Structs

A counter per key
A drop-in replacement for BTreeMap, where the keys have the property of being array indices. Some values may be missing. Much more efficient at operations on individual objects, because it just becomes a simple array lookup.
Convenience functions around a string->string map
Use when your key is just PartialEq, not Ord or Hash.

Traits

Use with FixedMap. From a particular key, extract a usize. These values should be roughly contiguous; the space used by the FixedMap will be O(n) with respect to the largest value returned here.

Functions