TypeScript build info cache

Restoring each package's *.tsbuildinfo would make the typecheck job incremental. Measured locally, that takes the packages from most of two minutes to seconds and roughly halves the app's pass — a real saving in the currency CI proposals are judged in, and it is still refused.

Why not

Only a prefix restore saves anything. An exact-key hit means this tree already typechecked green, which is a marker's answer and needs no compiler to give it.

A prefix hit cannot be shown correct. It would be the one gate here whose correctness rests on a compiler's own dependency graph rather than on a hash of its inputs — every other one is checkable by hashing what went in and looking at what came out. TypeScript versions each file by content, so a stale entry still reports an error introduced in a file someone edited; what no sampling establishes is that every indirect invalidation is caught, and the compiler here is a native implementation far younger than the format it writes. Getting it wrong buys a silent green on a gate.

The ecosystem does not lean on it either. The monorepo caches people do trust — this repository's own included — key inputs to outputs and never restore a compiler's internal state.

The revisit trigger

Nothing about the saving; only the proof. A restore whose correctness follows from a hash of inputs rather than from the compiler's invalidation — which is a marker, not this.