Merge export * tables without per-name collision trackers

[Gavin Kline (gwkline)]

Fixes #64719

Analysis

getExportsOfModuleWorker resolves a module's exports by cloning its own table. For each export * declaration it merges the re-exported module's table into a scratch table, then merges the scratch table into the result. To report TS2308 ("Module ... has already exported a member named ...") it allocates an ExportCollision struct, holding the module specifier text, for every name contributed by every export *. Afterwards it iterates that per-name table again to find the entries that actually collided. Every visited module is cloned as well, and every visited module's export names are copied into a set that is only consulted when an export type * was seen.

So each export * layer costs several allocations per re-exported name. That holds even for a module 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 on every layer. The generator in the issue spends nearly all of its 0.23 s check here.

Fix

• Each re-exported table is merged straight into the module's result table, which is pre-sized to the sum of the tables being merged.
• Only the first contributing export * per name is recorded, and only when the module has more than one export *. Names that do collide go into a map that is almost always empty. The specifier text is computed only when a diagnostic is reported.
• Modules without export * are no longer cloned when visited from an importer. The clone is kept for the module whose table is cached, since getExportsOfModule may extend it.
• Instead of copying every visited module's names into a set, the worker records the modules reached through a non-type-only path. It applies them to typeOnlyExportStarMap only when that map exists.

Semantics are unchanged. The first contributing declaration still wins, and a module's own exports still shadow re-exports without an error. default and export= are handled as before, and so are the export type * override rules. extendExportSymbols and ExportCollisionTable had no other users and are removed.

New test exportStarNameCollisions covers each case the merge distinguishes: 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, and a chain of re-exporting modules. Its baselines were generated with the old code and are unchanged. No existing baseline changes.

Results on an M3 Pro, with medians of alternating runs:

Program	Check time	Allocations
generator from the issue (207 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

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