Rollup merge of rust-lang#132612 - compiler-errors:async-trait-bounds, r=lcnr

Gate async fn trait bound modifier on `async_trait_bounds`

This PR moves `async Fn()` trait bounds into a new feature gate: `feature(async_trait_bounds)`. The general vibe is that we will most likely stabilize the `feature(async_closure)` *without* the `async Fn()` trait bound modifier, so we need to gate that separately.

We're trying to work on the general vision of `async` trait bound modifier general in: rust-lang/rfcs#3710, however that RFC still needs more time for consensus to converge, and we've decided that the value that users get from calling the bound `async Fn()` is *not really* worth blocking landing async closures in general.

Author: matthiaskrgr

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -516,6 +516,11 @@ 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

@@ -394,6 +394,8 @@ 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

@@ -940,7 +940,7 @@ 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()
@@ -951,7 +951,7 @@ 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

@@ -468,6 +468,7 @@ symbols! {
         async_for_loop,
         async_iterator,
         async_iterator_poll_next,
+        async_trait_bounds,
         atomic,
         atomic_mod,
         atomics,

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

@@ -794,7 +794,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                     closure_def_id,
                     found_kind,
                     expected_kind,
-                    "async ",
+                    "Async",
                 );
                 self.note_obligation_cause(&mut err, &obligation);
                 self.point_at_returns_when_relevant(&mut err, &obligation);

src/tools/miri/tests/pass/async-closure-captures.rs

@@ -1,6 +1,6 @@
 // Same as rustc's `tests/ui/async-await/async-closures/captures.rs`, keep in sync
 
-#![feature(async_closure, noop_waker)]
+#![feature(async_closure, noop_waker, async_trait_bounds)]
 
 use std::future::Future;
 use std::pin::pin;

src/tools/miri/tests/pass/async-closure-drop.rs

@@ -1,4 +1,4 @@
-#![feature(async_closure, noop_waker, async_fn_traits)]
+#![feature(async_closure, noop_waker, async_trait_bounds)]
 
 use std::future::Future;
 use std::pin::pin;

tests/codegen/async-closure-debug.rs

@@ -9,7 +9,7 @@
 
 #![feature(async_closure)]
 
-fn async_closure_test(upvar: &str) -> impl async Fn() + '_ {
+fn async_closure_test(upvar: &str) -> impl AsyncFn() + '_ {
     async move || {
         let hello = String::from("hello");
         println!("{hello}, {upvar}");

tests/coverage/async_closure.cov-map

@@ -1,19 +1,19 @@
 Function name: async_closure::call_once::<async_closure::main::{closure#0}>
-Raw bytes (9): 0x[01, 01, 00, 01, 01, 07, 01, 00, 2c]
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 07, 01, 00, 2b]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 1
-- Code(Counter(0)) at (prev + 7, 1) to (start + 0, 44)
+- Code(Counter(0)) at (prev + 7, 1) to (start + 0, 43)
 Highest counter ID seen: c0
 
 Function name: async_closure::call_once::<async_closure::main::{closure#0}>::{closure#0}
-Raw bytes (14): 0x[01, 01, 00, 02, 01, 07, 2c, 01, 0e, 05, 02, 01, 00, 02]
+Raw bytes (14): 0x[01, 01, 00, 02, 01, 07, 2b, 01, 0e, 05, 02, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 2
-- Code(Counter(0)) at (prev + 7, 44) to (start + 1, 14)
+- Code(Counter(0)) at (prev + 7, 43) to (start + 1, 14)
 - Code(Counter(1)) at (prev + 2, 1) to (start + 0, 2)
 Highest counter ID seen: c1
 

tests/coverage/async_closure.coverage

@@ -4,7 +4,7 @@
    LL|       |//@ aux-build: executor.rs
    LL|       |extern crate executor;
    LL|       |
-   LL|      1|async fn call_once(f: impl async FnOnce()) {
+   LL|      1|async fn call_once(f: impl AsyncFnOnce()) {
    LL|      1|    f().await;
    LL|      1|}
    LL|       |

tests/coverage/async_closure.rs

@@ -4,7 +4,7 @@
 //@ aux-build: executor.rs
 extern crate executor;
 
