rustdoc should show associated types in `impl` documentation

[SimonSapin]

For example, the std::ops::Index trait has an associated Output type. Of course, that type is potentially different for each impl of the trait. impl are documented at http://localhost/rust/std/ops/trait.Index.html#implementors , but there is apparently now way to know what is the Output type for a given implementation. (Other than finding the source code, which is not necessarily easy because of #15744.)

[tomjakubowski]

You should be able (for most impls at least) to follow the link for the Self type on the impl to see the associated types. I agree it would be nice to have them listed in the Trait Implementations section, though I can't think of a pleasant way to show it.

[SimonSapin]

I didn’t know that, thanks. Though there is apparently another bug for re-exported items (like in the std facade) similar to #20203:

http://doc.rust-lang.org/core/slice/struct.Iter.html#implementations

impl<'a, T> Index<Range<usize>> for Iter<'a, T>
  type Output = [T]
  fn index(&self, index: &Range<usize>) -> &[T]

http://doc.rust-lang.org/std/slice/struct.Iter.html#implementations

impl<'a, T> Index<Range<usize>> for Iter<'a, T>
  fn index(&self, index: &Range<usize>) -> &[T]
  type Output

[tomjakubowski]

I believe that's #20727

[tomjakubowski]

Related to #21348 perhaps (although this issue is about "trait implementations" on the trait's page, not the implementor's.)

[alexcrichton]

This appears to have been fixed at some point, yay!

[alexcrichton]

Ah but I'll add a test for it.