`alloc`-only `HashMap`?

JarredAllen:

What stops us from just moving the type definition to alloc and removing the default type parameter there, and providing the default again via a type alias in std?

I don't actually know if that works. If it does, then I agree this seems obvious.

(I feel like people think it doesn't, and that's why it hasn't been done already, but maybe the gestalt is wrong and we can just do it?)