-async fn call_once(f: impl async FnOnce()) {
+async fn call_once(f: impl AsyncFnOnce()) {
     f().await;
 }
 

tests/crashes/124020.rs

@@ -1,7 +1,7 @@
 //@ known-bug: #124020
 //@ compile-flags: -Zpolymorphize=on --edition=2018 --crate-type=lib
 
-#![feature(async_closure, noop_waker, async_fn_traits)]
+#![feature(async_closure, noop_waker, async_trait_bounds)]
 
 use std::future::Future;
 use std::pin::pin;
@@ -19,7 +19,7 @@ pub fn block_on<T>(fut: impl Future<Output = T>) -> T {
     }
 }
 
-async fn call_once(f: impl async FnOnce(DropMe)) {
+async fn call_once(f: impl AsyncFnOnce(DropMe)) {
     f(DropMe("world")).await;
 }
 

tests/ui/async-await/async-closures/async-fn-mut-for-async-fn.rs

@@ -10,7 +10,7 @@ fn main() {
     block_on::block_on(async {
         let x = async || {};
 
-        async fn needs_async_fn_mut(mut x: impl async FnMut()) {
+        async fn needs_async_fn_mut(mut x: impl AsyncFnMut()) {
             x().await;
         }
         needs_async_fn_mut(x).await;

tests/ui/async-await/async-closures/async-fn-once-for-async-fn.rs

@@ -8,7 +8,7 @@ extern crate block_on;
 
 fn main() {
     block_on::block_on(async {
-        async fn needs_async_fn_once(x: impl async FnOnce()) {
+        async fn needs_async_fn_once(x: impl AsyncFnOnce()) {
             x().await;
         }
 

tests/ui/async-await/async-closures/auxiliary/foreign.rs

@@ -2,6 +2,6 @@
 
 #![feature(async_closure)]
 
-pub fn closure() -> impl async Fn() {
+pub fn closure() -> impl AsyncFn() {
     async || { /* Don't really need to do anything here. */ }
 }

tests/ui/async-await/async-closures/body-check-on-non-fnmut.rs

@@ -11,7 +11,7 @@ extern crate block_on;
 
 async fn empty() {}
 
-pub async fn call_once<F: async FnOnce()>(f: F) {
+pub async fn call_once<F: AsyncFnOnce()>(f: F) {
     f().await;
 }
 

tests/ui/async-await/async-closures/box-deref-in-debuginfo.rs

@@ -16,7 +16,7 @@ impl Trait for (i32,) {
     }
 }
 
-async fn call_once(f: impl async FnOnce()) {
+async fn call_once(f: impl AsyncFnOnce()) {
     f().await;
 }
 

tests/ui/async-await/async-closures/brand.rs

@@ -13,7 +13,7 @@ struct S;
 struct B<'b>(PhantomData<&'b mut &'b mut ()>);
 
 impl S {
-    async fn q<F: async Fn(B<'_>)>(self, f: F) {
+    async fn q<F: AsyncFn(B<'_>)>(self, f: F) {
         f(B(PhantomData)).await;
     }
 }

tests/ui/async-await/async-closures/captures.rs

@@ -13,11 +13,11 @@ fn main() {
     block_on::block_on(async_main());
 }
 
-async fn call<T>(f: &impl async Fn() -> T) -> T {
+async fn call<T>(f: &impl AsyncFn() -> T) -> T {
     f().await
 }
 
-async fn call_once<T>(f: impl async FnOnce() -> T) -> T {
+async fn call_once<T>(f: impl AsyncFnOnce() -> T) -> T {
     f().await
 }
 
@@ -80,7 +80,7 @@ async fn async_main() {
         call_once(c).await;
     }
 
-    fn force_fnonce<T>(f: impl async FnOnce() -> T) -> impl async FnOnce() -> T {
+    fn force_fnonce<T>(f: impl AsyncFnOnce() -> T) -> impl AsyncFnOnce() -> T {
         f
     }
 

tests/ui/async-await/async-closures/clone-closure.rs

@@ -7,7 +7,7 @@
 
 extern crate block_on;
 
