[FR]: type-check dependent projects in parallel

[alexeagle]

IMPORTANT: This depends on an unimplemented feature request in TypeScript: --isolatedDeclarations microsoft/TypeScript#47947

What is the current behavior?

Currently, type-checking a tree of targets like app <-- counter <-- textutils/splitter requires that they are run in serial, because the results of type-checking a library are needed by all transitive dependents as inputs to their type-check action.

Quoting from the --isolatedDeclarations proposal:

To compile, we need to first compile textutils/splitter, wait for that to complete, e.g. 5s, then compile counter, wait e.g. 3s, then compile app (6s). Total compilation wall time is |app| + |counter| + |textutils/splitter|, in our example 5s + 3s + 6s = 16 seconds.

Describe the feature

Again, quoting from the --isolatedDeclarations proposal:

Now assume we could produce .d.ts files without requiring transitive inputs. That'd mean we could, in parallel, produce the .d.ts files for textutils/splitter, counter (and app, though we don't need that). After that, we could, in parallel, type check and compile textutils/splitter, counter, and app. Assuming sufficient available parallelism (which seems reasonable, given how common multicore CPUs are), total compilation wall time is the maximum time to extract .d.ts files, plus the time for the slowest compile. Assuming .d.ts extraction is purely syntactical, i.e. does not need type checking nor symbol resolution, it shouldn't add more overhead than a few hundred ms. Under these assumptions, the wall time to wait for the project to compile would be 500 ms + 6s = 6.5 seconds, i.e. more than a x2 speedup.

When the --isolatedDeclarations feature is available in some version of TypeScript, then rules_ts should support it with an action graph like the one implied there.

[alexeagle]

Note, this is unblocked and work likely to begin in the next month or so.

[binoche9]

TypeScript 5.5 is out with --isolatedDeclarations support!

[alexeagle]

Work on this has begun, thanks to folks from several companies for helping to vet designs! (and of course @mprobst for getting the ball rolling!)
Note that TypeScript 5.5 only gave us the beginning of the story: enforcing that source files have enough type annotations to be able to emitDeclarations from them without type-checking.

• Mirror Typescript 5.5 to this repo
• Upgrade examples to build with TS 5.5
• Add the isolated_declarations attribute, validate that tsconfig.json matches: Add example for isolatedDeclarations #647
• Choose one or more declaration emit implementations: swc, oxc, or tsc (nightly) and demonstrate a declaration emit (feat: support --noCheck for parallel transpiling without type-checking #679)
• Update logic in our action graph to emit declarations independently of typechecking (feat: support --noCheck for parallel transpiling without type-checking #679)
• Document this enough that most users are able to get the performance benefit

[gzm0]

TS 5.6 with --noCheck is out \o/ (sorry for the spam, this is exciting 🤩)

[jbedard]

See #679 for the first big step 😄