VacantEntry should provide an accessor for the key

[kmcallister]

So that I can use it (by reference) to compute the value to store.

[huonw]

Any particular reason you're restricting to only VacantEntry?

[Gankra]

This was part of my initial proposal, we cut it out for complexity reasons. Easy to provide, although might add overhead to some which could ditch the key (potentially SmallIntMap).

CC @aturon
CC rust-lang/rfcs/issues/353

[Gankra]

Also an into_key method was proposed.

[apasel422]

CC rust-lang/rfcs#1194.

[apasel422]

Fixed by #32248.