Currently, Rustdoc documents auto-trait impls for types, displaying them along with manually written impls. However, there is currently no way of telling what auto traits are implemented by the impl Future type returned by an async fn.

I'm not quite sure what the best way of displaying this is. We need to make it clear to users that the auto trait impls apply to opaque return type - not to the function itself, or to the underlying type returned by the future (e.g. the u8 in async fn foo() -> u8).