[heft] heft-jest-plugin watch mode fails on Jest 30: Cannot find module '@jest/core/build/watch.js' #6199

Summary

Since @rushstack/heft-jest-plugin moved to Jest 30 (#5739), watch mode no longer works. heft test-watch (and heft run-watch with the test phase) fails as soon as the test phase starts:

Cannot find module '<...>/node_modules/@jest/core/build/watch.js'

Normal heft test is unaffected, because it only calls the public runCLI export of @jest/core.

Repro steps

1. Use any project with @rushstack/heft and @rushstack/heft-jest-plugin 2.x (Jest 30) that has a test phase using the Jest plugin.
2. Run heft test-watch --clean.

Expected result: The build finishes, Jest runs the tests, and Heft waits for changes and reruns the affected tests.

Actual result: The build finishes, then the test phase fails right away:

 ---- build finished (2.925s) ----
 ---- test started ----
[test:jest] Error: Cannot find module '<...>/node_modules/.pnpm/@jest+core@30.3.0_<...>/node_modules/@jest/core/build/watch.js'
[test:jest] Require stack:
[test:jest] - <...>/node_modules/@rushstack/heft-jest-plugin/lib-commonjs/JestPlugin.js
[test:jest] - <...>/node_modules/@rushstack/heft/lib-commonjs/configuration/HeftPluginDefinition.js
[test:jest] - <...>/node_modules/@rushstack/heft/lib-commonjs/configuration/HeftPluginConfiguration.js
[test:jest] - <...>/node_modules/@rushstack/heft/lib-commonjs/pluginFramework/HeftLifecycle.js
[test:jest] - <...>/node_modules/@rushstack/heft/lib-commonjs/pluginFramework/InternalHeftSession.js
[test:jest] - <...>/node_modules/@rushstack/heft/lib-commonjs/cli/HeftCommandLineParser.js
[test:jest] - <...>/node_modules/@rushstack/heft/lib-commonjs/start.js
[test:jest] - <...>/node_modules/@rushstack/heft/lib-commonjs/startWithVersionSelector.js
[test:jest] - <...>/node_modules/@rushstack/heft/bin/heft
 ---- test encountered an error (0.060s) ----
-------------------- Failed (3.034s) --------------------
Encountered 1 error
  [test:jest] Cannot find module '<...>/@jest/core/build/watch.js'

Details

In watch mode, JestPlugin._runJestWatchAsync patches two internal modules of @jest/core so it can control reruns and capture output:

• path.resolve(jestCoreDir, 'watch.js'): replaces default with patchedWatch, which wraps the output stream and listens to haste map change events.
• path.resolve(jestCoreDir, 'runJest.js'): replaces default with patchedRunJest, which defers test execution until Heft requests a run.

(heft-plugins/heft-jest-plugin/src/JestPlugin.ts, around lines 355 and 406 on main.)

Jest 30 ships @jest/core as a single bundle. build/ now contains only index.js, index.mjs and index.d.ts, and package.json exports only exposes . and ./package.json. The public API is just SearchSource, createTestScheduler, getVersion and runCLI, so watch and runJest can't be reached as separate modules anymore and the patch can't be applied.

I checked the latest published version (2.0.22) and the current main source, and both still resolve watch.js and runJest.js, so this affects every 2.x release built on Jest 30. I couldn't find an existing issue for it.

Possible directions (I'm not familiar enough with the plugin internals to recommend one):

• Run watch mode without patching Jest internals. For example, Heft could drive reruns itself by calling runCLI with the changed files (findRelatedTests / --onlyChanged-style options) for each watch iteration, instead of using Jest's own watch loop.
• Until watch mode is supported again, detect Jest 30 in _runJestWatchAsync and fail with a clear message instead of a Cannot find module error.

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