Fix bound var resolution for trait aliases by rynewang · Pull Request #152309 · rust-lang/rust

rynewang · 2026-02-07T19:31:28Z

Fixes #152158
Fixes #152244

fmease · 2026-02-07T21:02:15Z

r? me

fmease

Preliminary notes.

View changes since this review

compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs

fmease · 2026-02-07T21:53:03Z

compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs

Your simple patch does seem to fix #152244, too. I didn't expect it to be that simple… I'm still gonna investigate a bit more whether it truly fixes the reported issue at its core.

Ah, now I see. I've missed that supertrait_hrtb_vars uses explicit_supertraits_containing_assoc_item under the hood which "expands" / deals with trait aliases just fine.

fmease · 2026-02-07T22:01:22Z

compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs

                | DefKind::TyAlias
-                | DefKind::Trait,
+                | DefKind::Trait
+                | DefKind::TraitAlias,


Adding TraitAlias to this list does constitute a behavioral change. Namely, it affects trait object lifetime defaulting. Namely, it makes trait alias refs eligible containers which would sort of fix #140710, too, except that there's still the same off-by-one error that would be fixed by my PR #129543.

I want to leave that error in for now since I hope that that PR will go into FCP soon. It just means that your PR leads us to reject certain programs that use the unstable feature trait_alias. E.g., it'll reject:

trait Trait<'a, T: ?Sized + 'a> = std::fmt::Debug; type X<'r> = dyn Trait<'r, dyn std::fmt::Debug>;

That's fine by me. I'll get fixed by me soon anyway 🤞

fmease

Thanks! Could you please

also add Fixes #152244 to the PR description
generalize the PR title as it actually fixes the RBV trait alias issue as a whole (not just for RTNs but for all assoc item kinds)
add the two reproducers from #152244 as test cases
basically remove all sections from the PR description (Summary + Problem + Root Cause + Fix) and only leave the two Fixes #NNN
- currently, the whole description is extremely verbose and its contents self-evident from the patch

Regarding (3), I'm thinking of something like

// Check that we're successfully collecting bound vars behind trait aliases.
// Regression test for <https://github.com/rust-lang/rust/issues/152244>.
//@ check-pass
//@ needs-rustc-debug-assertions
#![feature(trait_alias)]

trait A<'a> { type X; }
trait B: for<'a> A<'a> {}
trait C = B;

fn f<T>() where T: C<X: Copy> {}
fn g<T>() where T: C<X: for<'r> A<'r>> {}

Re. #140710, while your PR does render trait aliases eligible containers wrt. trait object lifetime defaulting (which the linked issue requests), it ultimately doesn't fix the provided snippet due to the preexisting off-by-one error that'll be fixed by me in #129543. Therefore, let's keep that open, I'll update it once this PR is merged and add the actual regression tests to PR #129543.

View changes since this review

tests/ui/associated-type-bounds/return-type-notation/trait-alias.rs

rynewang · 2026-02-07T23:17:05Z

Updated PR description and title, fixed repro format and added new repro.

fmease · 2026-02-07T23:23:47Z

(You've linked to the wrong PRs but I've fixed it myself)

@bors r+ rollup

rust-bors · 2026-02-07T23:23:51Z

📌 Commit 9433d7b has been approved by fmease

It is now in the queue for this repository.

…alias, r=fmease Fix bound var resolution for trait aliases Fixes rust-lang#152158 Fixes rust-lang#152244

fmease · 2026-02-07T23:53:00Z

Ah, my bad. The new test needs a fn main() {}

@bors r-

rust-bors · 2026-02-07T23:53:08Z

Commit 9433d7b has been unapproved.

This PR was contained in a rollup (#152312), which was also unapproved.

Handle DefKind::TraitAlias in resolve_bound_vars so that associated item constraints and return type notation work through trait aliases.

fmease · 2026-02-08T09:37:08Z

@bors r+

rust-bors · 2026-02-08T09:37:11Z

📌 Commit 7c5ea7f has been approved by fmease

It is now in the queue for this repository.

