unit-test builder (Vitest): key fsModuleCache on the served build output instead of forcing it off #34443

Command

test

Is this a regression?

• Yes, this behavior used to work in the previous version

The previous version in which this bug was not present was

No response

Description

Since f0a74e8 (released in 22.2.3), @angular/build:unit-test forces fsModuleCache: false, at the root and on the test project, overriding the runnerConfig. The reason is sound: Vitest keys its file system module cache on the file found on disk, while a test entry point is served from its bundle, which also holds the application code the spec imports. Changing an imported file without touching the spec left the key unchanged, and the cache served a stale bundle.

Vitest provides a hook for exactly this case: a plugin can add its own part to the cache key with defineCacheKeyGenerator, from configureVitest. The angular:test-in-memory-provider plugin already implements configureVitest and holds buildResultFiles, the files its load() serves, each with a content hash. Keying the cache on the served build output (the output file hash plus its source map hash) removes the staleness at its root, so the cache does not need to be turned off.

Since #34337 (also in 22.2.3) the test bundles are byte-identical from one build to the next, so such a key is stable and the cache can be reused across runs and CI pipelines.

Measured on a workspace application with 255 spec files (warm Angular cache, macOS, runs alternated on an idle machine), the builder patched as described:

The gain is modest here, since execution dominates this suite; it grows with the share of transform time. On the same workspace, changing a file imported by a spec (without touching the spec) makes that spec fail as expected, and reverting the change reuses the original cache entry.

I have a fix ready and will open a PR referencing this issue.

Minimal Reproduction

1. In a project using @angular/build:unit-test with Vitest 5, set fsModuleCache: true and fsModuleCachePath: './.vitest-cache' in the runnerConfig.
2. Run ng test --no-watch.
3. No .vitest-cache directory is created: the builder overrides the option.

Exception or Error

No response

Your Environment

Angular CLI       : 22.2.3
Angular           : 22.2.3
Node.js           : 24.21.0
Package Manager   : npm 11.19.0
Operating System  : darwin arm64

@angular/build    : 22.2.3
vitest            : 5.0.4

Anything else relevant?

Follow-up of #34335 / #34337 (deterministic test discovery) and of f0a74e8 (fsModuleCache disabled to avoid stale bundles).