AFIT: confusing error message with supertraits

[bugadani]

I tried this code godbolt:

#![feature(async_fn_in_trait)]

pub trait Bar: Foo {
    async fn bar(&mut self) -> Result<(), ()>;
}

pub trait Foo {
    async fn foo(&mut self) -> Result<(), ()>;
}

impl<B: Bar> Foo for B {
    async fn foo(&mut self) -> Result<(), ()> {
        self.bar().await
    }
}

I expected to see this happen: my code compiles (as it does without AFIT), or the compiler tells me something useful.

Instead, this happened: rustc gave me a very confusing error:

error[E0053]: method `foo` has an incompatible type for trait
  --> <source>:12:32
   |
12 |     async fn foo(&mut self) -> Result<(), ()> {
   |                                ^^^^^^^^^^^^^^ expected associated type, found future
   |
note: type in trait
  --> <source>:8:32
   |
8  |     async fn foo(&mut self) -> Result<(), ()>;
   |                                ^^^^^^^^^^^^^^
   = note: expected signature `fn(&mut B) -> impl Future<Output = Result<(), ()>>`
              found signature `fn(&mut B) -> impl Future<Output = Result<(), ()>>`

Meta

rustc --version --verbose:

rustc 1.72.0-nightly (371994e0d 2023-06-13)

[bugadani]

Duplicate of #112626