cannot implement async trait that returning "impl Trait"

[nicholas-1vbw]

Since rust 1.75, we are able to write functions that return impl Trait in trait.

I'd like to use it with async_trait, like below:

#[async_trait::async_trait]
pub trait X {
    type Error;
    async fn x(&self) -> Result<impl AsRef<str> + Send + Sync, Self::Error>;
}

The code upon can pass the compilation, but implementing X cannot.

struct T;
#[async_trait::async_trait]
impl X for T {
   type Error = ();
   async fn x(&self) -> Result<impl AsRef<str> + Send + Sync, Self::Error> {
      Ok("Hello World")
   }
}

Complation error:

error[E0562]: `impl Trait` is not allowed in paths
  --> scratch/src/main.rs:11:33
   |
11 |     async fn x(&self) -> Result<impl AsRef<str> + Send + Sync, Self::Error> {
   |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `impl Trait` is only allowed in arguments and return types of functions and methods

error[E0562]: `impl Trait` is not allowed in the type of variable bindings
  --> scratch/src/main.rs:11:33
   |
11 |     async fn x(&self) -> Result<impl AsRef<str> + Send + Sync, Self::Error> {
   |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `impl Trait` is only allowed in arguments and return types of functions and methods

After inspecting code generated by async_trait macro:

impl X for T {
    type Error = ();
    #[allow(
        elided_named_lifetimes,
        clippy::async_yields_async,
        clippy::diverging_sub_expression,
        clippy::let_unit_value,
        clippy::needless_arbitrary_self_type,
        clippy::no_effect_underscore_binding,
        clippy::shadow_same,
        clippy::type_complexity,
        clippy::type_repetition_in_bounds,
        clippy::used_underscore_binding
    )]
    fn x<'life0, 'async_trait>(
        &'life0 self,
    ) -> ::core::pin::Pin<
        Box<
            dyn ::core::future::Future<
                Output = Result<impl AsRef<str>, Self::Error>,
            > + ::core::marker::Send + 'async_trait,
        >,
    >
    where
        'life0: 'async_trait,
        Self: 'async_trait,
    {
        Box::pin(async move {
            if let ::core::option::Option::Some(__ret) = ::core::option::Option::None::<
                Result<impl AsRef<str> + Send + Sync, Self::Error>,
            > {
                #[allow(unreachable_code)] return __ret;
            }
            let __self = self;
            let __ret: Result<impl AsRef<str> + Send + Sync, Self::Error> = { Ok("Hello World") };
            #[allow(unreachable_code)] __ret
        })
    }
}

Upon inspecting the generated code by the async_trait macro, the impl Trait appears in type annotations, causing the issue. Replacing these occurrences of impl Trait with _ resolves the compilation errors.

Can the macro automatically replace all instances of impl Trait with _ to bypass this issue?

[dtolnay]

Fixed in 0.1.84.