Cannot implement an associated async trait

[HaoboGu]

I tried this code:

trait MyTraitA {
    fn run_a(&mut self) -> impl futures::Future<Output = ()>;
}

trait MyTraitB: MyTraitA {
    fn run_b(&mut self) -> impl futures::Future<Output = ()>;
}

impl<T: MyTraitB> MyTraitA for T {
    async fn run_a(&mut self) {
        self.run_b().await;
    }
}

I expected to see this happen: I want every struct that implements MyTraitB automatically implements MyTraitA.

Instead, this happened: I got the following error message:

   Compiling playground v0.0.1 (/playground)
error[E0053]: method `run_a` has an incompatible type for trait
  --> src/main.rs:10:5
   |
10 |     async fn run_a(&mut self) {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^ expected associated type, found future
   |
note: type in trait
  --> src/main.rs:2:28
   |
2  |     fn run_a(&mut self) -> impl futures::Future<Output = ()>;
   |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = note: expected signature `fn(&mut _) -> impl futures::Future<Output = ()>`
              found signature `fn(&mut _) -> impl futures::Future<Output = ()>`

For more information about this error, try `rustc --explain E0053`.
error: could not compile `playground` (bin "playground") due to 1 previous error

The error message is not clear, and the expected & found signature are same. If it's not correct, I'd expect a hint and a suggestion that tell me why I can't do this and how can I achieve it.

Meta

rustc --version --verbose:

rustc 1.83.0 (90b35a623 2024-11-26)
binary: rustc
commit-hash: 90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf
commit-date: 2024-11-26
host: aarch64-apple-darwin
release: 1.83.0
LLVM version: 19.1.1

Backtrace

<backtrace>

[theemathas]

Minimized:

trait Foo {}
impl Foo for i32 {}

trait MyTraitA {
    fn run_a() -> impl Foo;
}

trait MyTraitB: MyTraitA {}

impl<T: MyTraitB> MyTraitA for T {
    fn run_a() -> impl Foo {
        1
    }
}

Error output:

error[E0053]: method `run_a` has an incompatible type for trait
  --> src/lib.rs:11:19
   |
11 |     fn run_a() -> impl Foo {
   |                   ^^^^^^^^ expected associated type, found opaque type
   |
note: type in trait
  --> src/lib.rs:5:19
   |
5  |     fn run_a() -> impl Foo;
   |                   ^^^^^^^^
   = note: expected signature `fn() -> impl Foo`
              found signature `fn() -> impl Foo`

For more information about this error, try `rustc --explain E0053`.

[theemathas]

Duplicate of #124729