`HashMap` should not take ownership of its keys

[Binero]

Right now, to insert a new value into a std::collections::HashMap, one must transfer ownership of the key to the map. The map however does not seem to require ownership of the key.

The HashMap does not keep around a copy of the key however. It only stores its hash. This means that there is no way to get the key out of the HashMap again.

For example, take the signature of the insert method:

fn insert(&mut self, k: K, v: V) -> Option<V>

If the value is updated, it returns the old value, thus giving the user ownership of said value again. The key however, is lost forever. There is no way to get it back.

[codyps]

HashMap does store the key, and references to that key can be retrieved by using HashMap::entry() and HashMap::keys() (among others).

One can use the HashMap::entry() api to retrieve the stored key on removal as well with remove_entry.

That said, getting an entry currently requires another owned key to pass to entry(). There is an rfc working on fixing that.

[Binero]

Aha, my bad. I got confused just now because surely it must store the key SOMEWHERE, otherwise the HashMap wouldn't be able to check within a bucket. The entry is exactly what I was looking for, thanks a ton!