Trait implementation type elision

[scottmcm]

@CAD97

That said, it could be interesting to allow trait implemetations to “underbound” types to just what they use locally (say, Vec<_> if you only need .len() ), which means local types could elide what information isn’t needed locally.

Interesting thought. Has the nice property that { unimplemented!() } wouldn't need any parameter types.

[gnzlbg]

This sounds similar to: https://github.com/rust-lang/rfcs/pull/2063