Feature: `OccupiedEntry::into_key`

[Imberflur]

I have a method that takes a value, creates a new key, inserts those into the map, and returns the key to the caller. I can't find a way to just return a reference to the key since the reference from OccupiedEntry::key can't outlive the current function. In this situation, it seems like I need to either clone the key or return the whole OccupiedEntry.

It seems like I could solve this if OccupiedEntry had an into_key method be that consumes the entry and returns a reference to the key with the lifetime of the original borrow from the hashmap (like OccupiedEntry::into_mut does for the value). Is this a reasonable solution? Is there a reason this can't exist or an existing way to achieve this? I wasn't able to find anything on the topic from searching existing issues.

[cuviper]

You want fn into_key(self) -> &'a K, right? That should be fine, but note that the borrow checker will continue to treat the map as mutably borrowed.

We could also (or instead) have into_key_value, mutable on the latter.

[clarfonthey]

We already have into_key_value, actually; #670 adds OccupiedEntry::into_entry which returns (&'a K, &'a mut V). into_key would just discard the value portion, which would be a reasonable addition IMHO.

This was something I found was necessary to make HashSet's implementation rely entirely on HashMap, without accessing the internals.

[Imberflur]

Oh awesome! Personally, I don't need the into_key method if into_entry exists.

[clarfonthey]

This can be closed then, unless you want to keep the feature request for into_key open. I personally would rather not add just into_key since it keeping the table mutably borrowed isn't very intuitive, whereas it is intuitive if into_entry returns a mutable value too.