indexmap-rs
Support `get_or_insert_with`
Could you please support something like this? https://docs.rs/hashbrown/latest/hashbrown/struct.HashSet.html#method.get_or_insert_with
I would prefer to wait for the std version to stabilize, so we can make sure to match its final API.
https://github.com/rust-lang/rust/issues/60896
(I authored that initially, but anyone could chase that to completion.)
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.
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()).
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.
GIven how long it takes to stabilize the API in Rust,I think it would be beneficial to provide method to get_or_insert today and deal with deprecation later. To be frank I had to implement fn get_or_insert_default_with<Q, F>(&mut self, key: &Q, default: F) -> &mut V like three times by now for each container I work with and that's not fun -- don't get actual things done.
