Hi Marshall, Eric,
Currently the libcxx configuration option LIBCXX_ENABLE_THREADS serves two purposes:
Enabling C++11 threading constructs like std::thread and std::mutex in the library.
Ensuring certain constructs are safe to be used in multi-threaded environments. This is about ensuring that any storage allocated by the library itself is properly protected using mutexes and/or atomic operations. A good example for this is the ref counting mechanism of shared_prt.
(2) is a pre-requisite for (1). But I think (2) should be able to stand on its own even without (1) - which is currently not possible.
The use case for us is that there are users who want to use shared_prt (and others) in multi-threaded environments without having to enable high-level threading constructs like std::thread and std::mutex. I would also put std::atomic into the same category; these should be available even if std::mutex and std::thread are explicitly disabled in the library.
Do you think this would be useful to the general audience of libc++? I can spin some patches if so.