Which matches the description of that library:

https://github.com/Amanieu/parking_lot

"This library provides implementations of Mutex, RwLock, Condvar and Once that are smaller, faster and more flexible than those in the Rust standard library"

"Uncontended lock acquisition and release is done through fast inline paths which only require a single atomic operation.

Microcontention (a contended lock with a short critical section) is efficiently handled by spinning a few times while trying to acquire a lock.

The locks are adaptive and will suspend a thread after a few failed spin attempts."

The only thing that I'm missing is how often the sleeping threads wake, as a bad constant can increase the CPU use.