Comparator for BTree(Map|Set)s

[CAD97]

As I said the previous time this came up

@CAD97

Rather than storing a comparator, the more likely choice is probably something like the hashmap raw_entry API , where you provide the hash and an equality check callback, and insertion takes a hashing function.

For BTreeMap IIUC you'd just need impl Fn(&Key) -> Ordering for lookup and Key, Value, impl Fn(&Key, &Key) -> Ordering for insertion.

A stored comparator object is certainly more convenient, but providing one only when it's needed is more general and Rust has already biased for intrinsic ordering (e.g. cmp::Reverse<T>) rather than comparators.

@CAD97

I'd be up to mentoring a raw_cursor implementation. Open an API Change Proposal and ping me as a willing mentor (though note I don't have authority to second the proposal).

Though I've heard thought the grapevine that the std hashmap unstable raw_entry API might be slated for removal? I think a btree raw_cursor might be slightly easier to support, though, due to the more common use case of using a custom comparison, raw lookup in the btree being simpler than in the hashmap, and being able to provide cursor functionality through the raw API not available in the standard API.