…alias, r=fmease Fix bound var resolution for trait aliases Fixes rust-lang#152158 Fixes rust-lang#152244

…uwer Rollup of 7 pull requests Successful merges: - #151455 (Fix `SourceFile::normalized_byte_pos`) - #152250 (Remove support for slugs in diagnostic messages) - #152322 (Replace some `feature(core_intrinsics)` with stable hints) - #152328 (Fix a few diagnostics) - #151640 (Cleanup offload datatransfer) - #152212 (Port some attributes to the attr parser) - #152309 (Fix bound var resolution for trait aliases)

…alias, r=fmease Fix bound var resolution for trait aliases Fixes rust-lang#152158 Fixes rust-lang#152244

…uwer Rollup of 8 pull requests Successful merges: - #151455 (Fix `SourceFile::normalized_byte_pos`) - #152250 (Remove support for slugs in diagnostic messages) - #152322 (Replace some `feature(core_intrinsics)` with stable hints) - #151640 (Cleanup offload datatransfer) - #152212 (Port some attributes to the attr parser) - #152309 (Fix bound var resolution for trait aliases) - #152339 (diagnostics: fix ICE in closure signature mismatch) - #152341 (`cfg_select!`: allow optional comma after `{ /* ... */ }`)

…uwer Rollup of 9 pull requests Successful merges: - #151455 (Fix `SourceFile::normalized_byte_pos`) - #152250 (Remove support for slugs in diagnostic messages) - #152322 (Replace some `feature(core_intrinsics)` with stable hints) - #152328 (Fix a few diagnostics) - #151640 (Cleanup offload datatransfer) - #152212 (Port some attributes to the attr parser) - #152309 (Fix bound var resolution for trait aliases) - #152339 (diagnostics: fix ICE in closure signature mismatch) - #152341 (`cfg_select!`: allow optional comma after `{ /* ... */ }`)

Rollup merge of #152309 - rynewang:fix/ice-152158-rtn-trait-alias, r=fmease Fix bound var resolution for trait aliases Fixes #152158 Fixes #152244

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rynewang force-pushed the fix/ice-152158-rtn-trait-alias branch 2 times, most recently from 78141c3 to 6c244da Compare February 7, 2026 20:03

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rynewang force-pushed the fix/ice-152158-rtn-trait-alias branch from 6c244da to 867b1a8 Compare February 7, 2026 22:18

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tests/ui/associated-type-bounds/return-type-notation/trait-alias.rs Outdated Show resolved Hide resolved

fmease added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Feb 7, 2026

rynewang force-pushed the fix/ice-152158-rtn-trait-alias branch from 867b1a8 to 9433d7b Compare February 7, 2026 23:05

rynewang changed the title ~~Fix ICE when using return type notation through trait alias~~ Fix bound var resolution for trait aliases Feb 7, 2026

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Rollup merge of rust-lang#152309 - rynewang:fix/ice-152158-rtn-trait-…

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…alias, r=fmease Fix bound var resolution for trait aliases Fixes rust-lang#152158 Fixes rust-lang#152244

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Rollup of 2 pull requests #152312

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Fix bound var resolution for trait aliases

7c5ea7f

Handle DefKind::TraitAlias in resolve_bound_vars so that associated item constraints and return type notation work through trait aliases.

rynewang force-pushed the fix/ice-152158-rtn-trait-alias branch from 9433d7b to 7c5ea7f Compare February 8, 2026 02:43

rust-bors bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Feb 8, 2026

JonathanBrouwer added a commit to JonathanBrouwer/rust that referenced this pull request Feb 8, 2026

Rollup merge of rust-lang#152309 - rynewang:fix/ice-152158-rtn-trait-…

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…alias, r=fmease Fix bound var resolution for trait aliases Fixes rust-lang#152158 Fixes rust-lang#152244

JonathanBrouwer mentioned this pull request Feb 8, 2026

Rollup of 3 pull requests #152333

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Rollup merge of rust-lang#152309 - rynewang:fix/ice-152158-rtn-trait-…

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…alias, r=fmease Fix bound var resolution for trait aliases Fixes rust-lang#152158 Fixes rust-lang#152244

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Rollup of 5 pull requests #152343

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Rollup merge of rust-lang#152309 - rynewang:fix/ice-152158-rtn-trait-…

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…alias, r=fmease Fix bound var resolution for trait aliases Fixes rust-lang#152158 Fixes rust-lang#152244

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Rollup of 7 pull requests #152345

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Rollup merge of rust-lang#152309 - rynewang:fix/ice-152158-rtn-trait-…

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…alias, r=fmease Fix bound var resolution for trait aliases Fixes rust-lang#152158 Fixes rust-lang#152244

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Rollup of 8 pull requests #152352

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Rollup merge of #152309 - rynewang:fix/ice-152158-rtn-trait-alias, r=fmease Fix bound var resolution for trait aliases Fixes #152158 Fixes #152244

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