Skip to content

Simplify the HIR ty lowering of trait object lifetime bounds #157591

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
rust-bors merged 2 commits into from
Jun 9, 2026
+27 −22
Merged

Simplify the HIR ty lowering of trait object lifetime bounds #157591

Changes from all commits
Commits
Show all changes
2 commits
Select commit Hold shift + click to select a range
207667b
Simplify the HIR ty lowering of elided trait object lifetime bounds
fmease Jun 8, 2026
6b234eb
Match on `lifetime.kind` explicitly instead of calling `lifetime.is_i…
fmease Jun 8, 2026
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
49 changes: 27 additions & 22 deletions compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_trait.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -434,36 +434,41 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {

// N.b. principal, projections, auto traits
// FIXME: This is actually wrong with multiple principals in regards to symbol mangling
let mut v = principal_trait_ref
let mut predicates = principal_trait_ref
.into_iter()
.chain(existential_projections)
.chain(auto_trait_predicates)
.collect::<SmallVec<[_; 8]>>();
v.sort_by(|a, b| a.skip_binder().stable_cmp(tcx, &b.skip_binder()));
let existential_predicates = tcx.mk_poly_existential_predicates(&v);
predicates.sort_by(|a, b| a.skip_binder().stable_cmp(tcx, &b.skip_binder()));
let predicates = tcx.mk_poly_existential_predicates(&predicates);

// Use explicitly-specified region bound, unless the bound is missing.
let region_bound = if !lifetime.is_elided() {
self.lower_lifetime(lifetime, RegionInferReason::ExplicitObjectLifetime)
let region_bound = self.lower_trait_object_lifetime(lifetime, predicates, span);

Ty::new_dynamic(tcx, predicates, region_bound)
}

fn lower_trait_object_lifetime(
&self,
lifetime: &hir::Lifetime,
predicates: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
span: Span,
) -> ty::Region<'tcx> {
// Curiously, we also use the *object region bound* for `Infer` (`'_`)
// while we obviously don't use the *object lifetime default* for it...
Comment on lines +456 to +457

@fmease fmease Jun 8, 2026 •
edited
Loading

Copy link
Copy Markdown
Member Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

For a tiny bit more background information, see the large paragraph in the PR description of rust-lang/reference#2281 (the one about compute_object_lifetime_bound vs. RBV's named_bound_var).

View changes since the review

@Kivooeo Kivooeo Jun 8, 2026

Copy link
Copy Markdown
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Thanks for pointing this out, I'll take a closer look at this reference tomorrow

if let hir::LifetimeKind::ImplicitObjectLifetimeDefault | hir::LifetimeKind::Infer =
lifetime.kind
&& let Some(region) = self.compute_object_lifetime_bound(span, predicates)
{
return region;
}

let reason = if let hir::LifetimeKind::ImplicitObjectLifetimeDefault = lifetime.kind {
RegionInferReason::ObjectLifetimeDefault(span.shrink_to_hi())
} else {
self.compute_object_lifetime_bound(span, existential_predicates).unwrap_or_else(|| {
// Curiously, we prefer object lifetime default for `+ '_`...
if tcx.named_bound_var(lifetime.hir_id).is_some() {
self.lower_lifetime(lifetime, RegionInferReason::ExplicitObjectLifetime)
} else {
let reason =
if let hir::LifetimeKind::ImplicitObjectLifetimeDefault = lifetime.kind {
RegionInferReason::ObjectLifetimeDefault(span.shrink_to_hi())
} else {
RegionInferReason::ExplicitObjectLifetime
};
self.re_infer(span, reason)
}
})
RegionInferReason::ExplicitObjectLifetime
};
debug!(?region_bound);

Ty::new_dynamic(tcx, existential_predicates, region_bound)
self.lower_lifetime(lifetime, reason)
}

/// Check that elaborating the principal of a trait ref doesn't lead to projections
Expand Down
Loading