HashMap has no way to avoid key copy or double hash for insertion

[nagisa]

Currently all HashMap APIs which make insertion into a map possible (insert, entry) require the key by-value. If avoiding a clone is desired, the user must instead use something like if !map.contains(key) { map.insert(key.clone(), val); }, which results in key getting hashed twice.

Ideally, something like HashMap::insert_clone(key: &K, val: V) where K: Clone or HashMap::entry_clone(key: &K) where K: Clone would exist.

[nagisa]

Note that internal APIs have the ways to achieve this. HashMap::search takes a key by reference and returns InternalEntry, which, if converted to plain entry, gets provided the key by value.

[nagisa]

Another option is to have a method like HashMap::occupied_entry(key: &K) -> Option<OccupiedEntry>.

[Stebalien]

Personally, I'd use ToOwned instead of Clone. IIRC, I believe there was extensive discussion about this in some RFC issues.

[nagisa]

Ah, right, as always, I don’t care about bounds very much as long as it is possible to avoid both extra allocation when hashmap already has the key and double hashing when it doesn’t.

[dtolnay]

I believe this is going to require an RFC. There is already rust-lang/rfcs#1203 tracking a similar idea so let's continue the discussion there.