There are several discussions in this forum about lacking data structures in Swift: Adding more data structures to the standard library, Data structures as functions? (Priority Queue) - #7 by rumnat
For me personally, it is the PriorityQueue, which I need to implement PrioritizedReplayBuffer in Swift.
I took a cursory look at SwiftNIO's PriorityQueue, Heap and CircularBuffer implementation, it seems to be struct based and generally conforms to Sequence protocol. Anything else you feel that can be improved before we move it into separate repo and somehow find interests within Swift Core team to have a blessed library for data structures based on these?