once_map

这个 crate 提供 OnceMap，一种可以写入共享引用的 HashMap 类型，但只能写入一次。这与 once_cell 类似，但具有映射功能。这允许在映射的生命周期内引用映射内的值，而不需要进一步加锁。

这使得这种类型非常适合实现缓存。为此提供了 LazyMap 类型。

这个 crate 提供了这种高度优化以用于并发的映射，同时也提供了单线程版本。

示例

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"));

assert_eq!(roses, "red");
assert_eq!(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
assert_eq!(roses, "red");