A concurrent, lock-free version of HashMap<K, Mutex<V>> and HashMap<K, RwLock<V>>.

It automatically allocates mutexes and rwlocks for you, and it automatically deallocates them when they are no longer in use. The "no longer in use" condition is when the last guard is dropped AND [Empty::is_empty] returns true.

It's a bit like Web Locks API when V`` is ()`.

Example

use std::sync::Arc;

fn main() {
let locks = KeyMutex::<u32, BTreeSet<String>>::new();
let mut lock = locks.lock(1).unwrap();
lock.insert("Hello".to_owned());
lock.insert("World".to_owned());
drop(lock);

// Value is not empty and thus is not dropped
assert_eq!(locks.len(), 1);

let mut lock = locks.lock(1).unwrap();
assert_eq!(lock.len(), 2);
lock.clear();
drop(lock);

// Should be dropped now
assert_eq!(locks.len(), 0);
}