Add gate for precise capturing in traits

compiler/rustc_ast_lowering/src/lib.rs

@@ -1573,11 +1573,20 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         // Feature gate for RPITIT + use<..>
         match origin {
             rustc_hir::OpaqueTyOrigin::FnReturn { in_trait_or_impl: Some(_), .. } => {
-                if let Some(span) = bounds.iter().find_map(|bound| match *bound {
-                    ast::GenericBound::Use(_, span) => Some(span),
-                    _ => None,
-                }) {
-                    self.tcx.dcx().emit_err(errors::NoPreciseCapturesOnRpitit { span });
+                if !self.tcx.features().precise_capturing_in_traits
+                    && let Some(span) = bounds.iter().find_map(|bound| match *bound {
+                        ast::GenericBound::Use(_, span) => Some(span),
+                        _ => None,
+                    })
+                {
+                    let mut diag =
+                        self.tcx.dcx().create_err(errors::NoPreciseCapturesOnRpitit { span });
+                    add_feature_diagnostics(
+                        &mut diag,
+                        self.tcx.sess,
+                        sym::precise_capturing_in_traits,
+                    );
+                    diag.emit();
                 }
             }
             _ => {}

compiler/rustc_feature/src/unstable.rs

@@ -565,6 +565,8 @@ declare_features! (
     (incomplete, pin_ergonomics, "CURRENT_RUSTC_VERSION", Some(130494)),
     /// Allows postfix match `expr.match { ... }`
     (unstable, postfix_match, "1.79.0", Some(121618)),
+    /// Allows `use<..>` precise capturign on impl Trait in traits.
+    (unstable, precise_capturing_in_traits, "CURRENT_RUSTC_VERSION", Some(130044)),
     /// Allows macro attributes on expressions, statements and non-inline modules.
     (unstable, proc_macro_hygiene, "1.30.0", Some(54727)),
     /// Makes `&` and `&mut` patterns eat only one layer of references in Rust 2024.

compiler/rustc_span/src/symbol.rs

@@ -1479,6 +1479,7 @@ symbols! {
         powif64,
         pre_dash_lto: "pre-lto",
         precise_capturing,
+        precise_capturing_in_traits,
         precise_pointer_size_matching,
         pref_align_of,
         prefetch_read_data,

tests/ui/feature-gates/feature-gate-precise_capturing_in_traits.rs

@@ -0,0 +1,6 @@
+trait Foo {
+    fn test() -> impl Sized + use<Self>;
+    //~^ ERROR `use<...>` precise capturing syntax is currently not allowed in return-position
+}
+
+fn main() {}

tests/ui/feature-gates/feature-gate-precise_capturing_in_traits.stderr

@@ -0,0 +1,13 @@
+error: `use<...>` precise capturing syntax is currently not allowed in return-position `impl Trait` in traits
+  --> $DIR/feature-gate-precise_capturing_in_traits.rs:2:31
+   |
+LL |     fn test() -> impl Sized + use<Self>;
+   |                               ^^^^^^^^^
+   |
+   = note: currently, return-position `impl Trait` in traits and trait implementations capture all lifetimes in scope
+   = note: see issue #130044 <https://github.com/rust-lang/rust/issues/130044> for more information
+   = help: add `#![feature(precise_capturing_in_traits)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 1 previous error
+

tests/ui/impl-trait/precise-capturing/forgot-to-capture-type.stderr

@@ -5,6 +5,9 @@ LL |     fn bar() -> impl Sized + use<>;
    |                              ^^^^^
    |
    = note: currently, return-position `impl Trait` in traits and trait implementations capture all lifetimes in scope
+   = note: see issue #130044 <https://github.com/rust-lang/rust/issues/130044> for more information
+   = help: add `#![feature(precise_capturing_in_traits)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error: `impl Trait` must mention all type parameters in scope in `use<...>`
   --> $DIR/forgot-to-capture-type.rs:1:23

tests/ui/impl-trait/precise-capturing/redundant.rpitit.stderr

@@ -5,6 +5,9 @@ LL |     fn in_trait() -> impl Sized + use<'a, Self>;
    |                                   ^^^^^^^^^^^^^
    |
    = note: currently, return-position `impl Trait` in traits and trait implementations capture all lifetimes in scope
+   = note: see issue #130044 <https://github.com/rust-lang/rust/issues/130044> for more information
+   = help: add `#![feature(precise_capturing_in_traits)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error: `use<...>` precise capturing syntax is currently not allowed in return-position `impl Trait` in traits
   --> $DIR/redundant.rs:21:35
@@ -13,6 +16,9 @@ LL |     fn in_trait() -> impl Sized + use<'a> {}
    |                                   ^^^^^^^
    |
    = note: currently, return-position `impl Trait` in traits and trait implementations capture all lifetimes in scope
+   = note: see issue #130044 <https://github.com/rust-lang/rust/issues/130044> for more information
+   = help: add `#![feature(precise_capturing_in_traits)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error: aborting due to 2 previous errors
 

tests/ui/impl-trait/precise-capturing/rpitit-captures-more-method-lifetimes.stderr

@@ -5,6 +5,9 @@ LL |     fn bar<'tr: 'tr>(&'tr mut self) -> impl Sized + use<Self>;
    |                                                     ^^^^^^^^^
    |
    = note: currently, return-position `impl Trait` in traits and trait implementations capture all lifetimes in scope
+   = note: see issue #130044 <https://github.com/rust-lang/rust/issues/130044> for more information
+   = help: add `#![feature(precise_capturing_in_traits)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error: return type captures more lifetimes than trait definition
   --> $DIR/rpitit-captures-more-method-lifetimes.rs:11:40

tests/ui/impl-trait/precise-capturing/rpitit.stderr

@@ -5,6 +5,9 @@ LL |     fn hello() -> impl PartialEq + use<Self>;
    |                                    ^^^^^^^^^
    |
    = note: currently, return-position `impl Trait` in traits and trait implementations capture all lifetimes in scope
+   = note: see issue #130044 <https://github.com/rust-lang/rust/issues/130044> for more information
+   = help: add `#![feature(precise_capturing_in_traits)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error: `impl Trait` captures lifetime parameter, but it is not mentioned in `use<...>` precise captures list
   --> $DIR/rpitit.rs:9:19

tests/ui/impl-trait/precise-capturing/self-capture.stderr

@@ -5,6 +5,9 @@ LL |     fn bar<'a>() -> impl Sized + use<Self>;
    |                                  ^^^^^^^^^
    |
    = note: currently, return-position `impl Trait` in traits and trait implementations capture all lifetimes in scope
+   = note: see issue #130044 <https://github.com/rust-lang/rust/issues/130044> for more information
+   = help: add `#![feature(precise_capturing_in_traits)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error: aborting due to 1 previous error