Further tighten up relaxed bounds #147734

fmease · 2025-10-15T18:00:03Z

Follow-up to #142693, #135331 and #135841.
Fixes #143122.

Reject relaxed bounds ?Trait in the bounds of trait aliases.
Just like trait Trait {} doesn't mean trait Trait: Sized {} and we therefore reject trait Trait: ?Sized {}, trait Trait =; (sic!) doesn't mean trait Trait = Sized; (never did!) and as logical consequence trait Trait = ?Sized; is meaningless and should be forbidden.
Don't permit ?Sized in more places (e.g., supertrait bounds, trait object types) if feature more_maybe_bounds is enabled.
That internal feature is only meant to allow the user to define & use new default traits (that have fewer rules to follow for now to ease experimentation).
Slightly improve diagnostics.
Unconditionally check that the Trait in ?Trait is a default trait.
Previously, we would only perform this check in selected places which was very brittle and led to bugs slipping through.

fmease · 2025-10-15T18:00:18Z

Further tighten up relaxed bounds

rust-bors · 2025-10-15T20:10:53Z

☀️ Try build successful (CI)
Build commit: 9ed9a73 (9ed9a7337cbbb431708bcd7085fadfe1719c28c3, parent: 28d0a4a205f9e511ad2f51ee79a4aa19a704a455)

rust-timer · 2025-10-15T23:51:40Z

Finished benchmarking commit (9ed9a73): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results (primary -3.8%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.8%	[-6.7%, -1.0%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.8%	[-6.7%, -1.0%]	2

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 474.854s -> 475.36s (0.11%)
Artifact size: 388.09 MiB -> 388.06 MiB (-0.01%)

fmease · 2025-10-16T15:38:22Z

@bors rollup-

… was enabled The internal feature `more_maybe_bounds` doesn't influence sized elaboration in HIR ty lowering and therefore doesn't get to dictate where `?Sized` is allowed.

fmease · 2025-10-16T17:24:31Z

tests/ui/sized-hierarchy/trait-aliases.rs

"Completely" unrelated to feature sized_hierarchy. Well, it must've once ICE'd during the development of sized_hierarchy and the PR that introduced it did affect stable behavior.

In any case, since ?Sized is now illegal inside trait alias bounds, this is now covered by test tests/ui/traits/alias/relaxed-bounds.rs.

fmease · 2025-10-16T17:26:28Z

tests/ui/traits/alias/empty.rs

Indeed, we did not have any UI test for trait Empty =; prior.

fmease · 2025-10-16T17:27:44Z

tests/ui/traits/alias/only-maybe-bound.rs

Superseded by tests/ui/traits/alias/effectively-empty-trait-object-type.rs.

fmease · 2025-10-16T17:28:27Z

tests/ui/traits/wf-object/only-maybe-bound.rs

Moved into tests/ui/unsized/relaxed-bounds-invalid-places.rs.

fmease · 2025-10-16T17:28:53Z

tests/ui/traits/alias/maybe-bound.rs

Superseded by tests/ui/traits/alias/relaxed-bounds.rs.

fmease · 2025-10-16T17:33:56Z

tests/ui/feature-gates/feature-gate-more-maybe-bounds.rs

+//~| ERROR bound modifier `?` can only be applied to `Sized`
+//~| ERROR bound modifier `?` can only be applied to `Sized`
+//~| ERROR bound modifier `?` can only be applied to `Sized`


Duplicate diagnostics (under -Zdeduplicate-diagnostics=no) because we lower the supertraits multiple times during HIR ty lowering likely under different predicate filters (HIR ty lowering needs to do this sort of thing in a bunch of situations to avoid query cycles).

fmease · 2025-10-16T17:35:07Z

tests/ui/trait-bounds/more_maybe_bounds.rs

(This test is just a mixed bag, always has been, and that's fine, the feature is internal and still evolving)

fmease · 2025-10-16T17:35:26Z

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

-            trait_ref.path.segments.split_last().unwrap().1.iter(),
-            GenericsArgsErrExtend::None,
-        );
+        let [leading_segments @ .., segment] = trait_ref.path.segments else { bug!() };


Drive-by refactoring

fmease · 2025-10-16T17:54:36Z

r? compiler

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rust-bors bot added a commit that referenced this pull request Oct 15, 2025

Auto merge of #147734 - fmease:tighten-relaxed, r=<try>

9ed9a73

Further tighten up relaxed bounds

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Oct 15, 2025

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Oct 15, 2025

fmease force-pushed the tighten-relaxed branch 2 times, most recently from 2453034 to 5fab794 Compare October 16, 2025 14:19

fmease added the rla-silenced Silences rust-log-analyzer postings to the PR it's added on. label Oct 16, 2025

fmease force-pushed the tighten-relaxed branch from 5fab794 to c67d187 Compare October 16, 2025 15:39

fmease removed the rla-silenced Silences rust-log-analyzer postings to the PR it's added on. label Oct 16, 2025

fmease force-pushed the tighten-relaxed branch 2 times, most recently from 79993ba to 86cd4b0 Compare October 16, 2025 17:20

fmease added 5 commits October 16, 2025 19:53

Tweak diagnostics for relaxed bounds in invalid positions

690a8a6

Reject relaxed bounds inside trait alias bounds

ce68cd3

Don't permit ?Sized in more places just because more_maybe_bounds…

aee9d3b

… was enabled The internal feature `more_maybe_bounds` doesn't influence sized elaboration in HIR ty lowering and therefore doesn't get to dictate where `?Sized` is allowed.

More robustly reject relaxing non-default trait bounds

03dfb84

Guard us against degenerate default traits

65814c8

fmease force-pushed the tighten-relaxed branch from 86cd4b0 to 65814c8 Compare October 16, 2025 17:53

fmease commented Oct 16, 2025

View reviewed changes

fmease marked this pull request as ready for review October 16, 2025 17:54

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Oct 16, 2025

rustbot assigned SparrowLii Oct 16, 2025

rustbot removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Oct 16, 2025

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rustbot assigned petrochenkov and unassigned SparrowLii Oct 16, 2025

Further tighten up relaxed bounds #147734

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Further tighten up relaxed bounds #147734

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