Gate async fn trait bound modifier on async_trait_bounds

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -515,6 +515,11 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
         "async closures are unstable",
         "to use an async block, remove the `||`: `async {`"
     );
+    gate_all!(
+        async_trait_bounds,
+        "`async` trait bounds are unstable",
+        "use the desugared name of the async trait, such as `AsyncFn`"
+    );
     gate_all!(async_for_loop, "`for await` loops are experimental");
     gate_all!(
         closure_lifetime_binder,

compiler/rustc_feature/src/unstable.rs

@@ -387,6 +387,8 @@ declare_features! (
     (unstable, async_fn_track_caller, "1.73.0", Some(110011)),
     /// Allows `for await` loops.
     (unstable, async_for_loop, "1.77.0", Some(118898)),
+    /// Allows `async` trait bound modifier.
+    (unstable, async_trait_bounds, "CURRENT_RUSTC_VERSION", Some(62290)),
     /// Allows using C-variadics.
     (unstable, c_variadic, "1.34.0", Some(44930)),
     /// Allows the use of `#[cfg(<true/false>)]`.

compiler/rustc_parse/src/parser/ty.rs

@@ -943,7 +943,7 @@ impl<'a> Parser<'a> {
         let asyncness = if self.token.uninterpolated_span().at_least_rust_2018()
             && self.eat_keyword(kw::Async)
         {
-            self.psess.gated_spans.gate(sym::async_closure, self.prev_token.span);
+            self.psess.gated_spans.gate(sym::async_trait_bounds, self.prev_token.span);
             BoundAsyncness::Async(self.prev_token.span)
         } else if self.may_recover()
             && self.token.uninterpolated_span().is_rust_2015()
@@ -954,7 +954,7 @@ impl<'a> Parser<'a> {
                 span: self.prev_token.span,
                 help: HelpUseLatestEdition::new(),
             });
-            self.psess.gated_spans.gate(sym::async_closure, self.prev_token.span);
+            self.psess.gated_spans.gate(sym::async_trait_bounds, self.prev_token.span);
             BoundAsyncness::Async(self.prev_token.span)
         } else {
             BoundAsyncness::Normal

compiler/rustc_span/src/symbol.rs

@@ -465,6 +465,7 @@ symbols! {
         async_for_loop,
         async_iterator,
         async_iterator_poll_next,
+        async_trait_bounds,
         atomic,
         atomic_mod,
         atomics,

tests/ui/async-await/async-fn/dyn-pos.rs

@@ -2,7 +2,7 @@
 
 #![feature(async_closure)]
 
-fn foo(x: &dyn async Fn()) {}
+fn foo(x: &dyn AsyncFn()) {}
 //~^ ERROR the trait `AsyncFn` cannot be made into an object
 //~| ERROR the trait `AsyncFnMut` cannot be made into an object
 //~| ERROR the trait `AsyncFnMut` cannot be made into an object

tests/ui/async-await/async-fn/dyn-pos.stderr

@@ -1,8 +1,8 @@
 error[E0038]: the trait `AsyncFnMut` cannot be made into an object
   --> $DIR/dyn-pos.rs:5:16
    |
-LL | fn foo(x: &dyn async Fn()) {}
-   |                ^^^^^^^^^^ `AsyncFnMut` cannot be made into an object
+LL | fn foo(x: &dyn AsyncFn()) {}
+   |                ^^^^^^^^^ `AsyncFnMut` cannot be made into an object
    |
 note: for a trait to be "dyn-compatible" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
   --> $SRC_DIR/core/src/ops/async_function.rs:LL:COL
@@ -16,8 +16,8 @@ note: for a trait to be "dyn-compatible" it needs to allow building a vtable to
 error[E0038]: the trait `AsyncFnMut` cannot be made into an object
   --> $DIR/dyn-pos.rs:5:16
    |
-LL | fn foo(x: &dyn async Fn()) {}
-   |                ^^^^^^^^^^ `AsyncFnMut` cannot be made into an object
+LL | fn foo(x: &dyn AsyncFn()) {}
+   |                ^^^^^^^^^ `AsyncFnMut` cannot be made into an object
    |
 note: for a trait to be "dyn-compatible" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
   --> $SRC_DIR/core/src/ops/async_function.rs:LL:COL
@@ -32,8 +32,8 @@ note: for a trait to be "dyn-compatible" it needs to allow building a vtable to
 error[E0038]: the trait `AsyncFnMut` cannot be made into an object
   --> $DIR/dyn-pos.rs:5:16
    |
-LL | fn foo(x: &dyn async Fn()) {}
-   |                ^^^^^^^^^^ `AsyncFnMut` cannot be made into an object
+LL | fn foo(x: &dyn AsyncFn()) {}
+   |                ^^^^^^^^^ `AsyncFnMut` cannot be made into an object
    |
 note: for a trait to be "dyn-compatible" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
   --> $SRC_DIR/core/src/ops/async_function.rs:LL:COL
@@ -48,8 +48,8 @@ note: for a trait to be "dyn-compatible" it needs to allow building a vtable to
 error[E0038]: the trait `AsyncFn` cannot be made into an object
   --> $DIR/dyn-pos.rs:5:12
    |
-LL | fn foo(x: &dyn async Fn()) {}
-   |            ^^^^^^^^^^^^^^ `AsyncFn` cannot be made into an object
+LL | fn foo(x: &dyn AsyncFn()) {}
+   |            ^^^^^^^^^^^^^ `AsyncFn` cannot be made into an object
    |
 note: for a trait to be "dyn-compatible" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
   --> $SRC_DIR/core/src/ops/async_function.rs:LL:COL

