Yes, it will do exactly that.
https://github.com/rust-lang/rust/blob/master/src/libstd/col...
It would be cool to see if the "we'll manage our own pointers, thanks" approach makes less sense with this optimization in place. It would be nice; the source is pretty inscrutable, mostly in the interest of performance, I think.
Edit: Or, maybe more importantly: in the untyped memory they store hashes and (K,V) pairs separately, so I think the only opportunity is to swap (K,V) for (V,K).
Rustc special cases them in a lot of places to do things that I will not even pretend to understand, and not reordering 2-element structs keeps them working. Since this already took 6 months of my free time (I have a lot of free time, though not 40 hours a week admittedly), this was for the best.
You can make an argument that we should have, but it doesn't matter from the user's perspective. (a, b) is the same size as (b, a) no matter what a and b are.