Merge export * tables without per-name collision trackers

[Gavin Kline (gwkline)]

Fixes #64719

Analysis

For each module with export *, getExportsOfModuleWorker cloned the module's own table and every visited module's table. It merged each re-exported table into a scratch table, and allocated an ExportCollision record for every distinct re-exported name so it could report TS2308 later. It then walked that table again to find collisions and merged the scratch table into the result. It also copied every visited module's export names into a set that only mattered when an export type * was present.

So every export * layer cost several allocations per re-exported name, even with a single export *, where nothing can collide. Barrels over large modules (generated clients, or date-fns- and AWS-SDK-style entry points) pay this at every layer. The generator in the issue spends most of its check time here.

Fix

• Re-exported tables are merged directly into the module's result table (new getExportsWithExportStars). The table is pre-sized for the module's own exports plus its largest re-exported table, so barrels that re-export the same names through many modules don't over-allocate.
• Collision bookkeeping only exists when a module has more than one export *. It records the first contributing declaration per name, and names that actually collide go into a map that is almost always empty.
• Modules without export * are no longer cloned when visited from an importer, since their tables are only read during the merge. The top-level module's table is still cloned, because it is cached and the CommonJS export= typedef loop writes into it.
• Instead of a set of names, the worker records the modules reached through a non-type-only path. It applies them to typeOnlyExportStarMap only when that map exists.
• ExportCollision, ExportCollisionTable and extendExportSymbols had no other users and are removed.

Semantics

Unchanged. The first contributing export * still wins. A module's own exports still shadow re-exports without an error. default is skipped, export= is never reported as TS2308, and the export type * / export * override rules are the same.

New test exportStarNameCollisions covers:

• collisions from two and three declarations
• the same symbol reached twice
• a local export silencing a collision
• a single export *
• export type * on either side
• a chain of barrels

Its baselines were generated with the old code and are unchanged. No existing baseline changes.

Results

M3 Pro, medians of alternating runs:

Program	Check time	Allocations
generator from the issue (207 source files)	0.234 s → 0.052 s	1.58M → 0.24M
private monorepo app (9.6k files)	2.98 s → 2.84 s (−4.7%)	30.3M → 28.9M
private monorepo package (8.6k files)	4.10 s → 3.95 s (−3.6%)	46.1M → 44.7M

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

[Copilot]

🔵 Needs a closer look

The optimization changes core recursive module-export resolution and warrants final human validation despite strong regression coverage.

0 open findings

What changed in this PR

Optimizes compiler resolution of export * tables while preserving collision and type-only export semantics.

Changes:

• Merges re-export tables directly with preallocated capacity.
• Defers collision tracking and diagnostic text creation.
• Adds comprehensive compiler regression coverage and baselines.

File	Description
tsc/​internal/​checker/​checker.go	Optimizes export-table merging and collision tracking.
tsc/​testdata/​tests/​cases/​compiler/​exportStarNameCollisions.ts	Adds export-star collision scenarios.
tsc/​testdata/​baselines/​reference/​compiler/​exportStarNameCollisions.errors.txt	Captures expected diagnostics.
tsc/​testdata/​baselines/​reference/​compiler/​exportStarNameCollisions.js	Captures emit and declarations.
tsc/​testdata/​baselines/​reference/​compiler/​exportStarNameCollisions.symbols	Captures resolved symbols.
tsc/​testdata/​baselines/​reference/​compiler/​exportStarNameCollisions.types	Captures inferred types.

🧠 Review effort: Balanced

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[Gavin Kline (gwkline)]

@microsoft-github-policy-service agree