './driver/webpack-errors-extractor';
+
+describe('TypeScript Tracing', () => {
+  it.each([{ build: true, typescript: '~4.1.0' }])(
+    'can generate trace files for %p',
+    async ({ build, ...dependencies }) => {
+      await sandbox.load(path.join(__dirname, 'fixtures/typescript-basic'));
+      await sandbox.install('yarn', { ...dependencies });
+
+      // enable tracing
+      await sandbox.patch(
+        'tsconfig.json',
+        '"outDir": "./dist"',
+        '"outDir": "./dist",\n"generateTrace": "./traces"'
+      );
+
+      await sandbox.write(
+        'fork-ts-checker.config.js',
+        `module.exports = ${JSON.stringify({ typescript: { build } })};`
+      );
+
+      const webpackResult = await sandbox.exec('yarn webpack --mode=development');
+      const errors = extractWebpackErrors(webpackResult);
+      expect(errors).toEqual([]);
+
+      expect(await sandbox.exists('dist')).toEqual(true);
+
+      expect(await sandbox.list('./traces')).toEqual(
+        expect.arrayContaining([
+          expect.objectContaining({ name: expect.stringMatching(/types.*\.json/) }),
+          expect.objectContaining({ name: expect.stringMatching(/trace.*\.json/) }),
+        ])
+      );
+    }
+  );
+});