2661: Implement infer await from async function r=flodiebold a=edwin0cheng
This PR is my attempt for trying to add support for infer `.await` expression from an `async` function, by desugaring its return type to `Impl Future<Output=RetType>`.
Note that I don't know it is supposed to desugaring it in that phase, if it is not suitable in current design, just feel free to reject it :)
r=@flodiebold
Co-authored-by: Edwin Cheng <edwin0cheng@gmail.com>
When calling a function, argument-position impl Trait is transparent; same for
return-position impl Trait when inside the function. So in these cases, we need
to represent that type not by `Ty::Opaque`, but by a type variable that can be
unified with whatever flows into there.
