`hash_map::OccupiedEntry` should provide original key back.

[kevincox]

My understanding is that when .entry() returns Occupied the key provided is not used as the key was already in the map. There should then be a way to get the original key back. The current workaround is .key().clone() if they key type is clonable.

The following APIs could have an alternate version that provided the original key:

• std::collections::hash_map::Entry::and_modify
• std::collections::hash_map::OccupiedEntry::insert
• std::collections::hash_map::OccupiedEntry::into_mut
• std::collections::hash_map::OccupiedEntry::remove
• std::collections::hash_map::OccupiedEntry::remove_entry

Now it may not make sense to add alternatives to all of these but it should be considered. Alternatives would be infeasible such as adding a key member to OccupiedEntry or making Entry::Occupied(OccupiedEntry, K).

The best way to do this isn't clear, but it would be nice to be able to work with non-clonable types and avoid unnecessary clones.

[cuviper]

OccupiedEntry::replace_key and replace_entry both return the old key.

[kevincox]

That is different. Those consume the new key. The other methods listed do not. Only one of the new or old keys will end up in the map. It should be possible to get the other key back.

[cuviper]

I think I see what you mean -- e.g. remove_entry also does give you the old key, but then the new one is lost.

[clubby789]

@rustbot label +T-libs-api