Implement Hash for TreeSet

[nham]

cc #15294

[sfackler]

It seems like we should be able to derive Hash instead.

[nham]

I'm not really sure how #[deriving] works, but I'm guessing that would require that TreeMap implement Hash as well? (And the TreeNode structure that TreeMap uses as well). I'll try it.

[alexcrichton]

This may actually have problems with #[deriving(Hash)] because you can have two equivalent trees with different structures (but they should have the same hash).

[sfackler]

TreeMap would have to have a manual implementation, but TreeSet should be able to derive it since it'll defer to TreeMap's implementation.

[alexcrichton]

Aha, indeed!

[nham]

I'm having some difficulty understanding how to do this. Is there a way to manually implement Hash for TreeMap without using iter()? Because iter() is only implemented for keys that implement Ord, which means that Hash is only implemented for TreeMap's where the key implements Ord. This yields the following error when trying to derive Hash for TreeSet:

/home/travis/build/rust-lang/rust/src/libcollections/treemap.rs:592:5: 592:24 error: failed to find an implementation of trait core::cmp::Ord for T

/home/travis/build/rust-lang/rust/src/libcollections/treemap.rs:592 map: TreeMap<T, ()>

[sfackler]

Ah, yeah, you may have to manually implement Hash for both TreeMap and TreeSet then.

[alexcrichton]

Looks good to me! Could you squash the commits as well?

[nham]

Done.