The Multi-Level Smart Pointer uses an atomic global reference counter and per-thread non-atomic reference counters. The goal is that an MLSP should outperform a purely-atomically counted smart pointer when enough local copies are performed.

The Mlsp type can be used like Rc for sharing memory within one thread. It does not implement Send and cannot be sent between threads, so any clone() and drop() operations performed on it use the local counter (except the last drop in a given thread).

The benefit of an Mlsp over an Rc is that repackaging it to share it with another thread does not require copying or moving the underlying data, it is already being stored in a way and with a counter that can be used to share between threads.

To send an Mlsp to another thread, you must make an MlspPackage. The MlspPackage type does implement Send and is ready to be sent to other threads. Receiving threads then use to create a new Mlsp with its own local reference counter.

let a = Mlsp::new(1u8).package();

thread::spawn(move || {
    let a2 = a; // Valid because MlspPackage implements Send
    println!("{:?}", a.unpackage().borrow());
})

This library is still in need of extensive benchmarking and testing to demonstrate that it is robust and effective.