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.