library/std/src/sys/unix/locks/mod.rs

cfg_if::cfg_if! {
    if #[cfg(any(
        target_os = "linux",
        target_os = "android",
        all(target_os = "emscripten", target_feature = "atomics"),
        target_os = "freebsd",
        target_os = "openbsd",
        target_os = "dragonfly",
    ))] {
        mod futex;
        mod futex_rwlock;
        pub use futex::{Mutex, MovableMutex, Condvar, MovableCondvar};
        pub use futex_rwlock::{RwLock, MovableRwLock};
    } else {
        mod pthread_mutex;
        mod pthread_rwlock;
        mod pthread_condvar;
        pub use pthread_mutex::{Mutex, MovableMutex};
        pub use pthread_rwlock::{RwLock, MovableRwLock};
        pub use pthread_condvar::{Condvar, MovableCondvar};
    }
}