support partial type hints

[thestinger]

I want to be able to write this:

let xs: HashSet<_> = iter.collect();

rather than:

let xs: HashSet<SomeLongType<A, B>> = iter.collect();

I really don't like the idea of having special cases like to_owned_vec in std::iter to work around this issue:

fn to_owned_vec(&mut self) -> ~[A] {
    self.collect()
}

It doesn't fix the pain in the general case, and encourages using a specific container we special case rather than the right container for the use case. I think this problem will come up frequently with generic code, and the _ syntax would feel right at home with the usage elsewhere.

[nikomatsakis]

This seems useful and like it would not be too hard to implement -- we just have to be sure to disallow such hints in places where they are not legal. Would we permit you to omit the list of type parameters altogether? (i.e., HashSet or do we require HashSet<_>?) Should we extend this to lifetime names Foo<'_>? The latter looks really ugly to me :)

[thestinger]

@nikomatsakis: I think it would probably make sense to start with the minimal feature (individual parameters) and enhance it if compelling use cases come up.

[nikomatsakis]

I have had some thoughts in the meantime as part of my work on #4846 and have a larger proposal to make that would encompass this issue.

[nikomatsakis]

See proposal for #5121 described here: http://smallcultfollowing.com/babysteps/blog/2013/10/29/intermingled-parameter-lists/

[metajack]

+1. I recent ran into some code in Servo that would benefit from this.

[Kimundi]

This is supported now.

[alexcrichton]

Closed by #12764, thanks @Kimundi!