#
std/sys/sync/once/
mod.rs

    1// A "once" is a relatively simple primitive, and it's also typically provided
    2// by the OS as well (see `pthread_once` or `InitOnceExecuteOnce`). The OS
    3// primitives, however, tend to have surprising restrictions, such as the Unix
    4// one doesn't allow an argument to be passed to the function.
    5//
    6// As a result, we end up implementing it ourselves in the standard library.
    7// This also gives us the opportunity to optimize the implementation a bit which
    8// should help the fast path on call sites.
    9
    10cfg_if::cfg_if! {
    11    if #[cfg(any(
    12        all(target_os = "windows", not(target_vendor="win7")),
    13        target_os = "linux",
    14        target_os = "android",
    15        all(target_arch = "wasm32", target_feature = "atomics"),
    16        target_os = "freebsd",
    17        target_os = "openbsd",
    18        target_os = "dragonfly",
    19        target_os = "fuchsia",
    20        target_os = "hermit",
    21    ))] {
    22        mod futex;
    23        pub use futex::{Once, OnceState};
    24    } else if #[cfg(any(
    25        windows,
    26        target_family = "unix",
    27        all(target_vendor = "fortanix", target_env = "sgx"),
    28        target_os = "solid_asp3",
    29        target_os = "xous",
    30    ))] {
    31        mod queue;
    32        pub use queue::{Once, OnceState};
    33    } else {
    34        mod no_threads;
    35        pub use no_threads::{Once, OnceState};
    36    }
    37}