Compatibility with specialization

[jhpratt]

In Rocket, there's currently the following function:

impl<'r, R: Responder<'r> + Send + 'r, E: fmt::Debug + Send + 'r> Responder<'r> for Result<R, E> {
    default fn respond_to(self, req: &'r Request<'_>) -> response::ResultFuture<'r> {
        Box::pin(async move {
            match self {
                Ok(r) => r.respond_to(req).await,
                Err(e) => {
                    error_!("Response was a non-`Responder` `Err`: {:?}.", e);
                    Err(Status::InternalServerError)
                }
            }
        })
    }
}

I'm looking into rewriting the traits using this crate, and would like to write the function as

#[async_trait]
impl<'r, R: Responder<'r> + Send + 'r, E: fmt::Debug + Send + 'r> Responder<'r> for Result<R, E> {
    default async fn respond_to(self, req: &'r Request<'_>) -> response::Result<'r> {
        match self {
            Ok(r) => r.respond_to(req).await,
            Err(e) => {
                error_!("Response was a non-`Responder` `Err`: {:?}.", e);
                Err(Status::InternalServerError)
            }
        }
    }
}

When doing this, I get an error.

error: missing `fn`, `type`, or `const` for impl-item declaration
   --> core/lib/src/response/responder.rs:296:99
    |
296 |   impl<'r, R: Responder<'r> + Send + 'r, E: fmt::Debug + Send + 'r> Responder<'r> for Result<R, E> {
    |  ___________________________________________________________________________________________________^
297 | |     default async fn respond_to(self, req: &'r Request<'_>) -> response::Result<'r> {
    | |____^ missing `fn`, `type`, or `const`

I presume this is because of specialization, as it seems to work everywhere else.

[dtolnay]

This looks like a compiler bug -- it fails in rustc's parser before even hitting async_trait.

macro_rules! item {
    ($i:item) => {}
}

item! {
    trait Trait {
        async default fn f() {}
    }
}

item! {
    trait Trait {
        default async fn f() {}
    }
}

error: expected one of `extern`, `fn`, or `unsafe`, found `default`
 --> src/main.rs:7:15
  |
7 |         async default fn f() {}
  |               ^^^^^^^ expected one of `extern`, `fn`, or `unsafe` here

error: missing `fn`, `type`, or `const` for trait-item declaration
  --> src/main.rs:12:18
   |
12 |       trait Trait {
   |  __________________^
13 | |         default async fn f() {}
   | |________^ missing `fn`, `type`, or `const`

error: aborting due to 2 previous errors

Could you follow up with a bug report in rust-lang/rust?

[jhpratt]

Filed as rust-lang/rust#63716.

[dtolnay]

Specialization works as of the rustc parser fix in rust-lang/rust#63759. The Responder<'r> trait would probably still be blocked on #28.

[dtolnay]

Should work as of async-trait 0.1.12 thanks to #30.