If the map is known at compile time, and it does not change during the runtime, there is a very, very fast system that can improve by orders of magnitude the use of maps. It's called perfect hash functions, and that's the key to them: they perform the minimum required computation for a hash to know whether it's stored in the hash map. This is because it maps one, and only one, integer for each element. And it has no collisions. Of course, this requires a constant, known hash map at compilation time.
To use them, you will need the phf crate. With this crate, you will be able to define a hash map at compile time in the build.rs file at the same level as the Cargo.toml file and use it with no more overhead than a comparison ...
