Make --build --watch iterations reuse programs (30s → ~2s per file change)

[NullVoxPopuli]

Important

made with claude

I'm not an expert in this codebase, and could be way off base, and I apologize if this is a waste of time / completely wrong

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Fixes #1199 (upstream: volarjs/volar.js#314).

In a TS project-references monorepo (59 composite projects, 19,286-file app), a one-line change under ember-tsc --build --watch took ~30s to report — the same cost as the project's initial cold build. Plain --watch on the same project: ~2s.

phase	stock -b --watch	this PR	plain --watch
Parse	7.29s	0.00s	0.00s
ResolveModule	7.22s	0.00s	0.03s
Program	27.57s	1.74s	1.80s
Check	0.14s	0.13s	0.15s
Total	29.98s	2.08s	2.14s

Why it was slow (three compounding causes, proven by instrumentation)

1. TS's solution builder unconditionally releases each project's Program after building (afterProgramDone → releaseProgram()), even in watch mode — so oldProgram never reaches createProgram and SourceFile/resolution reuse is impossible.
2. Its compiler host (unlike plain watch's) has no SourceFile cache across iterations — every file re-parses, and the new object identities also evict volar's parsedSourceFiles WeakMap, so every .gts re-transforms.
3. volar's proxyCreateProgram keeps its language/snapshot/moduleResolutionCache state in a single slot keyed on the last options — any multi-project -b solution thrashes it every iteration.

The patches

Same playbook as the existing proxyCreateProgram/tryGetJSExtensionForFile source patches in run-volar-tsc.ts (exact-shape matching, loud error if the dep changes — verified matching on both TS 5.9.3 and 6.0.3):

• afterProgramDone: keep programs alive in watch mode (~+1.4GB RSS on the 19k-file project).
• setGetSourceFileAsHashVersioned: version-validated SourceFile cache, scoped per project — with project references the same physical file is a redirect in one program and a root in another, so SourceFile objects must never be shared across projects (sharing them crashes buildinfo emit).
• tryAddRoot: with a partially reused program, source-of-project-reference redirect files can lose their fileIncludeReasons keying and buildinfo emit crashed (Cannot read properties of undefined (reading 'some')). A file without include reasons can't be a RootFile, so skipping it is the correct result.
• proxyCreateProgram: key volar's caches per configFilePath.

Tests

New build-watch-mode.test.ts + a two-project fixture (composite emitter + noEmit/incremental consumer importing the emitter's source through a project-reference redirect — the exact shape that crashed). Spawns --build --watch, asserts diagnostics appear and clear across iterations against a reused program, no crash. Existing build-mode tests still pass (6/6 in ts-extensionless-app); the 5 failing language-server tests in package-test-core fail identically on main (pre-existing, CLI path untouched).

🤖 Generated with Claude Code