mutexes should be separate from condition variables

[thestinger]

It's very common to use a single mutex with many condition variables.

Windows Vista and later have support for native condition variables without emulation. Windows XP support ends in April 2014 so there's no need for Rust to cripple the API to support it.

A simple example: https://github.com/thestinger/rust-core/blob/master/core/concurrent.rs#L73

[sfackler]

extra::Mutex can be initialized with multiple condition variables, though you can't use the Condvar without holding the associated lock first. The API's also a bit awkward.

[thestinger]

To clarify, I'm talking about the native std::unstable::Mutex type. The type in extra is a slow implementation on top of the scheduler. A high quality mutex like the ones provided by glibc and Windows will do a micro-optimized spin before falling back to a fast system call to block. If there isn't a need to block, there will be no system call or context switch.

[alexcrichton]

Fixed in #19274.