-async fn for_each(f: impl async FnOnce(&str) + Clone) {
+async fn for_each(f: impl AsyncFnOnce(&str) + Clone) {
     f.clone()("world").await;
     f.clone()("world2").await;
 }

tests/ui/async-await/async-closures/constrained-but-no-upvars-yet.rs

@@ -6,15 +6,15 @@
 
 #![feature(async_closure)]
 
-fn constrain<T: async FnOnce()>(t: T) -> T {
+fn constrain<T: AsyncFnOnce()>(t: T) -> T {
     t
 }
 
 fn call_once<T>(f: impl FnOnce() -> T) -> T {
     f()
 }
 
-async fn async_call_once<T>(f: impl async FnOnce() -> T) -> T {
+async fn async_call_once<T>(f: impl AsyncFnOnce() -> T) -> T {
     f().await
 }
 

tests/ui/async-await/async-closures/debuginfo-by-move-body.rs

@@ -7,7 +7,7 @@
 
 extern crate block_on;
 
-async fn call_once(f: impl async FnOnce()) {
+async fn call_once(f: impl AsyncFnOnce()) {
     f().await;
 }
 

tests/ui/async-await/async-closures/drop.rs

@@ -16,7 +16,7 @@ impl Drop for DropMe {
     }
 }
 
-async fn call_once(f: impl async FnOnce()) {
+async fn call_once(f: impl AsyncFnOnce()) {
     println!("before call");
     let fut = Box::pin(f());
     println!("after call");

tests/ui/async-await/async-closures/fn-exception-target-features.rs

@@ -10,7 +10,7 @@ use std::future::Future;
 #[target_feature(enable = "sse2")]
 fn target_feature()  -> Pin<Box<dyn Future<Output = ()> + 'static>> { todo!() }
 
-fn test(f: impl async Fn()) {}
+fn test(f: impl AsyncFn()) {}
 
 fn main() {
     test(target_feature); //~ ERROR the trait bound

tests/ui/async-await/async-closures/fn-exception-target-features.stderr

@@ -9,8 +9,8 @@ LL |     test(target_feature);
 note: required by a bound in `test`
   --> $DIR/fn-exception-target-features.rs:13:17
    |
-LL | fn test(f: impl async Fn()) {}
-   |                 ^^^^^^^^^^ required by this bound in `test`
+LL | fn test(f: impl AsyncFn()) {}
+   |                 ^^^^^^^^^ required by this bound in `test`
 
 error: aborting due to 1 previous error
 

tests/ui/async-await/async-closures/fn-exception.rs

@@ -13,7 +13,7 @@ unsafe extern "C" {
     pub safe fn abi() -> Pin<Box<dyn Future<Output = ()> + 'static>>;
 }
 
-fn test(f: impl async Fn()) {}
+fn test(f: impl AsyncFn()) {}
 
 fn main() {
     test(unsafety); //~ ERROR the trait bound

tests/ui/async-await/async-closures/fn-exception.stderr

@@ -9,8 +9,8 @@ LL |     test(unsafety);
 note: required by a bound in `test`
   --> $DIR/fn-exception.rs:16:17
    |
-LL | fn test(f: impl async Fn()) {}
-   |                 ^^^^^^^^^^ required by this bound in `test`
+LL | fn test(f: impl AsyncFn()) {}
+   |                 ^^^^^^^^^ required by this bound in `test`
 
 error[E0277]: the trait bound `extern "C" fn() -> Pin<Box<(dyn Future<Output = ()> + 'static)>> {abi}: AsyncFn()` is not satisfied
   --> $DIR/fn-exception.rs:20:10
@@ -23,8 +23,8 @@ LL |     test(abi);
 note: required by a bound in `test`
   --> $DIR/fn-exception.rs:16:17
    |
-LL | fn test(f: impl async Fn()) {}
-   |                 ^^^^^^^^^^ required by this bound in `test`
+LL | fn test(f: impl AsyncFn()) {}
+   |                 ^^^^^^^^^ required by this bound in `test`
 
 error: aborting due to 2 previous errors
 

tests/ui/async-await/async-closures/force-move-due-to-inferred-kind.rs

@@ -6,7 +6,7 @@
 
 extern crate block_on;
 
