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Rollup merge of #145627 - compiler-errors:const-supertrait-dyn-compat, r=fee1-dead

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Unconditionally-const supertraits are considered not dyn compatible

Let's save some space in the design of const traits by making `dyn Trait` where `trait Trait: const Super` not dyn compatible.

Such a trait cannot satisfy `dyn Trait: Trait`; we could in the future make this dyn compatible but *NOT* implement `Trait`, but that's a bit weird and seems like it needs to be independently justified moving forward.

Fixes https://github.com/rust-lang/rust/issues/145198

r? fee1-dead
This commit is contained in:
Jacob Pratt
2025-08-21 01:12:21 -04:00
committed by GitHub
parent 803456aeff e57e5f02b8
commit 64c43edffe
4 changed files with 75 additions and 11 deletions
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27
compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs
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@@ -106,6 +106,10 @@ fn dyn_compatibility_violations_for_trait(
if !spans.is_empty() {
violations.push(DynCompatibilityViolation::SupertraitNonLifetimeBinder(spans));
}
let spans = super_predicates_are_unconditionally_const(tcx, trait_def_id);
if !spans.is_empty() {
violations.push(DynCompatibilityViolation::SupertraitConst(spans));
}
violations
}
@@ -247,16 +251,31 @@ fn super_predicates_have_non_lifetime_binders(
tcx: TyCtxt<'_>,
trait_def_id: DefId,
) -> SmallVec<[Span; 1]> {
// If non_lifetime_binders is disabled, then exit early
if !tcx.features().non_lifetime_binders() {
return SmallVec::new();
}
tcx.explicit_super_predicates_of(trait_def_id)
.iter_identity_copied()
.filter_map(|(pred, span)| pred.has_non_region_bound_vars().then_some(span))
.collect()
}
/// Checks for `const Trait` supertraits. We're okay with `[const] Trait`,
/// supertraits since for a non-const instantiation of that trait, the
/// conditionally-const supertrait is also not required to be const.
fn super_predicates_are_unconditionally_const(
tcx: TyCtxt<'_>,
trait_def_id: DefId,
) -> SmallVec<[Span; 1]> {
tcx.explicit_super_predicates_of(trait_def_id)
.iter_identity_copied()
.filter_map(|(pred, span)| {
if let ty::ClauseKind::HostEffect(_) = pred.kind().skip_binder() {
Some(span)
} else {
None
}
})
.collect()
}
fn trait_has_sized_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
tcx.generics_require_sized_self(trait_def_id)
}