tests/ui/async-await/async-fn/edition-2015.rs

@@ -1,8 +1,8 @@
 fn foo(x: impl async Fn()) -> impl async Fn() { x }
 //~^ ERROR `async` trait bounds are only allowed in Rust 2018 or later
 //~| ERROR `async` trait bounds are only allowed in Rust 2018 or later
-//~| ERROR async closures are unstable
-//~| ERROR async closures are unstable
+//~| ERROR `async` trait bounds are unstable
+//~| ERROR `async` trait bounds are unstable
 //~| ERROR use of unstable library feature 'async_closure'
 //~| ERROR use of unstable library feature 'async_closure'
 

tests/ui/async-await/async-fn/edition-2015.stderr

@@ -16,27 +16,27 @@ LL | fn foo(x: impl async Fn()) -> impl async Fn() { x }
    = help: pass `--edition 2021` to `rustc`
    = note: for more on editions, read https://doc.rust-lang.org/edition-guide
 
-error[E0658]: async closures are unstable
+error[E0658]: `async` trait bounds are unstable
   --> $DIR/edition-2015.rs:1:16
    |
 LL | fn foo(x: impl async Fn()) -> impl async Fn() { x }
    |                ^^^^^
    |
    = note: see issue #62290 <https://github.com/rust-lang/rust/issues/62290> for more information
-   = help: add `#![feature(async_closure)]` to the crate attributes to enable
+   = help: add `#![feature(async_trait_bounds)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-   = help: to use an async block, remove the `||`: `async {`
+   = help: use the desugared name of the async trait, such as `AsyncFn`
 
-error[E0658]: async closures are unstable
+error[E0658]: `async` trait bounds are unstable
   --> $DIR/edition-2015.rs:1:36
    |
 LL | fn foo(x: impl async Fn()) -> impl async Fn() { x }
    |                                    ^^^^^
    |
    = note: see issue #62290 <https://github.com/rust-lang/rust/issues/62290> for more information
-   = help: add `#![feature(async_closure)]` to the crate attributes to enable
+   = help: add `#![feature(async_trait_bounds)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-   = help: to use an async block, remove the `||`: `async {`
+   = help: use the desugared name of the async trait, such as `AsyncFn`
 
 error[E0658]: use of unstable library feature 'async_closure'
   --> $DIR/edition-2015.rs:1:42

tests/ui/async-await/async-fn/mbe-async-trait-bound-theoretical-regression.rs

@@ -13,9 +13,9 @@ macro_rules! demo {
 }
 
 demo! { impl async Trait }
-//~^ ERROR async closures are unstable
+//~^ ERROR `async` trait bounds are unstable
 
 demo! { dyn async Trait }
-//~^ ERROR async closures are unstable
+//~^ ERROR `async` trait bounds are unstable
 
 fn main() {}

tests/ui/async-await/async-fn/mbe-async-trait-bound-theoretical-regression.stderr

@@ -20,27 +20,27 @@ LL | demo! { dyn async Trait }
    |
    = note: this error originates in the macro `demo` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error[E0658]: async closures are unstable
+error[E0658]: `async` trait bounds are unstable
   --> $DIR/mbe-async-trait-bound-theoretical-regression.rs:15:14
    |
 LL | demo! { impl async Trait }
    |              ^^^^^
    |
    = note: see issue #62290 <https://github.com/rust-lang/rust/issues/62290> for more information
-   = help: add `#![feature(async_closure)]` to the crate attributes to enable
+   = help: add `#![feature(async_trait_bounds)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-   = help: to use an async block, remove the `||`: `async {`
+   = help: use the desugared name of the async trait, such as `AsyncFn`
 
