Thread-unsafe C functions are a problem for Rust indeed. For example, C `setenv()` is unfixably broken for multi-threaded programs, and C code invoking it can c

Thread-unsafe C functions are a problem for Rust indeed. For example, C `setenv()` is unfixably broken for multi-threaded programs, and C code invoking it can crash Rust programs. Currently it's an open question what Rust should do about that. However, for Rust functions there's no hole, because Rust doesn't allow thread-unsafe global state. All globals always have to be immutable or use some synchronization primitive. Therefore, a safe Rust version of the `setlocale()` function would be forced to use atomics or mutexes to set the locale without data races. It's still a bad API, and Rust can't fix that.