LRU Cache with compile-time reference stability

The main codes are copy from lru-rs, very grateful for the project.

The main motivation for implementing this project is that LRUCache should allow multiple immutable references obtained through get method.

The main idea is separating the value operating permissions from the data structure itself. I'll give an elaboration in my blog post later.

Example

Below is a simple example shows the main feature of LRUCache.

new_lru_cache(NonZeroUsize::new(2).unwrap(), |mut perm, mut cache| {
    assert_eq!(cache.put("apple", "red", &mut perm), None);
    assert_eq!(cache.put("banana", "yellow", &mut perm), None);
    assert_eq!(cache.put("lemon", "yellow", &mut perm), Some("red"));

    let colors: Vec<_> = ["apple", "banana", "lemon", "watermelon"]
        .iter()
        .map(|k| cache.get(k, &perm))
        .collect();

    // Any modification will cause compilation failure here since `&perm` were held by `colors`.
    // cache.put("watermelon", "green", &mut perm);
    assert!(colors[0].is_none());
    assert_opt_eq(colors[1], "yellow");
    assert_opt_eq(colors[2], "yellow");
    assert!(colors[3].is_none());

    drop(colors);

    // Modifications here will success.
    cache.put("watermelon", "green", &mut perm);

    (perm, cache)
});