fix(@angular/build): introduce vitest-base.config for test configuration

When using the Vitest runner, a standard `vitest.config.js` file can cause conflicts with other tools or IDE extensions that might assume it represents a complete, standalone configuration. However, the builder uses this file only as a *base* configuration, which is then merged with its own internal setup.

To avoid this ambiguity, the builder now searches for a `vitest-base.config.(js|ts|...etc)` file when the `runnerConfig` option is set to `true`. This makes the intent clear that the file provides a base configuration specifically for the Angular CLI builder.

The search order is as follows:
1. Project Root
2. Workspace Root

If no `vitest-base.config.*` file is found, the builder proceeds with its default in-memory configuration. Vitest's default behavior of searching for `vitest.config.*` is explicitly disabled in this mode to ensure predictable and consistent test execution.

(cherry picked from commit 0fe6db9)

Author: clydin

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

@@ -0,0 +1,55 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.dev/license
+ */
+
+/**
+ * @fileoverview
+ * This file contains utility functions for finding the Vitest base configuration file.
+ */
+
+import { readdir } from 'node:fs/promises';
+import path from 'node:path';
+
+/**
+ * A list of potential Vitest configuration filenames.
+ * The order of the files is important as the first one found will be used.
+ */
+const POTENTIAL_CONFIGS = [
+  'vitest-base.config.ts',
+  'vitest-base.config.mts',
+  'vitest-base.config.cts',
+  'vitest-base.config.js',
+  'vitest-base.config.mjs',
+  'vitest-base.config.cjs',
+];
+
+/**
+ * Finds the Vitest configuration file in the given search directories.
+ *
+ * @param searchDirs An array of directories to search for the configuration file.
+ * @returns The path to the configuration file, or `false` if no file is found.
+ * Returning `false` is used to disable Vitest's default configuration file search.
+ */
+export async function findVitestBaseConfig(searchDirs: string[]): Promise<string | false> {
+  const uniqueDirs = new Set(searchDirs);
+  for (const dir of uniqueDirs) {
+    try {
+      const entries = await readdir(dir, { withFileTypes: true });
+      const files = new Set(entries.filter((e) => e.isFile()).map((e) => e.name));
+
+      for (const potential of POTENTIAL_CONFIGS) {
+        if (files.has(potential)) {
+          return path.join(dir, potential);
+        }
+      }
+    } catch {
+      // Ignore directories that cannot be read
+    }
+  }
+
+  return false;
+}

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

@@ -22,6 +22,7 @@ import {
 import { NormalizedUnitTestBuilderOptions } from '../../options';
 import type { TestExecutor } from '../api';
 import { setupBrowserConfiguration } from './browser-provider';
+import { findVitestBaseConfig } from './configuration';
 import { createVitestPlugins } from './plugins';
 
 type VitestCoverageOption = Exclude<InlineConfig['coverage'], undefined>;
@@ -207,11 +208,16 @@ export class VitestExecutor implements TestExecutor {
         }
       : {};
 
+    const runnerConfig = this.options.runnerConfig;
+
     return startVitest(
       'test',
       undefined,
       {
-        config: this.options.runnerConfig === true ? undefined : this.options.runnerConfig,
+        config:
+          runnerConfig === true
+            ? await findVitestBaseConfig([this.options.projectRoot, this.options.workspaceRoot])
+            : runnerConfig,
         root: workspaceRoot,
         project: ['base', this.projectName],
         name: 'base',

packages/angular/build/src/builders/unit-test/tests/options/runner-config_spec.ts

@@ -44,7 +44,7 @@ describeBuilder(execute, UNIT_TEST_BUILDER_INFO, (harness) => {
       });
 
       it('should search for a config file when `true`', async () => {
-        harness.writeFile('vitest.config.ts', VITEST_CONFIG_CONTENT);
+        harness.writeFile('vitest-base.config.ts', VITEST_CONFIG_CONTENT);
         harness.useTarget('test', {
           ...BASE_OPTIONS,
           runnerConfig: true,
@@ -57,7 +57,7 @@ describeBuilder(execute, UNIT_TEST_BUILDER_INFO, (harness) => {
       });
 
       it('should ignore config file when `false`', async () => {
-        harness.writeFile('vitest.config.ts', VITEST_CONFIG_CONTENT);
+        harness.writeFile('vitest-base.config.ts', VITEST_CONFIG_CONTENT);
         harness.useTarget('test', {
           ...BASE_OPTIONS,
           runnerConfig: false,
@@ -70,9 +70,53 @@ describeBuilder(execute, UNIT_TEST_BUILDER_INFO, (harness) => {
       });
 
       it('should ignore config file by default', async () => {
+        harness.writeFile('vitest-base.config.ts', VITEST_CONFIG_CONTENT);
+        harness.useTarget('test', {
+          ...BASE_OPTIONS,
+        });
+
+        const { result } = await harness.executeOnce();
+
+        expect(result?.success).toBeTrue();
+        harness.expectFile('vitest-results.xml').toNotExist();
+      });
+
+      it('should find and use a `vitest-base.config.mts` in the project root', async () => {
+        harness.writeFile('vitest-base.config.mts', VITEST_CONFIG_CONTENT);
+        harness.useTarget('test', {
+          ...BASE_OPTIONS,
+          runnerConfig: true,
+        });
+
+        const { result } = await harness.executeOnce();
+
+        expect(result?.success).toBeTrue();
+        harness.expectFile('vitest-results.xml').toExist();
+      });
+
+      it('should find and use a `vitest-base.config.js` in the workspace root', async () => {
+        // This file should be ignored because the new logic looks for `vitest-base.config.*`.
+        harness.writeFile('vitest.config.ts', VITEST_CONFIG_CONTENT);
+        // The workspace root is the directory containing the project root in the test harness.
+        harness.writeFile('vitest-base.config.js', VITEST_CONFIG_CONTENT);
+        harness.useTarget('test', {
+          ...BASE_OPTIONS,
+          runnerConfig: true,
+        });
+
+        const { result } = await harness.executeOnce();
+
+        expect(result?.success).toBeTrue();
+        harness.expectFile('vitest-results.xml').toExist();
+      });
+
+      it('should fallback to in-memory config when no base config is found', async () => {
+        // This file should be ignored because the new logic looks for `vitest-base.config.*`
+        // and when `runnerConfig` is true, it should not fall back to the default search.
         harness.writeFile('vitest.config.ts', VITEST_CONFIG_CONTENT);
         harness.useTarget('test', {
           ...BASE_OPTIONS,
+          runnerConfig: true,
         });
 
         const { result } = await harness.executeOnce();