Fix extract_type_alias and provide inlay hints for const `_` placeholders by nicolas-guichard · Pull Request #22211 · rust-lang/rust-analyzer

nicolas-guichard · 2026-04-29T18:33:33Z

This is basically the extension of #20125 to const generics.

Before:

After:

Blocked on #22237.

ChayimFriedman2 · 2026-05-01T08:41:33Z

I have a branch that rewrites how we collect placeholders to be more principled (pass the table and the InferCtxt to TyLoweringContext), so it should be merged before this.

nicolas-guichard · 2026-05-01T12:15:17Z

Oh right, thanks!

nicolas-guichard · 2026-06-04T12:52:29Z

@ChayimFriedman2 Now that your other PRs are merged, would you have some time to review this?

This extends InferenceResult with a new const_of_const_placeholders map to record which const a given const or type placeholder gets resolved to. This is basically the same as type_of_type_placeholder, but for consts. Note that while surprising, TypeRef::Placeholder may indeed resolve to a const because except for arrays the parser doesn't know ahead of type if a _ will resolve to a type or a const. In the case of an array, we have an Expr::Underscore: ``` let a: [_; _] = [4, 5, 6]; //^ TypeRef::Placeholder that resolves to the type i32 //^ Expr::Underscore that resolves to the const 3 ``` But for a struct we have a TypeRef::Placeholder: ``` struct S<T, const N: usize>([T; N]); let s: S<_, _> = S([1, 2, 3]); //^ TypeRef::Placeholder that resolves to the type i32 //^ TypeRef::Placeholder that resolves to the const 3 ```

This fixes SourceAnalyzer::type_of_type to replace const infer vars in types with their resolved value.

This provides an inlay hint for _ in const generic arguments.

rustbot · 2026-07-10T14:58:26Z

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Apr 29, 2026

nicolas-guichard force-pushed the push-qltvotzxsuqw branch 2 times, most recently from aac49b9 to 5980265 Compare April 29, 2026 18:47

nicolas-guichard marked this pull request as draft May 1, 2026 12:15

rustbot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label May 1, 2026

nicolas-guichard force-pushed the push-qltvotzxsuqw branch from 5980265 to 0a4871a Compare May 6, 2026 09:56