Repository: rust
Issue: `dyn AsyncFn` generates many independent errors
Labels: A-diagnostics,T-compiler,F-async_closure,D-verbose,A-trait-objects

### Code

```rust
#![feature(async_closure)]

use std::ops::AsyncFn;

async fn foo(x: &dyn AsyncFn()) {
    x().await;
}
```

### Current output

```
error[E0038]: the trait `AsyncFnMut` cannot be made into an object
  --> src/lib.rs:5:22
   |
5  | async 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>
  --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/async_function.rs:30:10
   |
30 |     type CallRefFuture<'a>: Future<Output = Self::Output>
   |          ^^^^^^^^^^^^^ the trait cannot be made into an object because it contains the generic associated type `CallRefFuture`
   = help: the following types implement the trait, consider defining an enum where each variant holds one of these types, implementing `AsyncFnMut` for this new enum and using it instead:
             &F
             &mut F
             std::boxed::Box<F, A>

error[E0038]: the trait `AsyncFn` cannot be made into an object
  --> src/lib.rs:5:18
   |
5  | async 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>
  --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/async_function.rs:30:10
   |
30 |     type CallRefFuture<'a>: Future<Output = Self::Output>
   |          ^^^^^^^^^^^^^ the trait cannot be made into an object because it contains the generic associated type `CallRefFuture`
   = help: the following types implement the trait, consider defining an enum where each variant holds one of these types, implementing `AsyncFn` for this new enum and using it instead:
             &F
             std::boxed::Box<F, A>

error[E0038]: the trait `AsyncFn` cannot be made into an object
  --> src/lib.rs:6:5
   |
6  |     x().await;
   |     ^^^ `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>
  --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/async_function.rs:30:10
   |
30 |     type CallRefFuture<'a>: Future<Output = Self::Output>
   |          ^^^^^^^^^^^^^ the trait cannot be made into an object because it contains the generic associated type `CallRefFuture`
   = help: the following types implement the trait, consider defining an enum where each variant holds one of these types, implementing `AsyncFn` for this new enum and using it instead:
             &F
             std::boxed::Box<F, A>

error[E0038]: the trait `AsyncFn` cannot be made into an object
  --> src/lib.rs:6:9
   |
6  |     x().await;
   |         ^^^^^ `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>
  --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/async_function.rs:30:10
   |
30 |     type CallRefFuture<'a>: Future<Output = Self::Output>
   |          ^^^^^^^^^^^^^ the trait cannot be made into an object because it contains the generic associated type `CallRefFuture`
   = help: the following types implement the trait, consider defining an enum where each variant holds one of these types, implementing `AsyncFn` for this new enum and using it instead:
             &F
             std::boxed::Box<F, A>

For more information about this error, try `rustc --explain E0038`.
error: could not compile `playground` (lib) due to 4 previous errors
```

### Desired output
```
error[E0038]: the trait `AsyncFn` cannot be made into an object
  --> src/lib.rs:5:22
   |
5  | async fn foo(x: &dyn AsyncFn()) {
   |                      ^^^^^^^^^ `AsyncFn` cannot be made into an object
   |
note: `async` closures are not yet usable with dynamic dispatch. See <link for more information>
error: could not compile `playground` (lib) due to 1 previous error
```

### Rationale and extra context

The current error output spits out four related issues where only one is needed. Additionally, none provides quite the right context, and all reference implementation details of `AsyncFn` that users may not be concerned with.

### Rust Version

2024-11-05 nightly