> Prior to that issue Rust was using something much worse, pthread_mutex_t

Presumably you're referring to this description, from the Github Issue:

> > On most platforms, these structures are currently wrappers around their pthread equivalent, such as pthread_mutex_t. These types are not movable, however, forcing us to wrap them in a Box, resulting in an allocation and indirection for our lock types. This also gets in the way of a const constructor for these types, which makes static locks more complicated than necessary.

pthread mutexes are const-constructible in a literal sense, just not in the sense Rust requires. In C you can initialize a pthread_mutex_t with the PTHREAD_MUTEX_INITIALIZER initializer list instead of pthread_mutex_init, and at least with glibc there's no subsequent allocation when using the lock. But Rust can't do in-place construction[1] (i.e. placement new in C++ parlance), which is why Rust needs to be able to "move" the mutex. Moving a mutex is otherwise non-sensical once the mutex is visible--it's the address of the mutex that the locking is built around.

The only thing you gain by not using pthread_mutex_t is a possible smaller lock--pthread_mutex_t has to contain additional members to support robust, recursive, and error checking mutexes, though altogether that's only 2 or 3 additional words because some are union'd. I guess you also gain the ability to implement locking, including condition variables, barriers, etc, however you want, though now you can't share those through FFI.

[1] At least not without unsafe and some extra work, which presumably is a non-starter for a library type where you want to keep it all transparent.

I’m actually thinking of the sheer size of pthread mutexes. They are giant. The issue says that they wanted something small, efficient, and const constructible. Pthread mutexes are too large for most applications doing fine-grained locking.