Implement unstable trait impl #140399

tiif · 2025-04-28T15:13:52Z

This PR allows marking impls of stable trait with stable type as unstable.

Approach

In std/core, an impl can be marked as unstable by annotating it with #[unstable_feature_bound(feat_name)]. This will add a ClauseKind::Unstable_Feature(feat_name) to the list of predicates in predicates_of .

When an unstable impl's function is called, we will first iterate through all the goals in param_env to check if there is any ClauseKind::UnstableFeature(feat_name) in param_env.

The existence of ClauseKind::Unstable_Feature(feat_name) in param_env means an#[unstable_feature_bound(feat_name)] is present at the call site of the function, so we allow the check to succeed in this case.

If ClauseKind::UnstableFeature(feat_name) does not exist in param_env, we will still allow the check to succeed for either of the cases below:

The feature is enabled through #[feature(feat_name)] outside of std / core.
We are in codegen because we may be monomorphizing a body from an upstream crate which had an unstable feature enabled that the downstream crate do not.

For the rest of the case, it will fail with ambiguity.

Limitation

In this PR, we do not support:

using items that need #[unstable_feature_bound] within stable APIs
annotate main function with #[unstable_feature_bound]
annotate #[unstable_feature_bound] on items other than free function and impl

Acknowledgement

The design and mentoring are done by @BoxyUwU

tests/ui/internal/unstable-impl-assoc-type.rs

bors · 2025-06-10T21:33:29Z

☔ The latest upstream changes (presumably #142299) made this pull request unmergeable. Please resolve the merge conflicts.

tests/ui/unstable-feature_bound/unstable-feature-cross-crate-multiple-symbol.rs

compiler/rustc_next_trait_solver/src/solve/mod.rs

tests/ui/unstable-feature_bound/unstable_impl_coherence_inherence.rs

lcnr

a few more nits

rustbot · 2025-07-03T18:52:46Z

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

Co-authored-by: lcnr <[email protected]>

tiif · 2025-07-08T12:38:20Z

tests/ui/unstable-feature_bound/unstable_feature_bound_propagate_bound.rs

+#[stable(feature = "a", since = "1.1.1")]
+#[unstable_feature_bound(feat_bar)]
+fn bar() {
+    Bar::foo();
+}


This does not error in current implementation, we probably should lint against this usage.

as in, have an #[unstable_feature_bound(feat_bar)] on a stable item?

yeah. Please add a fixme for this somewhere

Yes, using anything annotated with #[unstable_feature_bound] will need to be enabled through #[feature], so it will no longer be stable.

added a lint in 9cb97c2

compiler/rustc_next_trait_solver/src/solve/mod.rs

lcnr · 2025-07-08T14:53:25Z

compiler/rustc_trait_selection/Cargo.toml

@@ -20,6 +20,7 @@ rustc_parse_format = { path = "../rustc_parse_format" }
 rustc_session = { path = "../rustc_session" }
 rustc_span = { path = "../rustc_span" }
 rustc_transmute = { path = "../rustc_transmute", features = ["rustc"] }
+rustc_type_ir  = {path = "../rustc_type_ir"}


please go through rustc_middle instead, it should reexport all of rustc_type_ir in ty

lcnr · 2025-07-08T14:58:43Z

compiler/rustc_trait_selection/src/traits/fulfill.rs

+                    #[allow(rustc::usage_of_type_ir_traits)]
+                    if self.selcx.infcx.may_use_unstable_feature(obligation.param_env, symbol) {
+                        return ProcessResult::Changed(Default::default());
+                    } else {
+                        return ProcessResult::Unchanged;
+                    }


Suggested change

#[allow(rustc::usage_of_type_ir_traits)]

if self.selcx.infcx.may_use_unstable_feature(obligation.param_env, symbol) {

return ProcessResult::Changed(Default::default());

} else {

return ProcessResult::Unchanged;

}

if self.selcx.infcx.may_use_unstable_feature(obligation.param_env, symbol) {

ProcessResult::Changed(Default::default());

} else {

ProcessResult::Unchanged;

}

cc @compiler-errors I don't think we should use the InferCtxtLike trait here. However, I can't think of a good place to put this.

We could make it a free function and use the reexport from rustc_middle, but 🤷 idk, also feels kinda iffy

Ya, I don't see why it shouldn't just be a free function. Why does it use InferCtxtLike today?

So I basically need a place to put may_use_unstable_feature that is accessible by old and new solvers, ideally without too much change, and InferCtxtLike just happened to work in a7eada0. But yeah, free function should work too.

