fix: don't fire `explicit_outlives_requirements` on `?Sized` type params - #158615

Albab-Hasan · 2026-06-30T13:58:51Z

explicit_outlives_requirements uses tcx.inferred_outlives_of() to check whether an explicit T: 'a bound is structurally implied by struct fields. for a field r: &'a T, it is — so the lint fired and suggested removing it.

but RFC 599 object lifetime defaults are computed from explicit HIR bounds not inferred ones. removing an explicit T: 'a from a ?Sized type param silently changes every Struct<dyn Trait> use from dyn Trait + 'a to dyn Trait + 'static, breaking callers without any error.

added a guard: before emitting the lint for a type param outlives bound, check whether the param carries any ?Sized bound (BoundPolarity::Maybe in HIR). if so, skip — the bound may be the sole anchor for the object lifetime default.

also suppresses the pre-existing false positive in edition-lint-infer-outlives.rs (noted in issue #105150) which was an instance of the same bug.

closes #134902

rustbot · 2026-06-30T13:58:57Z

r? @jieyouxu

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Albab-Hasan · 2026-07-01T07:48:49Z

r? @folkertdev

folkertdev · 2026-07-01T09:01:56Z

r? types

fmease

Funnily, I just added a disclaimer in PR #157998.

Updating the lint itself I didn't dare to do since I was wondering whether this would require T-lang involvement (via T-lang-nomination). While it's a bug fix I feel like that this drastically cuts down the number of times this lints fires in practice. OTOH Sized type parameters bounded by 'a have a lot of use cases (see the stdlib), so I dunno.

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rustbot · 2026-07-01T09:19:22Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

Albab-Hasan · 2026-07-01T10:43:04Z

@rustbot ready

Albab-Hasan · 2026-07-08T06:27:37Z

@fmease hey, sorry for bothering you. just wanted to follow up on the pr

Albab-Hasan · 2026-07-18T03:34:56Z

@fmease ...

fmease

Thanks! Sorry for the delay, I didn't have much time for the project the last few weeks

One suggestion + squash.

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fmease · 2026-07-26T16:49:22Z

+// `T: 'a` is implied by the field, but removing it would change the object lifetime
+// default for `dyn Trait` uses from `'a` to `'static` — the lint must not fire here.
 struct InferredWhereBoundWithInlineBound<'a, T: ?Sized>
-//~^ ERROR outlives requirements can be inferred


Could you drop the : ?Sized instead since your change no longer exercises #105150.

When a type param `T` is not `Sized` (i.e. `is_sized` returns false), removing an explicit `T: 'r` bound can silently change trait object lifetime defaults per RFC 599 — `Struct<dyn Trait>` would shift from `dyn Trait + 'r` to `dyn Trait + 'static`. Suppress the lint in that case using `Ty::new_param(...).is_sized(tcx, typing_env)`. Fixes rust-lang#134902

Albab-Hasan · 2026-07-27T00:52:56Z

thanks for the review!

rustbot · 2026-07-27T00:53:43Z

This PR was rebased onto a different main 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.

…licit_outlives_requirements`

fmease · 2026-07-28T21:40:01Z

I've pushed a commit that adds back a lengthy note about trait object lifetime defaulting to the description of the lint to address this part of #158615 (comment):

Ideally, you'd preserve parts of the disclaimer and turn it into a normal paragraph that justifies the fact that this lint doesn't fire if the type parameter isn't bounded by Sized since it's not unlikely that users will stumble upon this in the future and wonder if this is a false negative.

Apart from this reason I just didn't want my efforts from #157998 to go to waste ;)

@bors r+ rollup

rust-bors · 2026-07-28T21:40:04Z

📌 Commit bf49633 has been approved by fmease

It is now in the queue for this repository.

Rollup of 11 pull requests Successful merges: - #158168 (Added implementation on `set_permissions_nofollow` for all primary platforms) - #160055 (Simplify `MaybeRequiresStorage`) - #157226 (Partially stabilize `box_vec_non_null`) - #158879 (simplify `slice::Iter[Mut]::next_chunk` implementation) - #159413 (Enable `#[diagnostic::on_unknown]` during late res) - #160091 (Fix rustdoc toolbar height when title is taller than one line) - #158615 (fix: don't fire `explicit_outlives_requirements` on `?Sized` type params) - #159666 (fix(ld64.lld): route version mismatch warnings to linker_info on macOS) - #160032 (rustdoc-json: Make `Stability` compatible with non-self-describing serde formats) - #160039 (Add regression test for enum unconstrained parameter ) - #160049 (Use assert_eq! in splat codegen tests)

Rollup merge of #158615 - Albab-Hasan:fix-explicit-outlives-unsized, r=fmease fix: don't fire `explicit_outlives_requirements` on `?Sized` type params `explicit_outlives_requirements` uses `tcx.inferred_outlives_of()` to check whether an explicit `T: 'a` bound is structurally implied by struct fields. for a field `r: &'a T`, it is — so the lint fired and suggested removing it. but RFC 599 object lifetime defaults are computed from *explicit* HIR bounds not inferred ones. removing an explicit `T: 'a` from a `?Sized` type param silently changes every `Struct<dyn Trait>` use from `dyn Trait + 'a` to `dyn Trait + 'static`, breaking callers without any error. added a guard: before emitting the lint for a type param outlives bound, check whether the param carries any `?Sized` bound (`BoundPolarity::Maybe` in HIR). if so, skip — the bound may be the sole anchor for the object lifetime default. also suppresses the pre-existing false positive in `edition-lint-infer-outlives.rs` (noted in issue #105150) which was an instance of the same bug. closes #134902

rustbot assigned jieyouxu Jun 30, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jun 30, 2026

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Albab-Hasan force-pushed the fix-explicit-outlives-unsized branch from 2b2893b to 0013e7b Compare June 30, 2026 14:01

rustbot assigned folkertdev and unassigned jieyouxu Jul 1, 2026

fmease changed the title ~~fix: don't fire \explicit_outlives_requirements\ on \?Sized\ type params~~ fix: don't fire explicit_outlives_requirements on ?Sized type params Jul 1, 2026

rustbot added the T-types Relevant to the types team, which will review and decide on the PR/issue. label Jul 1, 2026

rustbot assigned oli-obk and unassigned folkertdev Jul 1, 2026

fmease requested changes Jul 1, 2026

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Albab-Hasan requested a review from fmease July 1, 2026 10:43

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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 Jul 26, 2026

Albab-Hasan force-pushed the fix-explicit-outlives-unsized branch from 44622b7 to 1d08359 Compare July 27, 2026 00:53

Albab-Hasan requested a review from fmease July 27, 2026 01:03

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Add note about object lifetime defaulting to description of lint `exp…

bf49633

…licit_outlives_requirements`

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-review Status: Awaiting review from the assignee but also interested parties. labels Jul 28, 2026

jhpratt mentioned this pull request Jul 29, 2026

Rollup of 11 pull requests #160112

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rust-bors Bot merged commit cc89039 into rust-lang:main Jul 29, 2026
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rustbot added this to the 1.99.0 milestone Jul 29, 2026

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