Fully generalize cmp instances

[aturon]

With cmp/ops reform, all of the comparison traits allow the types of the two sides to differ. However, the traits provide a default type for the right-hand size that is the same as the left-hand side. Thus, an impl like:

impl<T: PartialEq> PartialEq for Rc<T> { ... }

is more limited than it needs to be; it could instead be

impl<U, T: PartialEq<U>> PartialEq<Rc<U>> for Rc<T> { ... }

(Of course, you could imagine being even more general than that, allowing Rc values to be compared to other values.)

The various impls in the standard library should probably be generalized along these lines; currently a few are but most aren't.

[aturon]

cc @japaric @alexcrichton

I don't think we need to do this prior to stabilization, since we can always safely generalize in the future. I'd also like to put some thought into how general we want to make things, but the example above seems like a good minimum.

[aturon]

A bit of follow up: I'm seeing some cases where e.g. the PartialEq implementation has been generalized like the above, but the Eq hasn't.

[steveklabnik]

What's the status of this with #20065 merged?

[steveklabnik]

I am guessing the status is that it's done, so I'm giving it a close. Let me know if there's anything specific still missing.

[aturon]

@steveklabnik No, this hasn't been done yet. Basically it would require a sweep through all existing impls to generalize them.

I'm not sure where you want to track tickets like this. It's generally backwards compatible to make these changes, and not exactly high priority, but something nice for expressiveness and consistency (and definitely does not require an RFC).