-fn force_fnonce<T: async FnOnce()>(t: T) -> T { t }
+fn force_fnonce<T: AsyncFnOnce()>(t: T) -> T { t }
 
 fn main() {
     block_on::block_on(async {

tests/ui/async-await/async-closures/foreign.rs

@@ -12,7 +12,7 @@ extern crate foreign;
 
 struct NoCopy;
 
-async fn call_once(f: impl async FnOnce()) {
+async fn call_once(f: impl AsyncFnOnce()) {
     f().await;
 }
 

tests/ui/async-await/async-closures/implements-fnmut.rs

@@ -1,7 +1,7 @@
 //@ build-pass
 //@ edition: 2021
 
-// Demonstrates that an async closure may implement `FnMut` (not just `async FnMut`!)
+// Demonstrates that an async closure may implement `FnMut` (not just `AsyncFnMut`!)
 // if it has no self-borrows. In this case, `&Ty` is not borrowed from the closure env,
 // since it's fine to reborrow it with its original lifetime. See the doc comment on
 // `should_reborrow_from_env_of_parent_coroutine_closure` for more detail for when we

tests/ui/async-await/async-closures/inline-body.rs

@@ -24,7 +24,7 @@ pub fn block_on<T>(fut: impl Future<Output = T>) -> T {
     }
 }
 
-async fn call_once<T>(f: impl async FnOnce() -> T) -> T {
+async fn call_once<T>(f: impl AsyncFnOnce() -> T) -> T {
     f().await
 }
 

tests/ui/async-await/async-closures/mangle.rs

@@ -13,11 +13,11 @@ use std::future::Future;
 use std::pin::pin;
 use std::task::*;
 
-async fn call_mut(f: &mut impl async FnMut()) {
+async fn call_mut(f: &mut impl AsyncFnMut()) {
     f().await;
 }
 
-async fn call_once(f: impl async FnOnce()) {
+async fn call_once(f: impl AsyncFnOnce()) {
     f().await;
 }
 

tests/ui/async-await/async-closures/moro-example.rs

@@ -22,7 +22,7 @@ impl<'scope, 'env: 'scope> Scope<'scope, 'env> {
 
 fn scope_with_closure<'env, B>(_body: B) -> BoxFuture<'env, ()>
 where
-    for<'scope> B: async FnOnce(&'scope Scope<'scope, 'env>),
+    for<'scope> B: AsyncFnOnce(&'scope Scope<'scope, 'env>),
 {
     todo!()
 }

tests/ui/async-await/async-closures/move-is-async-fn.rs

@@ -19,7 +19,7 @@ fn main() {
         is_static(&c);
 
         // Check that `<{async fn} as AsyncFnOnce>::CallOnceFuture` owns its captures.
-        fn call_once<F: async FnOnce()>(f: F) -> F::CallOnceFuture { f() }
+        fn call_once<F: AsyncFnOnce()>(f: F) -> F::CallOnceFuture { f() }
         is_static(&call_once(c));
     });
 }

tests/ui/async-await/async-closures/mut-ref-reborrow.rs

@@ -9,7 +9,7 @@
 
 extern crate block_on;
 
-async fn call_once(f: impl async FnOnce()) { f().await; }
+async fn call_once(f: impl AsyncFnOnce()) { f().await; }
 
 pub async fn async_closure(x: &mut i32) {
     let c = async move || {

tests/ui/async-await/async-closures/no-borrow-from-env.rs

@@ -5,7 +5,7 @@
 
 fn outlives<'a>(_: impl Sized + 'a) {}
 
-async fn call_once(f: impl async FnOnce()) {
+async fn call_once(f: impl AsyncFnOnce()) {
     f().await;
 }