rustdoc: render references to associated type using short form when possible

[aturon]

Currently, rustdoc renders the Index trait as:

pub trait Index<Index> where Index: ?Sized {
    type Output;
    fn index(&'a self, index: &Index) -> &'a <Self as Index<Index>>::Output;
}

Ideally it would render as

pub trait Index<Index> where Index: ?Sized {
    type Output;
    fn index(&'a self, index: &Index) -> &'a Self::Output;
}

and eventually, when the scoping rules for associated items are settled, it could skip even the Self::

[aturon]

cc @tomjakubowski

[tomjakubowski]

Hmmm yep. We need to either pass the trait's type all the way down (maybe as some kind of QualifyContext type) when cleaning its items, or just tidy up any eligible QPaths afterwards.

[tomjakubowski]

So this particular bug only happens for cross-crate inlined traits. But it's broken in a different way in core, where Self is missing from the type entirely!

pub trait Index<Index: ?Sized> {
    type Output: ?Sized;

    fn index<'a>(&'a self, index: &Index) -> &'a Output;
}