Rust: Handle more explicit type arguments in type inference

[hvitved]

This PR improves handling of explicit type argument in type inference. For example, in

struct Foo<T>(T);

impl<T : Default> Foo<T> {
  fn default() -> Self {
    Foo(Default::default())
  }
}

Foo::<i32>::default();

we are now able to correctly infer the type of Foo::<i32>::default().

The implementation works by treating type qualifiers as self arguments, and then the type of the qualifier is matched against the Self type of the call target, just like we do for normal arguments. This also means that for associated functions, like default above, we pretend that there is a self parameter for the sake of matching up types.

Additionally, this PR also fixes an issue where type aliases were not resolved correctly, via a reimplementation of TypeMention.resolveTypeAt.

DCA looks good; we get extra call targets at an acceptable slowdown.

[geoffw0]

There are only a handful in the tests, but there are a lot of new type inference inconsistencies in the DCA run. Can we do / plan to do anything about this?

[hvitved]

Good catch; I have pushed a commit that should remove these overlapping results.

[geoffw0]

Thanks for addressing this.

I'm happy with the DCA results and tests - but I didn't get far figuring out how the code changes actually work. Up to you if you want to try and explain the changes broken down in more detail, or wait for someone else (e.g. @paldepind).

[hvitved]

I would prefer to have it merged soonish, as I have other PRs lined up that build on top.

[aibaars]

Looks reasonable to me.