Skip .d.ts checks from referenced projects

[vthib]

Suggestion

From what i have seen, it appears that declaration files generated from a project are re-checked when compiling projects referencing it.

Here is an example:

tsconfig-a.json : include a.ts
tsconfig-b.json: include b.ts

when building tsconfig-b.json, we can see:

• building project a => a.ts is type-checked
• building project b => outDir/a.d.ts and b.ts is type-checked.

(I can provide a repository with this example if needed)

This is an issue because when using many projects to split a big codebase, those output files are re-checked again and again,
leading to a huge time loss.

This is evident when using the skipLibCheck. In our repository with about 900 typescript files and 15 projects:

• without skipLibCheck: 3 minutes to run tsc -b
• with skipLibCheck: 1 minute 15 seconds to run tsc -b

Unless i'm missing something, this shows clearly that declaration files from referenced projects are type-checked again and again, and it isn't just declaration files from node_modules which are skipped.

The issue with skipLibCheck is that it skips declaration files from the repository, and not just from node_modules or projects output directories.

Forgive me if i'm wrong, or if this has been talked about already, I have not found anything about this in my searches.

🔍 Search Terms

project references, skipLibCheck, declaration files

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

My suggestion is to skip checking declaration files when they come from a referenced project, as we already know it has been type checked.

📃 Motivating Example

This change would:

• Improve the speed of building project references even more
• Allow type-checking declaration files in the project without sacrificing build time
• Fix a performance issue where a declaration file is type-checked X times (max being the number of projects) instead of once.

💻 Use Cases

Use-case is using project references to split a big codebase into chunks and improve build time, without being forced to use skipLibCheck.
I don't think there is any drawback to the change.
Workaround is using skipLibCheck, but it disables type-checking on declaration files that are inside a project.

[RyanCavanaugh]

We've gotten the request to "Skip lib check, but don't skip my lib checks" before. Skipping referenced projects and node_modules while still checking local .d.ts files should be done concurrently with #30511.

[vthib]

Thank you for you quick answer.
I was talking about always skipping checks for declaration files produced by projects, even if skipLibCheck is not set, as i'm not sure i see a valid reason for not skipping them, and it does introduce a big time loss (which including type checks of node_modules does not introduce).
So imho this is not exactly the same as #30511 (which is about changing the behavior of skipLibCheck). I do see however the link between the two, and a solution for #30511 would indeed answer this ticket as well.
Just wanted to make sure this feature request was not misunderstood. If you tell me that it should be solved by fixing #30511 i will close this ticket (or you can close it directly).
Thanks for your time and your work on Typescript!

[RyanCavanaugh]

Right. What I'm saying is, if we have any feature like this, it should apply equally to node_modules and upstream project reference outputs.

[vthib]

I understand. I'll close this and will follow the other ticket. Thanks!