once_map
This crate provides OnceMap, a type of HashMap where entries can be written with a shared reference,
but can be written ony once. This is similar to [once_cell], but with a map.
This enables to reference values inside the map for the lifetime of the map, without the need
of further locks.
This makes this type perfect for implementation of caches. A type LazyMap is provided for such cases.
This crate provides such a map heavily optimized for concurrent use, but also a single-threaded version.
```rust let map = OnceMap::new();
// All these are &str pointing directly in the map.
// Note that we don't need a mutable reference, so we can have several of
// them at the same time.
let roses = map.insert(String::from("rose"), || String::from("red"));
let violets = map.insert(String::from("violets"), || String::from("blue"));
let sugar = map.insert(String::from("sugar"), |_| String::from("sweet"));
asserteq!(roses, "red"); asserteq!(violets, "blue"); assert_eq!(sugar, "sweet");
// The closure is never run here, because we already have a value for "rose" let roses = map.insert(String::from("rose"), || String::from("green")); // The old value did not change asserteq!(roses, "red"); ```