fix: prevent false Vitest import resolution

packages/vitest/src/runtime/moduleRunner/cachedResolver.ts

@@ -9,6 +9,14 @@ const normalizedDistDir = normalize(distDir)
 const relativeIds: Record<string, string> = {}
 const externalizeMap = new Map<string, string>()
 
+function getRelativeDistDir(root: string): string {
+  const normalizedRoot = normalize(root).replace(/\/+$/, '') || '/'
+  const rootPrefix = normalizedRoot === '/' ? normalizedRoot : `${normalizedRoot}/`
+  return normalizedDistDir.startsWith(rootPrefix)
+    ? normalizedDistDir.slice(normalizedRoot.length)
+    : ''
+}
+
 // all Vitest imports always need to be externalized
 export function getCachedVitestImport(
   id: string,
@@ -24,7 +32,7 @@ export function getCachedVitestImport(
   // always externalize Vitest because we import from there before running tests
   // so we already have it cached by Node.js
   const root = state().config.root
-  const relativeRoot = relativeIds[root] ?? (relativeIds[root] = normalizedDistDir.slice(root.length))
+  const relativeRoot = relativeIds[root] ?? (relativeIds[root] = getRelativeDistDir(root))
   if (id.includes(distDir) || id.includes(normalizedDistDir)) {
     const { file, postfix } = splitFileAndPostfix(id)
     const externalize = id.startsWith('file://')

test/unit/test/cached-resolver.test.ts

@@ -0,0 +1,63 @@
+import type { WorkerGlobalState } from '../../../packages/vitest/src/types/worker'
+import { pathToFileURL } from 'node:url'
+import { dirname, normalize } from 'pathe'
+import { describe, expect, test } from 'vitest'
+import { distDir } from '../../../packages/vitest/src/paths'
+import { getCachedVitestImport } from '../../../packages/vitest/src/runtime/moduleRunner/cachedResolver'
+
+const normalizedDistDir = normalize(distDir)
+
+function stateWithRoot(root: string): () => WorkerGlobalState {
+  return () => ({ config: { root } }) as WorkerGlobalState
+}
+
+function unrelatedRootForSuffix(suffix: string, fill: string): string {
+  return `/${fill.repeat(normalizedDistDir.length - suffix.length - 1)}`
+}
+
+describe('getCachedVitestImport', () => {
+  test('does not resolve a bare builtin relative to an unrelated root', () => {
+    const root = unrelatedRootForSuffix('t', 'a')
+    expect(normalizedDistDir.slice(root.length)).toBe('t')
+
+    expect(getCachedVitestImport('timers', stateWithRoot(root))).toBeNull()
+  })
+
+  test('does not resolve a builtin subpath relative to an unrelated root', () => {
+    const root = unrelatedRootForSuffix('st', 'b')
+    expect(normalizedDistDir.slice(root.length)).toBe('st')
+
+    expect(getCachedVitestImport('stream/promises', stateWithRoot(root))).toBeNull()
+  })
+
+  test('does not derive a relative path from a sibling root prefix', () => {
+    const root = normalizedDistDir.slice(0, -'st/dist'.length)
+    expect(normalizedDistDir.slice(root.length)).toBe('st/dist')
+
+    expect(getCachedVitestImport('st/dist/index.js', stateWithRoot(root))).toBeNull()
+  })
+
+  test('resolves a dist path relative to an ancestor root', () => {
+    const root = dirname(normalizedDistDir)
+    const id = `${normalizedDistDir.slice(root.length)}/index.js?relative`
+
+    expect(getCachedVitestImport(id, stateWithRoot(root))).toEqual({
+      externalize: `${pathToFileURL(`${normalizedDistDir}/index.js`)}?relative`,
+      type: 'module',
+    })
+  })
+
+  test('still resolves absolute and bare Vitest imports', () => {
+    const root = unrelatedRootForSuffix('test', 'c')
+    const id = `${normalizedDistDir}/index.js?absolute`
+
+    expect(getCachedVitestImport(id, stateWithRoot(root))).toEqual({
+      externalize: `${pathToFileURL(`${normalizedDistDir}/index.js`)}?absolute`,
+      type: 'module',
+    })
+    expect(getCachedVitestImport('vitest/config', stateWithRoot(root))).toEqual({
+      externalize: 'vitest/config',
+      type: 'module',
+    })
+  })
+})