Unconditionally-const supertraits are considered not dyn compatible #145627

compiler-errors · 2025-08-19T20:33:11Z

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 #145198

r? fee1-dead

fee1-dead · 2025-08-20T05:03:56Z

compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs

+    if !tcx.features().const_trait_impl() {
+        return SmallVec::new();
+    }


This seems wrong? If I am a downstream crate that attempts to use a trait upstream that has : const Tr, then I should see that the trait isn't dyn compatible no matter whether I enable const_trait_impl.

I checked the callers of this and it looks like it's not limited to just local items.

Yeah, I kinda don't care about crate composability when it comes to nightly features, but I think it's fine to just remove this check and always run it. There isn't much of an overhead.

compiler-errors · 2025-08-20T15:47:19Z

@bors try @rust-timer queue

…r=<try> Unconditionally-const supertraits are considered not dyn compatible

rust-bors · 2025-08-20T18:04:11Z

☀️ Try build successful (CI)
Build commit: 1d5de18 (1d5de1838cfad930bfe6fbdeaed6a5b875938a5b, parent: e8a792daf500b5ff8097896ddb6cc037abe92487)

rust-timer · 2025-08-20T19:15:09Z

Finished benchmarking commit (1d5de18): 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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Results (primary 1.6%, secondary -2.4%)

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	mean	range	count
Regressions ❌ (primary)	1.6%	[1.6%, 1.6%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.4%	[-2.4%, -2.4%]	1
All ❌✅ (primary)	1.6%	[1.6%, 1.6%]	1

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Results (secondary 3.0%)

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)	3.0%	[2.9%, 3.1%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

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Bootstrap: 471.66s -> 471.441s (-0.05%)
Artifact size: 378.20 MiB -> 378.13 MiB (-0.02%)

fee1-dead · 2025-08-21T01:55:49Z

@bors r+ rollup

bors · 2025-08-21T01:55:52Z

📌 Commit e57e5f0 has been approved by fee1-dead

It is now in the queue for this repository.

…yn-compat, r=fee1-dead 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 rust-lang#145198 r? fee1-dead

Rollup of 19 pull requests Successful merges: - #143383 (stabilize `const_array_each_ref`) - #144758 ([Doc] Add links to the various collections) - #144915 (Defer tail call ret ty equality to check_tail_calls) - #145256 (Add new `--test-codegen-backend` bootstrap option) - #145297 (fix(debuginfo): handle false positives in overflow check) - #145390 (Shorten some dependency chains in the compiler) - #145415 (std_detect: RISC-V: implement implication to "C") - #145525 (stdlib: Replace typedef -> type alias in doc comment) - #145590 (Prevent impossible combinations in `ast::ModKind`.) - #145593 (UnsafePinned::raw_get: sync signature with get) - #145621 (Fix some doc typos) - #145627 (Unconditionally-const supertraits are considered not dyn compatible) - #145642 (Do not use effective_visibilities query for Adt types of a local trait while proving a where-clause) - #145650 (Fix JS search scripts path) - #145654 (Download CI GCC into the correct directory) - #145662 (Enforce correct number of arguments for `"x86-interrupt"` functions) - #145673 (Add flock support for cygwin) - #145674 (Enable triagebot `[review-changes-since]` feature) - #145678 (Fix typo in docstring) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #145627 - compiler-errors:const-supertrait-dyn-compat, r=fee1-dead 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 #145198 r? fee1-dead

rustbot assigned fee1-dead Aug 19, 2025

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Aug 19, 2025

compiler-errors force-pushed the const-supertrait-dyn-compat branch from d367474 to a65a29d Compare August 19, 2025 20:43

fee1-dead requested changes Aug 20, 2025

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rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Aug 20, 2025

Unconditionally-const supertraits are considered not dyn compatible

e57e5f0

compiler-errors force-pushed the const-supertrait-dyn-compat branch from a65a29d to e57e5f0 Compare August 20, 2025 15:47

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Auto merge of #145627 - compiler-errors:const-supertrait-dyn-compat, …

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…r=<try> Unconditionally-const supertraits are considered not dyn compatible

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Aug 20, 2025

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bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Aug 21, 2025

This was referenced Aug 21, 2025

Rollup of 20 pull requests #145697

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Rollup of 19 pull requests #145701

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bors merged commit 64c43ed into rust-lang:master Aug 21, 2025
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Unconditionally-const supertraits are considered not dyn compatible #145627

Unconditionally-const supertraits are considered not dyn compatible #145627

compiler-errors commented Aug 19, 2025

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Unconditionally-const supertraits are considered not dyn compatible #145627

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