Deduplicate object safety errors on impl dyn Trait { .. }
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`impl dyn Trait {}` has an implicit `dyn Trait: 'static` bound. We add it in
`inferred_outlives_of` for more context in errors. With this we point at
`impl dyn Trait` and not to it's associated functions, deduplicating errors
to a single one per `impl dyn Trait` instead of one for it plus one for each
associated function.
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impl dyn Trait { .. }impl dyn Trait { .. }
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Deduplicate object safety errors on `impl dyn Trait { .. }`
`impl dyn Trait {}` has an implicit `dyn Trait: 'static` bound. We add it in `inferred_outlives_of` for more context in errors. With this we point at `impl dyn Trait` and not to it's associated functions, deduplicating errors to a single one per `impl dyn Trait` instead of one for it plus one for each associated function.
CC rust-lang#83246, as this is necessary but not sufficient to help with the lack of tracking of `'static` obligations.
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Feb 16, 2024
| | ----- this trait cannot be made into an object... | ||
| LL | const N: usize; | ||
| | ^ ...because it contains this associated `const` | ||
| = help: consider moving `N` to another trait |
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Totally unrelated but if tcx.features().generic_const_items we could suggest alternatively, consider constraining `N` so it does not apply to trait objects // const N: usize where Self: Sized;
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| @@ -46,6 +47,24 @@ fn inferred_outlives_of(tcx: TyCtxt<'_>, item_def_id: LocalDefId) -> &[(ty::Clau | |||
| &[] | |||
| } | |||
| } | |||
| DefKind::Impl { of_trait: false } | |||
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I'd prefer a solution that doesn't involve changing the rules for where we add implicit outlives obligations. This code is really delicate, and I'm worried this could turn out to be unsound without us really knowing why.
Is it possible to instead modify the object safety violation error reporting code to instead prefer pointing at Self type rather than arguments? We can get away with corner cases in diagnostics reporting code, but can't really get away with it in good-path code 😅
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Also, could you explain why adding this outlives obligation actually solves the problem? It's not necessarily clear why this is fixing what it does, and I'm not yet convinced its worth the changes to the inferred_outlives_of function for such a special case.
| // more context in errors. With this we point at `impl dyn Trait` and not to it's | ||
| // associated functions, deduplicating errors to a single one per `impl dyn Trait` instead | ||
| // of one for it plus one for each associated function. | ||
| let ty = tcx.type_of(item_def_id).instantiate_identity(); |
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Also, this should at least be asserting that the implicit object outlives is actually static:
let ty::Dynamic(_, outlives, _) = *ty.kind() else {
bug!("expected dyn self type");
};
assert_eq!(outlives, tcx.regions.re_static);
Then at least if the object outlives bound computation changes down the road, this will ICE.
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I'm not sure if that's actually true in general (*). Just so you know In any case I think it's the wrong approach to look at implicit outlives bounds. #83246 is strictly about default object lifetimes (which I'm fairly acquainted with by now). So any PR that wants to improve diagnostics for them should look at the trait object lifetime directly instead of any derived outlives bounds. (*) I'm saying that I don't know if (pseudo syntax): Γ, 'a0, …, T0, … ⊢ dyn for<'h0, …> Trait<P0('a0), …, Q0(T0), …, R0('h0), …> + 'r
'r ∊ {'static, 'a0, …, 'h0, …}
Pn, Qm, Ro parametrized lifetime/type
-------------------------------------------------------------------------------------
Γ, 'a0, …, T0, … ⊢ dyn for<'h0, …> Trait<P0('a0), …, Q0(T0), …, R0('h0), …> + 'r : 'rSo if |
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☔ The latest upstream changes (presumably #131511) made this pull request unmergeable. Please resolve the merge conflicts. |
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impl dyn Trait {}has an implicitdyn Trait: 'staticbound. We add it ininferred_outlives_offor more context in errors. With this we point atimpl dyn Traitand not to it's associated functions, deduplicating errors to a single one perimpl dyn Traitinstead of one for it plus one for each associated function.CC #83246, as this is necessary but not sufficient to help with the lack of tracking of
'staticobligations.