Cache exports lookups and path patterns in module specifier generation

[Gavin Kline (gwkline)]

Fixes #64721

Analysis

When declaration emit (or auto-import) needs a specifier for a file, it searches the mapping tables that could name it:

• exports. For a file in a package with an "exports" map, tryGetModuleNameFromExports walks every entry and condition, resolving a path and comparing it with the target for each. For every string entry it also recomputes the target's JS file name (and, for "imports", its output and declaration file names through outputpaths). It also joins the package name with every subpath before knowing whether the entry matches.
• paths. With "paths" (or a package's "typesVersions"), tryGetModuleNameFromPaths resolves every pattern against the table's base directory. For every pattern it also recomputes the candidate specifiers of the target file, although they don't depend on the pattern and processEnding can query the file system.

The nodebuilder caches the final specifier per importing file, so both scans repeat for every file that names the same target. The cost is importing files × table size. Emit holds a file's checker exclusively, so this sits on the emit critical path.

Fix

• Exports cache. ModuleSpecifierGenerationHost gains ModuleSpecifierGenerationCache(), returning a modulespecifiers.GenerationCache or nil. The compiler's Program owns one, which its emit host forwards to, and the auto-import alias resolver owns another. The cache memoizes the subpath an exports map publishes a target under. The key is the parsed *packagejson.PackageJson, the package directory and name, the target file, the resolution mode, and the compiler options. A re-read package.json therefore never serves a stale answer, and neither does a different option set. Negative results are cached too.
• Paths cache. The same cache keeps the resolved patterns of each paths / typesVersions table, keyed by the table and its base directory. A lookup becomes prefix and suffix compares against precomputed strings.
• Per-lookup work. The candidate specifiers in tryGetModuleNameFromPaths are computed once per lookup. exports/imports lookups compute the target's file names once per lookup, and build an entry's specifier only when that entry matches.
• typesVersions tables. GetVersionPaths returns VersionPaths by value, and GetPaths built the table lazily on the receiver. Every caller therefore rebuilt the table on its own copy, including module resolution on every resolution through such a package, and the table had no stable identity to cache by. The table is now built once, inside the sync.Once that selects the version paths. TestGetVersionPathsSharesTable pins this.

Results are unchanged: lookups return the same specifiers in the same table order, and emitted declarations are byte-identical on every project below. Reachability of node_modules is still checked outside the cached lookup, so this composes with #64599.

New tests in modulespecifiers/cache_test.go and packagejson/packagejson_test.go:

• They check cached against uncached results across conditions, custom conditions, resolution modes, patterns, entries with extensions, and absolute patterns resolved from different base directories.
• Every case runs twice against one cache, so a key that dropped any input fails. The custom-conditions case caught exactly that in an earlier version of this patch.
• Benchmarks cover both scans.

Benchmark	Time per lookup	Allocations per lookup
exports scan, 180 subpaths, uncached	371 µs → 298 µs	1,984 → 1,085
paths scan, 400 patterns, uncached	331 µs → 240 µs	7,201 → 6,004
paths scan, 400 patterns, cached	331 µs → 6.4 µs	7,201 → 3

Project	Emit time	Allocations
exports generator from the issue	0.188 s → 0.077 s	2.82M → 0.95M
paths generator from the issue	0.64 s → 0.096 s	25.2M → 1.24M
private monorepo package, declaration build (11k files)	2.48 s → 1.45 s (total 7.39 s → 6.47 s)	56.6M → 51.2M

The profiling, the patch and this description were produced with Claude Code (Claude Fable 5.1 and Claude Opus 5.5). I have read and understand the change and will handle review myself.

Copilot Checklist

I successfully ran the applicable command at the end of my session, and it completed without error:

• npx hereby validate
• npx hereby validate --api (for TypeScript API changes; no API changes here)

🤖 Generated with Claude Code

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