rustdoc: missing type parameters on impls of generic traits

[aturon]

Sometimes, but not always, rustdoc does not record the type parameters for a trait impl.

For example, the Rc type (http://static.rust-lang.org/doc/master/std/rc/struct.Rc.html) correctly shows the type parameter for the Hash impl but does not show it for Deref impl.

[sfackler]

Dup of #14462

[aturon]

@sfackler Are you sure this is a dup? It doesn't seem covered by the issue you referenced.

cc @alexcrichton

[sfackler]

Isn't this the first bullet? ("Lifetime parameters on functions are missing")

[sfackler]

Although that other issue may only explicitly cover the trait itself, not the impls.

[aturon]

No -- I was referring to the line that says what trait is being implemented. Currently, for Rc, you get:

impl<T> Deref for Rc<T>

but you should get

impl<T> Deref<T> for Rc<T>

These are regular type parameters to the trait itself, not lifetime parameters on a function.

[sfackler]

Oh!

[alexcrichton]

This is quite unusual. I will investigate.

[alexcrichton]

I have a fix, will open a PR soon with some other improvements.