Rustdoc: inline methods available through blanket impls

[SimonSapin]

On reddit: https://www.reddit.com/r/rust/comments/36f94o/how_do_i_read_the_api_docs/

I had a terrible time with the API docs. Take str... I know I can use to_string() on it because I saw a web page that said I could, but I can't seem to infer that from the docs. Then I search for to_string() and get this page, but it doesn't seem to apply to str types.

What’s going on is that the to_string() method is on the ToString trait which has a blanket impl

impl<T> ToString for T where T: Display + ?Sized

… and str implements Display.

So in the spirit of #19190 (show methods available through deref), rustdoc should also show methods available through blanket implementations.

[steveklabnik]

Triage: no change

[QuietMisdreavus]

Closing as duplicate in favor of #33772, which had more discussion.