I think in this case it doesn't, because the `HashMap` implementation just stores everything behind an untyped pointer.

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).