Render auto traits in RPIT

[A4-Tacks]

Similar #14055

trait Trait {}
impl Trait for T {}
fn foo() -> impl Trait {
    &raw const ()
}

Hover on impl outputs:

impl Trait + Unpin + UnwindSafe + RefUnwindSafe
impl Trait = *const ()
---
impl
---
keyword docs...

Maybe we can use inlayHints, but there are too many auto traits

Complex cases that should be implemented:

async foo() -> Result<(impl ToString, impl Fn()), impl Error> { /*...*/ }

[ChayimFriedman2]

Nice idea! Implementation wise, you can just check trait implementation like any IDE feature: the solver will handle the rest.

[A4-Tacks]

It seems that hir::Impl::all_for_type can only search for blanket implementation and cannot search for auto implementation of auto traits

[ChayimFriedman2]

You're not supposed to use it. Use the solver (Type::impls_trait() from hir, other things like ObligationCtxt from hir-ty).