I made may_use_unstable_feature a free function, https://github.com/rust-lang/rust/pull/140399/files/bd1f7d494b30b68978864a2565abe6886048402a..2eca4815ab43ffe9bd2deb6f357705a94f8dcb61 is what it looks like for now.

compiler/rustc_trait_selection/src/traits/select/mod.rs

lcnr · 2025-07-08T15:00:11Z

tests/ui/unstable-feature_bound/unstable_feature_bound_propagate_bound.rs

+#[stable(feature = "a", since = "1.1.1")]
+#[unstable_feature_bound(feat_bar)]
+fn bar() {
+    Bar::foo();
+}


as in, have an #[unstable_feature_bound(feat_bar)] on a stable item?

yeah. Please add a fixme for this somewhere

lcnr · 2025-07-08T15:01:09Z

tests/ui/unstable-feature_bound/unstable_impl_coherence_inherence.rs

@@ -0,0 +1,15 @@
+//@ aux-build:unstable_impl_coherence_inference_aux.rs


this file is called tests/ui/unstable-feature_bound/unstable_impl_coherence_inherence.rs rn, not inference. and why call it coherence 🤔 this is more about method selection, is it not?

yeah method selection is a better name, fixed :>

compiler-errors · 2025-07-08T16:29:35Z

Could you please do a perf run before this lands too? The extra call to hir_attrs in predicates_of may need to be querifyied and/or some fast path might need to be added for perf.

…he same item

Co-authored-by: lcnr <[email protected]>

tiif · 2025-07-09T13:43:46Z

Sure, I will do a rebase after lcnr finish reviewing, then request for a perf run.

tiif · 2025-07-10T12:11:01Z

@rustbot ready

hi @lcnr, this is ready for another round of review :>

rust-cloud-vms bot force-pushed the unstable_impl branch from d01f12a to 9e139e1 Compare May 23, 2025 10:26

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BoxyUwU self-assigned this May 27, 2025

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rust-cloud-vms bot force-pushed the unstable_impl branch from 486d3d3 to 02f780f Compare May 30, 2025 09:52

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BoxyUwU mentioned this pull request Jun 2, 2025

Impl stability is not checked #55436

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rust-cloud-vms bot force-pushed the unstable_impl branch from 567ec89 to 80fd239 Compare June 9, 2025 09:06

BoxyUwU reviewed Jun 10, 2025

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tests/ui/internal/unstable-impl-assoc-type.rs Outdated Show resolved Hide resolved

rust-cloud-vms bot force-pushed the unstable_impl branch from 850e3e1 to 27d9e21 Compare June 11, 2025 13:39

lcnr reviewed Jul 3, 2025

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tests/ui/unstable-feature_bound/unstable-feature-cross-crate-multiple-symbol.rs Outdated Show resolved Hide resolved

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compiler/rustc_next_trait_solver/src/solve/mod.rs Outdated Show resolved Hide resolved

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tests/ui/unstable-feature_bound/unstable_impl_coherence_inherence.rs Outdated Show resolved Hide resolved

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tiif added 2 commits July 3, 2025 18:51

Attempt to deduplicate the core logic

a7eada0

Update the lock file

8156918

tiif and others added 4 commits July 3, 2025 19:09

Manually apply comment suggestion

9f6c4f0

Improve comment for coherence inference test

0b4a751

Co-authored-by: lcnr <[email protected]>

Try to mirror existing error for non-std / core

cf4770d

Add more tests

25f21f9

tiif commented Jul 8, 2025

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lcnr reviewed Jul 8, 2025

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tiif and others added 5 commits July 9, 2025 12:26

Lint against having both #[unstable_feature_bound] and #[stable] on t…

9cb97c2

…he same item

Fixed some tests

b8fc551

Rename test dir to unstable-feature-bound

1361b66

fmt

c565255

Omit the return keyword

43f32dd

Co-authored-by: lcnr <[email protected]>

tiif added 7 commits July 9, 2025 13:45

Rename tests

88b35d1

Manually apply suggestion

bd1f7d4

Make may_use_unstable_feature a free function

5446ab7

Fix test name in comment

be3326f

fmt

cbc2cea

Remove rustc_type_ir import

a82acdc

fmt

2eca481

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 Jul 10, 2025

		@@ -0,0 +1,15 @@
		//@ aux-build:unstable_impl_coherence_inference_aux.rs

Implement unstable trait impl #140399

Are you sure you want to change the base?

Implement unstable trait impl #140399

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tiif commented Apr 28, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Approach

Limitation

Acknowledgement

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bors commented Jun 10, 2025

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compiler-errors commented Jul 8, 2025

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tiif commented Jul 9, 2025

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tiif commented Jul 10, 2025

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tiif commented Apr 28, 2025 •

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