Trait aliases: Also imply default trait bounds on type params other than `Self` by fmease · Pull Request #152688 · rust-lang/rust

fmease · 2026-02-16T02:08:04Z

Trait aliases already correctly imply default trait bounds on Self type params. However, due to an oversight, they didn't do that for normal type params.

Fixes #152687.

fmease · 2026-02-16T02:08:38Z

@bors try @rust-timer queue

[WIP] Trait aliases: Imply default trait bounds

rust-bors · 2026-02-16T04:19:24Z

☀️ Try build successful (CI)
Build commit: ffd787c (ffd787cdc83d953ee66126e6f9b32b4465636845, parent: 139651428df86cf88443295542c12ea617cbb587)

rust-timer · 2026-02-16T04:59:14Z

Finished benchmarking commit (ffd787c): 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

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	mean	range	count
Regressions ❌ (primary)	2.6%	[2.6%, 2.6%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.6%	[2.6%, 2.6%]	1

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Results (primary 0.2%, secondary 2.6%)

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

	mean	range	count
Regressions ❌ (primary)	2.6%	[2.6%, 2.6%]	1
Regressions ❌ (secondary)	2.6%	[2.6%, 2.6%]	1
Improvements ✅ (primary)	-2.2%	[-2.2%, -2.2%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[-2.2%, 2.6%]	2

Binary size

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Bootstrap: 482.657s -> 481.709s (-0.20%)
Artifact size: 397.96 MiB -> 395.94 MiB (-0.51%)

fmease · 2026-02-16T15:23:55Z

My change only affects trait aliases since implied_predicates_with_filter is only ever called with filter PredicateFilter::All for trait aliases (namely in explicit_implied_predicates_of). Consequently, my PR won't affect stable behavior.

fmease · 2026-02-16T15:31:35Z

    ) -> Vec<(ty::Clause<'tcx>, Span)> {
        let mut bounds = Vec::new();

+        if let PredicateFilter::All = filter {


Predicates where the bounded type is a HIR type param obviously never qualify for PredicateFilter::{SelfOnly, SelfTraitThatDefines, SelfAndAssociatedTypeBounds, SelfConstIfConst}.

Regarding PredicateFilter::{SelfConstIfConst, ConstIfConst}, (1) these shouldn't be collected here anyway (const conditions are gathered elsewhere), (2) we don't have any default const or const-if-const trait bounds at the time of writing. Sure, we might have const Sized and const MetaSized in the future (open RFC 3729 | Sized Hierarchy) but then that should be dealt with when that part gets implemented imo.

As a result, PredicateFilter::All is the only filter that makes sense.

fmease · 2026-02-16T15:34:37Z

+trait B1<T: ?Sized> =; // has a default `T: MetaSized` bound
+fn g1<T: std::marker::PointeeSized>() where (): B1<T> {}
+
+// For completeness, let's also check default trait bounds on `Self`.


We do have tests that exercise add_implicit_sizedness_bounds in implied_predicates_with_filter, namely tests/ui/sized-hierarchy/bound-on-assoc-type-projection.rs and
tests/ui/sized-hierarchy/elaboration-simple.rs. However a trait alias specific one doesn't hurt imo.

fmease · 2026-02-16T15:34:59Z

r? types

fmease · 2026-04-08T11:10:47Z

@jackh726 Friendly ping, it's been 7 weeks and 3 days. The changes are trivial btw.

jackh726

Apologies for the late review. I've been constantly trying to catch up on these.

So. do you know what PR added this implied behavior? I...almost think it's simpler/better to not imply default or explicit bounds on trait alias parameters. Perhaps, it's okay by me to land this, but I'd like to have a note of this behavior somewhere to ensure it gets revisited whenever we actually move towards trait alias stabilization.

Other, this looks good. Agreed that adding a test of the negative behavior here is good.

View changes since this review

jackh726 · 2026-04-11T23:40:24Z

+
+#![feature(trait_alias, more_maybe_bounds, lang_items, auto_traits)]
+
+#[lang = "default_trait1"]


Ha, this is funny that this exists.

fmease · 2026-04-12T12:36:04Z

I...almost think it's simpler/better to not imply default or explicit bounds on trait alias parameters. Perhaps, it's okay by me to land this, but I'd like to have a note of this behavior somewhere to ensure it gets revisited whenever we actually move towards trait alias stabilization.

I do agree that it's odd and surprising that parameter bounds get implied but I didn't feel like submitting a PR that drastically altered the semantics of trait aliases either, esp. since they're in limbo design-wise.

However, if we were to stop implying parameter bounds, I think we should stop implying any predicates whose self type isn't Self (or a projection thereof), i.e., copy the behavior of normal traits. Otherwise trait A<T> = where T: Copy; and trait A<T> = where (T,): Copy; would behave differently. Of course, that would diminish the power of trait aliases but maybe that'd be a good thing.

So. do you know what PR added this implied behavior?

No, I haven't checked yet, I might do so later.

