Design Meeting Notes, 1/17/2024

[DanielRosenwasser]

Mark call and new expressions as intra-expression inference sites

#54183

• Have a concept called intra-expression inference sites.
  • Allows us to walk within an expression to infer from inner expressions, in case one of the inner expressions relies on prior inner expressions.
• The change is to consider calls and news as possible locations that may contain intra-expression inference sites.
• Need time to look over and review this.

Isolated Declarations

• Isolated declarations #53463

• Design Meeting Notes, 3/24/2023 #53500

• --isolatedModules says: you may only use features that can be "trivially" compiled to JS.

• Idea: --isolatedDeclarations says: you may only declare entities that can be "trivially" transformed into declaration files.

• Benefits: alternative tools can generate declaration files, and the project system graph is not a bottle-neck in parallel checking.

  • Includes using very fast compilers, declaration file bundlers.
  • Allows writing tools to generate all the .d.ts files in a fully parallel manner, and then run however many type-checkers as you want on all of your projects.

• Some issues:

  • Subset is somewhat restrictive - can be painful.
    • Alleviated by carving out a subset of JSON-y expressions.
  • 3rd party transformers may want to build off a superset of features of --isolatedDeclarations.
    • However, people writing external transformers have provided explicit feedback that they would not want to implementa declaration file transformer.

• Given that there was a big feature list of other things, is this stable? Should it initially be marked as experimental?

  • Possibly!

• Coming back to a point: should this be an external tool?

  • Could see this being a compiler API rather than a top-level flag.
    • The biggest use-case is build tools, and many of them are relying on TypeScript itself too.
    • Having this at the API level.
  • You really do want diagnostics and quick fixes as you adopt this feature.

• Is this 5.4 ready?

  • Close the 5.4 beta deadline, so risky.
    • But there is time before RC/stable.
  • Some places where the implementation is not what we would've expected.
    • For example, isolatedDeclarations doesn't use a separate path, this does.
    • Effectively a separate binder.
    • Larger footprint in general.

• We have raised all of these points in the past. It's okay if we want to revisit, but need to make feedback actionable and avoid churn.

• Is isolatedModules a mistake even though we tell everyone to use it?

  • isolatedModules was based on 3rd party tools needing to emit
  • Maybe ahistorical - people might've just wanted this for transpileModule.
  • Regardless, there's a bit of chicken/egg.

• The feature is a few things

  • Errors
  • Quick fixes
  • Fast declaration emit via API
  • Fast declaration emit via tsc

• Has all the optimistic stuff been removed?

  • Mostly, yes.

• Can this exist out of band?

  • Feels like the concept doesn't keep its head above water if it's not in the compiler.

• One set of ideas:

  • Take the isolated declaration emitter and move it out of band.
  • Keep the isolatedDeclarations flag in the compiler in some capacity.