Add Return Type Tooltip For .await

[danii]

I think it would be really useful to be able to hover over .awaits and see the value they return.

Something among the lines of this should be enough:

[jonas-schievink]

This should work with any expression already, as long as you select it

[flodiebold]

It does make sense to also show it when hovering the await though.

[Veykril]

Would probably make sense to show the type of any expression a keyword is being hovered over for(where it makes sense that is)? That is if, loop, etc.

[Veykril]

keyword hovers are rendered here https://github.com/rust-analyzer/rust-analyzer/blob/93036aa0e201474d43c38be3862f4a899defe267/crates/ide/src/hover/render.rs#L232-L258

So all that has to be done is take the parent node of the token, check if its an ast::Expr then ask sema.type_of_expr(..) for the type and add that to the message appropriately(behind the keyword, as shown in the screenshot makes sense).

[danii]

I suppose anyone with a little bit of Rust knowledge could add that with that information...
Oh, hey! I know Rust! How do I self assign?