Support get_or_insert_with

[p4l1ly]

Could you please support something like this? https://docs.rs/hashbrown/latest/hashbrown/struct.HashSet.html#method.get_or_insert_with

[cuviper]

I would prefer to wait for the std version to stabilize, so we can make sure to match its final API.
rust-lang/rust#60896

(I authored that initially, but anyone could chase that to completion.)

[cuviper]

It's worth mentioning that the version by value, get_or_insert, is trivial with IndexSet::insert_full and then &set[i]. I'm sure we'll add that method too when the time comes, but it won't be any better than doing that yourself.

[p4l1ly]

My use case is kind of a micro-optimisation when I want to insert a possibly big borrowed value into the set. For efficiency reasons, I want to clone it only if it is not present. But IndexSet::insert_full takes ownership, so I have to clone it also if it is not stored. With get_or_insert_with I could do set.get_or_insert_with(value, |v| v.clone()).

[cuviper]

Yeah, understood. Another potential workaround is to make the clone cheaper, like wrapping the expensive part in Rc/Arc so it can be shared, but that's not always feasible.