Update: Arvid Norlander has gone through the trouble of refactoring this code into a crate and publishing it. Thank you, Arvid!
Rust’s BTreeMap and corresponding BTreeSet are excellent,
B-tree-based sorted map and key types. The map implements
the ergonomic entry API, more flexible
than other map APIs, made possible by the borrow checker.
They are implemented with the more performant but more gnarly
B-Tree data structure, rather than
the more common AVL trees or
red-black trees.
All in all, they are an excellent piece of engineering, and an excellent
standard library feature.