delegation: support self ty propagation for functions in free to trait reuse

[aerooneqq]

This PR adds support for self types specified in free to trait reuse. Up to this point we always generated Self despite the fact whether self type was specified or not. Now we use it in signature inheritance. Moreover we no more generate Self for static methods. Part of #118212.

trait Trait<T> {
  fn foo<const B: bool>(&self) {}
  fn bar() {}
}

impl<T> Trait<T> for usize {}

reuse <usize as Trait>::foo;

// Desugaring (no `Self` as usize is specified)
fn foo<T, const B: bool>(self: &usize) {
  <usize as Trait::<T>>::foo::<B>(self)
}

reuse Trait::bar;

// Desugaring (no `Self` as static method)
fn bar<T>() {
  Trait::<T>::bar(); //~ERROR: type annotations needed
}

r? @petrochenkov

[petrochenkov]

r=me with the tests updated #155644 (comment).
@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[aerooneqq]

@rustbot ready

[petrochenkov]

@bors r+

[rust-bors]

📌 Commit 78435a9 has been approved by petrochenkov

It is now in the queue for this repository.