Rollup merge of #121044 - compiler-errors:mbe-async-trait-bounds, r=fmease

Support async trait bounds in macros

r? fmease

This is similar to your work on const trait bounds. This theoretically regresses `impl async $ident:ident` in macros, but I doubt this is occurring in practice.

tests/ui/async-await/async-fn/impl-header.rs

@@ -3,6 +3,10 @@
 struct F;
 
 impl async Fn<()> for F {}
-//~^ ERROR expected type, found keyword `async`
+//~^ ERROR `async` trait implementations are unsupported
+//~| ERROR the precise format of `Fn`-family traits' type parameters is subject to change
+//~| ERROR manual implementations of `Fn` are experimental
+//~| ERROR expected a `FnMut()` closure, found `F`
+//~| ERROR not all trait items implemented, missing: `call`
 
 fn main() {}

tests/ui/async-await/async-fn/impl-header.stderr

@@ -1,8 +1,47 @@
-error: expected type, found keyword `async`
+error: `async` trait implementations are unsupported
   --> $DIR/impl-header.rs:5:6
    |
 LL | impl async Fn<()> for F {}
-   |      ^^^^^ expected type
+   |      ^^^^^
 
-error: aborting due to 1 previous error
+error[E0658]: the precise format of `Fn`-family traits' type parameters is subject to change
+  --> $DIR/impl-header.rs:5:12
+   |
+LL | impl async Fn<()> for F {}
+   |            ^^^^^^
+   |
+   = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
+   = help: add `#![feature(unboxed_closures)]` 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+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date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[E0183]: manual implementations of `Fn` are experimental
+  --> $DIR/impl-header.rs:5:12
+   |
+LL | impl async Fn<()> for F {}
+   |            ^^^^^^ manual implementations of `Fn` are experimental
+   |
+   = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
+
+error[E0277]: expected a `FnMut()` closure, found `F`
+  --> $DIR/impl-header.rs:5:23
+   |
+LL | impl async Fn<()> for F {}
+   |                       ^ expected an `FnMut()` closure, found `F`
+   |
+   = help: the trait `FnMut<()>` is not implemented for `F`
+   = note: wrap the `F` in a closure with no arguments: `|| { /* code */ }`
+note: required by a bound in `Fn`
+  --> $SRC_DIR/core/src/ops/function.rs:LL:COL
+
+error[E0046]: not all trait items implemented, missing: `call`
+  --> $DIR/impl-header.rs:5:1
+   |
+LL | impl async Fn<()> for F {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^ missing `call` in implementation
+   |
+   = help: implement the missing item: `fn call(&self, _: ()) -> <Self as FnOnce<()>>::Output { todo!() }`
+
+error: aborting due to 5 previous errors
+
+Some errors have detailed explanations: E0046, E0183, E0277, E0658.
+For more information about an error, try `rustc --explain E0046`.

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

@@ -0,0 +1,21 @@
+// Demonstrates and records a theoretical regressions / breaking changes caused by the
+// introduction of async trait bounds.
+
+// Setting the edition to 2018 since we don't regress `demo! { dyn async }` in Rust <2018.
+//@ edition:2018
+
+macro_rules! demo {
+    ($ty:ty) => { compile_error!("ty"); };
+    //~^ ERROR ty
+    //~| ERROR ty
+    (impl $c:ident Trait) => {};
+    (dyn $c:ident Trait) => {};
+}
+
+demo! { impl async Trait }
+//~^ ERROR async closures are unstable
+
+demo! { dyn async Trait }
+//~^ ERROR async closures are unstable
+
+fn main() {}

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

@@ -0,0 +1,47 @@
+error: ty
+  --> $DIR/mbe-async-trait-bound-theoretical-regression.rs:8:19
+   |
+LL |     ($ty:ty) => { compile_error!("ty"); };
+   |                   ^^^^^^^^^^^^^^^^^^^^
+...
+LL | demo! { impl async Trait }
+   | -------------------------- in this macro invocation
+   |
+   = note: this error originates in the macro `demo` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: ty
+  --> $DIR/mbe-async-trait-bound-theoretical-regression.rs:8:19
+   |
+LL |     ($ty:ty) => { compile_error!("ty"); };
+   |                   ^^^^^^^^^^^^^^^^^^^^
+...
+LL | demo! { dyn async Trait }
+   | ------------------------- in this macro invocation
+   |
+   = 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
+  --> $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
+   = 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 {`
+
+error[E0658]: async closures 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
+   = 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 {`
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

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

@@ -0,0 +1,12 @@
+//@ edition: 2021
+
+macro_rules! x {
+    ($x:item) => {}
+}
+
+x! {
+    async fn foo() -> impl async Fn() { }
+    //~^ ERROR async closures are unstable
+}
+
+fn main() {}

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

@@ -0,0 +1,14 @@
+error[E0658]: async closures 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
+   = 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 {`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.