This crate implements a small unordered-set data structure implemented using smallvec. It initially stores set elements in a simple unordered array. When the set is smaller than a parameterizable size, no allocations will be performed. The data structure is thus very space-efficient for sets of only a few elements, much more so than a tree-based or hash-table-based set data structure. It is also fast when the set is small: queries and inserts perform a linear scan, which is more cache-friendly than a pointer-chasing search through a tree.
However, as the set grows, it will transform internally into a std::collections::HashSet.
This is a fork of the original library here: rust-smallset. I have rewritten the internals completely to not have such a bad fallback mode and added more features (and their tests and documentations).