Advertise impls of out-of-create traits for third party types with some type parameters set to in-crate types in rustdoc

[reem]

As an example, in iron, the impls of Modifier<Response> for std types like String, File, and Path are not advertised anywhere in the generated documentation.

[jashephe]

Just wanted to add in, this would be useful for rust-gnome as well, because we're starting to have a few cross-crate trait impls, and, as of now, have no easy way of documenting them.

[steveklabnik]

Triage: no change. Given that this is an open set of things, I'm not sure it's something that can ever really be done...

[QuietMisdreavus]

Just to clarify this: You want to be able to show a trait implementation when one of its type parameters is a local type? i.e. impl From<LocalType> for ForeignType would show on LocalType's page? That might not take too much work, but i'd like to make sure that that's what's being requested.

[jethrogb]

Given that this is an open set of things, I'm not sure it's something that can ever really be done...

Because of the coherence rules, impls can only be defined in one place depending on the types they reference. That's the place they should be documented as well.

[jethrogb]

#42440 was labelled C-bug but this is labelled C-enhancement