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.

lcnr

pls add PR description providing an outline of the approach

apart from that, a few minor nits, after this r=me :3

Thank you for working on this ❤️

compiler/rustc_middle/src/ty/context.rs

compiler/rustc_next_trait_solver/src/solve/mod.rs

compiler/rustc_trait_selection/src/traits/fulfill.rs

lcnr · 2025-07-01T11:33:55Z

compiler/rustc_trait_selection/src/traits/fulfill.rs

+                    // enabled that we do not.
+                    //
+                    // Note: we don't consider a feature to be enabled
+                    // if we are in std/core even if there is a corresponding `feature` attribute on the crate.


future work: we likely want an #[rustc_allow_internal_unstable(foo)] attribute which only adds UnstableFeature clauses to the param_env, but not predicates_of.

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

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

tests/ui/unstable-feature_bound/unstable-feature-cross-crate-require-bound.rs

tests/ui/unstable-feature_bound/unstable_feature_bound_multi_attr.rs

tests/ui/unstable-feature_bound/auxiliary/unstable_impl_codegen_aux2.rs

tests/ui/unstable-feature_bound/unstable_impl_coherence.rs

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

compiler/rustc_trait_selection/src/error_reporting/traits/ambiguity.rs

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

lcnr · 2025-07-03T14:30:17Z

compiler/rustc_trait_selection/src/error_reporting/traits/ambiguity.rs

+
+                let mut err = self.dcx().struct_span_err(
+                    span,
+                    format!("unstable feature `{sym}` is used without being enabled."),


I would try to mirror the existing errors for unstable library features if staged_api is not enabled

error[E0658]: use of unstable library feature `slice_partition_dedup` --> src/main.rs:3:7 | 3 | x.partition_dedup(); | ^^^^^^^^^^^^^^^ | = note: see issue #54279 <https://github.com/rust-lang/rust/issues/54279> for more information = help: add `#![feature(slice_partition_dedup)]` to the crate attributes to enable = note: this compiler was built on 2025-07-02; consider upgrading it if it is out of date

not sure whehter you could even reuse the reporting machinery we already use for methods etc

This diagnostics can be reported through feature_err_issue, but it requires the issue number of the library feature. Unfortunately we don't have issue number information when we emit ambiguity error, and putting issue number into #[unstable_feature_bound] can be quite verbose because there can be multiple symbol at a time, for example: #[unstable_feature_bound(foo, bar)]

So I added feature_err_unstable_feature_bound in cf4770d, which is basically feature_err_issue but omitted the issue number note.

tests/ui/unstable-feature_bound/unstable-feature-bound-two-error.rs

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

lcnr · 2025-07-08T14:52:20Z

compiler/rustc_next_trait_solver/src/solve/mod.rs

+        if self.may_use_unstable_feature(param_env, symbol) {
+            return self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes);
+        } else {
+            return self.evaluate_added_goals_and_make_canonical_response(Certainty::Maybe(
+                MaybeCause::Ambiguity,
+            ));
+        }


Suggested change

if self.may_use_unstable_feature(param_env, symbol) {

return self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes);

} else {

return self.evaluate_added_goals_and_make_canonical_response(Certainty::Maybe(

MaybeCause::Ambiguity,

));

}

if self.may_use_unstable_feature(param_env, symbol) {

self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)

} else {

self.evaluate_added_goals_and_make_canonical_response(Certainty::Maybe(

MaybeCause::Ambiguity,

))

}

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?

lcnr · 2025-07-08T14:58:54Z

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

+                        return Ok(EvaluatedToOk);
+                    } else {
+                        return Ok(EvaluatedToAmbig);


Suggested change

return Ok(EvaluatedToOk);

} else {

return Ok(EvaluatedToAmbig);

Ok(EvaluatedToOk)

} else {

Ok(EvaluatedToAmbig)

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?

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.

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 1, 2025

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BoxyUwU 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 Jul 2, 2025

tiif and others added 5 commits July 2, 2025 13:19

Emit better ambiguity error for UnstableFeature

820dcf4

Use #[type_foldable(identity)] and #[type_visitable(ignore)] for Symbol

929baca

Apply suggestions from code review

e9b76a1

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

Make the test crate stable to remove the error

81deb7d

Slightly tweak the error message's wording

7f35491

BoxyUwU reviewed Jul 2, 2025

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compiler/rustc_trait_selection/src/error_reporting/traits/ambiguity.rs Outdated Show resolved Hide resolved

tiif and others added 5 commits July 3, 2025 05:28

Separate the error message non-staged-api

fcc0afd

Add explanation and test for ambiguity error

22f3a19

fmt

2e89657

style check

9ca956f

flip condition in the solver's logic

2c0e0f5

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