-error[E0658]: async closures are unstable
+error[E0658]: `async` trait bounds are unstable
   --> $DIR/mbe-async-trait-bound-theoretical-regression.rs:18:13
    |
 LL | demo! { dyn async Trait }
    |             ^^^^^
    |
    = note: see issue #62290 <https://github.com/rust-lang/rust/issues/62290> for more information
-   = help: add `#![feature(async_closure)]` to the crate attributes to enable
+   = help: add `#![feature(async_trait_bounds)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-   = help: to use an async block, remove the `||`: `async {`
+   = help: use the desugared name of the async trait, such as `AsyncFn`
 
 error: aborting due to 4 previous errors
 

tests/ui/async-await/async-fn/not-a-trait.rs

@@ -1,6 +1,6 @@
 //@ edition:2018
 
-#![feature(async_closure)]
+#![feature(async_trait_bounds)]
 
 struct S;
 

tests/ui/async-await/async-fn/sugar.rs

@@ -1,7 +1,7 @@
 //@ edition: 2021
 //@ check-pass
 
-#![feature(async_closure)]
+#![feature(async_closure, async_trait_bounds)]
 
 async fn foo() {}
 

tests/ui/async-await/async-fn/trait-bounds-in-macro.rs

@@ -6,7 +6,7 @@ macro_rules! x {
 
 x! {
     async fn foo() -> impl async Fn() { }
-    //~^ ERROR async closures are unstable
+    //~^ ERROR `async` trait bounds are unstable
 }
 
 fn main() {}

tests/ui/async-await/async-fn/trait-bounds-in-macro.stderr

@@ -1,13 +1,13 @@
-error[E0658]: async closures are unstable
+error[E0658]: `async` trait bounds are unstable
   --> $DIR/trait-bounds-in-macro.rs:8:28
    |
 LL |     async fn foo() -> impl async Fn() { }
    |                            ^^^^^
    |
    = note: see issue #62290 <https://github.com/rust-lang/rust/issues/62290> for more information
-   = help: add `#![feature(async_closure)]` to the crate attributes to enable
+   = help: add `#![feature(async_trait_bounds)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-   = help: to use an async block, remove the `||`: `async {`
+   = help: use the desugared name of the async trait, such as `AsyncFn`
 
 error: aborting due to 1 previous error
 

tests/ui/async-await/async-fn/wrong-trait.rs

@@ -1,6 +1,6 @@
 //@ edition:2018
 
-#![feature(async_closure)]
+#![feature(async_trait_bounds)]
 
 trait Foo {}
 

tests/ui/impl-trait/precise-capturing/bound-modifiers.rs

@@ -9,7 +9,7 @@ fn polarity() -> impl Sized + ?use<> {}
 fn asyncness() -> impl Sized + async use<> {}
 //~^ ERROR expected identifier, found keyword `use`
 //~| ERROR cannot find trait `r#use` in this scope
-//~| ERROR async closures are unstable
+//~| ERROR `async` trait bounds are unstable
 
 fn constness() -> impl Sized + const use<> {}
 //~^ ERROR expected identifier, found keyword `use`

tests/ui/impl-trait/precise-capturing/bound-modifiers.stderr

@@ -46,16 +46,16 @@ error[E0405]: cannot find trait `r#use` in this scope
 LL | fn binder() -> impl Sized + for<'a> use<> {}
    |                                     ^^^ not found in this scope
 
-error[E0658]: async closures are unstable
+error[E0658]: `async` trait bounds are unstable
   --> $DIR/bound-modifiers.rs:9:32
    |
 LL | fn asyncness() -> impl Sized + async use<> {}
    |                                ^^^^^
    |
    = note: see issue #62290 <https://github.com/rust-lang/rust/issues/62290> for more information
-   = help: add `#![feature(async_closure)]` to the crate attributes to enable
+   = help: add `#![feature(async_trait_bounds)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-   = help: to use an async block, remove the `||`: `async {`
+   = help: use the desugared name of the async trait, such as `AsyncFn`
 
 error[E0658]: const trait impls are experimental
   --> $DIR/bound-modifiers.rs:14:32

tests/ui/parser/bad-recover-kw-after-impl.rs

@@ -12,6 +12,6 @@ macro_rules! impl_primitive {
 
 impl_primitive!(impl async);
 //~^ ERROR expected identifier, found `<eof>`
-//~| ERROR async closures are unstable
+//~| ERROR `async` trait bounds are unstable
 
 fn main() {}

tests/ui/parser/bad-recover-kw-after-impl.stderr

@@ -7,16 +7,16 @@ LL |     ($ty:ty) => {
 LL | impl_primitive!(impl async);
    |                      ^^^^^ expected identifier
 
-error[E0658]: async closures are unstable
+error[E0658]: `async` trait bounds are unstable
   --> $DIR/bad-recover-kw-after-impl.rs:13:22
    |
 LL | impl_primitive!(impl async);
    |                      ^^^^^
    |
    = note: see issue #62290 <https://github.com/rust-lang/rust/issues/62290> for more information
-   = help: add `#![feature(async_closure)]` to the crate attributes to enable
+   = help: add `#![feature(async_trait_bounds)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-   = help: to use an async block, remove the `||`: `async {`
+   = help: use the desugared name of the async trait, such as `AsyncFn`
 
 error: aborting due to 2 previous errors