Tracking Issue for return type notation

[compiler-errors]

This is a tracking issue for the EXPERIMENTAL feature return type notation (RFC: pending).
The feature gate for the issue is #![feature(return_type_notation)].

Additional background: https://smallcultfollowing.com/babysteps/blog/2023/02/13/return-type-notation-send-bounds-part-2/

About experimental features

An experimental feature is one that has not yet had an RFC. The idea is to allow implementation work to proceed to better inform an upcoming RFC. Experimental features cannot be stabilized without first having an RFC. The existence of an experimental feature does not indicate lang team consensus that the feature is desirable, only that there is the problem is worthy of being solved and that the idea has enough merit to consider exploring. See the lang team process page for more details.

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Steps

• Implement the feature (Initial support for return type notation (RTN) #109010)
• Author RFC
• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide)

Experimental results

Part of the goal of the experimental process is to identify benefits but also concerns that need to be addressed in the RFC. This section catalogues notable findings thus far.

• Alternative: typeof syntax.
• Concerns raised by lcnr

Implementation history

• Initial support for return type notation (RTN) #109010

[VictorBulba]

hey

Is there a way to use this the following way?

#![feature(async_fn_in_trait)]
#![feature(return_type_notation)]

use std::future::Future;

trait Inner: Send + Sync {
    async fn do_async(&self);
}

trait Outer {
    type In: Inner<do_async(): Send>;
} 

fn foo<T>(inner: T::In) -> impl Future + Send
where
    T: Outer,
{
    async move {
        inner.do_async().await;
    }
}

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=d69310a05735ec25356754d4b8bdd708