Although it's true that all of Rust's standard library collections are generic over Allocator, Rust 1.100 only stabilizes useful allocator APIs for Box and Vec

If you want to use your MyCustomAllocator with a HashMap or a BinaryHeap or you've lost your mind and want a LinkedList none of that was stabilized yet.

Personally I am slightly interested in HashMap, VecDeque and String but I think Vec is much more heavily used than any of those and will illustrate just how thin the pickings are in this space for real.