This crate implements a small unordered-set data structure implemented using smallvec. It stores set elements in a simple unordered array, and when the set is smaller than a parameterizable size, the elements are stored completely inline (i.e., with zero heap allocations). 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.
smallset should be used where minimizing heap allocations is of primary
importance and where it is expected that no more than a few elements will be
present. If the set grows large, then it will exhibit poor (O(n) queries and
inserts) performance.