Of course, the behavior is perfectly consistent right now (apart from default bounds) since it's very intentional I'm pretty sure that given e.g., trait A = where for<'a> i32: Copy; (i.e., arbitrary bounds) trait alias ref A implies for<'a> i32: Copy (at least the behavior is well-known by users & devs and "PredicateFilter::All" says it all). Consequently trait A<T: Bound> =; should indeed imply T: Bound under these rules since it's equivalent to trait A<T> = where T: Bound;.

(I do wonder though if trait aliases used to behave differently wrt. parameter bounds when they were first added. Maybe <T: Bound> used to behave differently from <T> where T: Bound since they used to not share the same representation as you know 🤔)

(I remember Niko expressing his disappointment in some trait alias RFC thread regarding the fact that in Rust we're syntactically conflating ensures and requires by only utilizing : and where which are context dependent; trait X<T ensures Copy> =; vs. trait X<T requires Copy> =; would be grand albeit too lengthy ofc)

rustbot · 2026-04-13T15:28:26Z

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.

…Self`

fmease · 2026-04-13T15:36:33Z

I'd like to have a note of this behavior somewhere to ensure it gets revisited whenever we actually move towards trait alias stabilization.

I'm gonna add it to the tracking issue #41517 as an unresolved question.

Added tests for the negative behavior.

@rustbot review

jackh726 · 2026-04-13T16:53:17Z

Thanks!

@bors r+

rust-bors · 2026-04-13T16:53:20Z

📌 Commit 9d627dd has been approved by jackh726

It is now in the queue for this repository.

JonathanBrouwer · 2026-04-13T16:56:19Z

@bors rollup
I think this can be rolled up, right?

Rollup of 5 pull requests Successful merges: - #149357 (Implement `-Z allow-partial-mitigations` (RFC 3855)) - #154939 (Refactor: simplify report_selection_error) - #152688 (Trait aliases: Also imply default trait bounds on type params other than `Self`) - #154352 (rustdoc: dep-info for standalone markdown inputs) - #155195 (tidy: handle `#[cfg_attr(bootstrap, doc = "...")]` in `compiler/` comments)

Rollup merge of #152688 - fmease:implied-preds-default-bounds, r=jackh726 Trait aliases: Also imply default trait bounds on type params other than `Self` Trait aliases already correctly imply default trait bounds on `Self` type params. However, due to an oversight, they didn't do that for normal type params. Fixes #152687.

fmease added the T-types Relevant to the types team, which will review and decide on the PR/issue. label Feb 16, 2026

rustbot added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Feb 16, 2026

fmease added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Feb 16, 2026

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rust-bors Bot pushed a commit that referenced this pull request Feb 16, 2026

Auto merge of #152688 - fmease:implied-preds-default-bounds, r=<try>

ffd787c

[WIP] Trait aliases: Imply default trait bounds

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fmease force-pushed the implied-preds-default-bounds branch 2 times, most recently from eff5e38 to fdb25e1 Compare February 16, 2026 02:21

fmease added the rla-silenced Silences rust-log-analyzer postings to the PR it's added on. label Feb 16, 2026

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fmease changed the title ~~[WIP] Trait aliases: Imply default trait bounds~~ [WIP] Trait aliases: Imply default trait bounds on type params other than Self Feb 16, 2026

fmease force-pushed the implied-preds-default-bounds branch 2 times, most recently from 7269e75 to f61aed1 Compare February 16, 2026 15:19

fmease changed the title ~~[WIP] Trait aliases: Imply default trait bounds on type params other than Self~~ Trait aliases: Imply default trait bounds on type params other than Self Feb 16, 2026

fmease added F-trait_alias `#![feature(trait_alias)]` and removed rla-silenced Silences rust-log-analyzer postings to the PR it's added on. labels Feb 16, 2026

fmease commented Feb 16, 2026

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rustbot assigned jackh726 Feb 16, 2026

fmease marked this pull request as ready for review February 16, 2026 15:35

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Feb 16, 2026

fmease commented Feb 16, 2026

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Comment thread tests/ui/traits/alias/default-trait-bounds.rs

fmease changed the title ~~Trait aliases: Imply default trait bounds on type params other than Self~~ Trait aliases: Also imply default trait bounds on type params other than Self Feb 16, 2026

jackh726 reviewed Apr 11, 2026

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fmease force-pushed the implied-preds-default-bounds branch from f61aed1 to dce0935 Compare April 13, 2026 15:28

Trait aliases: Imply default trait bounds on type params other than `…

9d627dd

…Self`

fmease force-pushed the implied-preds-default-bounds branch from dce0935 to 9d627dd Compare April 13, 2026 15:35

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Apr 13, 2026

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fmease mentioned this pull request Apr 13, 2026

Tracking issue for trait alias implementation (RFC 1733) #55628

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jhpratt mentioned this pull request Apr 14, 2026

Rollup of 5 pull requests #155267

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rust-bors Bot merged commit 7e40efc into rust-lang:main Apr 14, 2026
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rustbot added this to the 1.97.0 milestone Apr 14, 2026

fmease deleted the implied-preds-default-bounds branch April 14, 2026 12:16

fmease mentioned this pull request Apr 14, 2026

Tracking issue for trait aliases #41517

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		#![feature(trait_alias, more_maybe_bounds, lang_items, auto_traits)]

		#[lang = "default_trait1"]

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