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.

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

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.

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.

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

This comment has been minimized.

Sign in to view

BoxyUwU self-assigned this May 27, 2025

This comment has been minimized.

Sign in to view

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

This comment has been minimized.

Sign in to view

BoxyUwU mentioned this pull request Jun 2, 2025

Impl stability is not checked #55436

Open

This comment has been minimized.

Sign in to view

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

View reviewed changes

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

flip condition in the solver's logic

2c0e0f5

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