fix(@angular/build): key the Vitest fsModuleCache on the served build output

packages/angular/build/src/builders/unit-test/runners/vitest/plugins.ts

@@ -281,7 +281,6 @@ export async function createVitestConfigPlugin(
         test: {
           name: projectName,
           include,
-          fsModuleCache: false,
           // CLI provider browser options override, if present
           ...(browser ? { browser } : {}),
           // Only override if the user explicitly set it via CLI
@@ -301,7 +300,6 @@ export async function createVitestConfigPlugin(
 
       return {
         test: {
-          fsModuleCache: false,
           coverage: await generateCoverageOption(
             options.coverage,
             testConfig?.coverage,
@@ -335,12 +333,42 @@ export function createVitestPlugins(pluginOptions: PluginOptions): Vite.Plugin[]
   );
   let vitestConfig: ResolvedConfig;
 
+  /** The build output path served for a module, whether it is a test entry point or a built artifact. */
+  const getOutputPath = (id: string): string => {
+    const entryPoint = testFileToEntryPoint.get(id);
+
+    return entryPoint ? entryPoint + '.js' : toPosixPath(path.relative(workspaceRoot, id));
+  };
+
   return [
     {
       name: 'angular:test-in-memory-provider',
       enforce: 'pre',
       configureVitest(context: VitestPluginContext) {
         vitestConfig = context.vitest.config;
+
+        // Vitest keys its file system module cache on the file found on disk. A test entry point is
+        // served from its bundle instead, which also holds the application code it imports, so the
+        // key must follow the served build output, or the cache would return a stale bundle.
+        // Vitest 4.1 only has the experimental name, and 4.0 has no file system module cache at all.
+        const pluginContext: Partial<VitestPluginContext> = context;
+        const defineCacheKeyGenerator =
+          pluginContext.defineCacheKeyGenerator ??
+          pluginContext.experimental_defineCacheKeyGenerator;
+        defineCacheKeyGenerator?.(({ id }) => {
+          const outputPath = getOutputPath(id);
+          const outputFile = buildResultFiles.get(outputPath);
+          if (!outputFile) {
+            return undefined;
+          }
+          if (outputFile.origin !== 'memory') {
+            return false;
+          }
+
+          const sourceMapFile = buildResultFiles.get(outputPath + '.map');
+
+          return `${outputFile.hash}:${sourceMapFile?.origin === 'memory' ? sourceMapFile.hash : ''}`;
+        });
       },
       resolveId: (id: string, importer: string | undefined) => {
         // Fast path for test entry points.
@@ -399,12 +427,9 @@ export function createVitestPlugins(pluginOptions: PluginOptions): Vite.Plugin[]
       async load(id: string) {
         assert(buildResultFiles.size > 0, 'buildResult must be available for in-memory loading.');
 
-        // Attempt to load as a source test file.
-        const entryPoint = testFileToEntryPoint.get(id);
-        let outputPath;
-        if (entryPoint) {
-          outputPath = entryPoint + '.js';
-
+        // Attempt to load as a source test file, or else as a built artifact.
+        const outputPath = getOutputPath(id);
+        if (testFileToEntryPoint.has(id)) {
           // Setup files must not be wrapped in a virtual import stub because Vitest only invalidates
           // the setup file itself between test files; wrapping it would cause the underlying bundle
           // to be cached, preventing per-test hooks from running on subsequent test files.
@@ -417,10 +442,6 @@ export function createVitestPlugins(pluginOptions: PluginOptions): Vite.Plugin[]
               code: `import "./${outputPath}";`,
             };
           }
-        } else {
-          // Attempt to load as a built artifact.
-          const relativePath = path.relative(workspaceRoot, id);
-          outputPath = toPosixPath(relativePath);
         }
 
         const outputFile = buildResultFiles.get(outputPath);

packages/angular/build/src/builders/unit-test/tests/behavior/runner-config-vitest_spec.ts

@@ -6,6 +6,7 @@
  * found in the LICENSE file at https://angular.dev/license
  */
 
+import { readdirSync } from 'node:fs';
 import { execute } from '../../index';
 import {
   BASE_OPTIONS,
@@ -650,7 +651,47 @@ describeBuilder(execute, UNIT_TEST_BUILDER_INFO, (harness) => {
 
       const secondRun = await harness.executeOnce();
       expect(secondRun.result?.success).toBeTrue();
-      harness.expectDirectory('.vitest-cache').toNotExist();
+      harness.expectDirectory('.vitest-cache').toExist();
+    });
+
+    it('should reuse the cached modules across runs when nothing changes', async () => {
+      harness.useTarget('test', {
+        ...BASE_OPTIONS,
+        runnerConfig: 'vitest.config.mts',
+      });
+
+      await harness.writeFiles({
+        'vitest.config.mts': `
+          import { defineConfig } from 'vitest/config';
+          export default defineConfig({
+            test: {
+              fsModuleCache: true,
+              fsModuleCachePath: './.vitest-cache',
+            },
+          });
+        `,
+        'src/app/value.ts': `export const VALUE = 'initial';`,
+        'src/app/app.component.spec.ts': `
+          import { test, expect } from 'vitest';
+          import { VALUE } from './value';
+
+          test('should use the value from imported module', () => {
+            expect(VALUE).toBe('initial');
+          });
+        `,
+      });
+      const cacheEntries = () =>
+        readdirSync(harness.resolvePath('.vitest-cache'), { recursive: true }).sort();
+
+      const firstRun = await harness.executeOnce();
+      expect(firstRun.result?.success).toBeTrue();
+      const entriesAfterFirstRun = cacheEntries();
+      expect(entriesAfterFirstRun.length).toBeGreaterThan(0);
+
+      // An unchanged build produces the same bundles, so no new cache entry is written.
+      const secondRun = await harness.executeOnce();
+      expect(secondRun.result?.success).toBeTrue();
+      expect(cacheEntries()).toEqual(entriesAfterFirstRun);
     });
   });
 });