1 files changed, 36 insertions, 0 deletions
diff --git a/library/std/src/sys/sync/once/mod.rs b/library/std/src/sys/sync/once/mod.rs
new file mode 100644
index 00000000000..61b29713fa1
--- /dev/null
+++ b/library/std/src/sys/sync/once/mod.rs
@@ -0,0 +1,36 @@
+// A "once" is a relatively simple primitive, and it's also typically provided
+// by the OS as well (see `pthread_once` or `InitOnceExecuteOnce`). The OS
+// primitives, however, tend to have surprising restrictions, such as the Unix
+// one doesn't allow an argument to be passed to the function.
+//
+// As a result, we end up implementing it ourselves in the standard library.
+// This also gives us the opportunity to optimize the implementation a bit which
+// should help the fast path on call sites.
+
+cfg_if::cfg_if! {
+    if #[cfg(any(
+        target_os = "linux",
+        target_os = "android",
+        all(target_arch = "wasm32", target_feature = "atomics"),
+        target_os = "freebsd",
+        target_os = "openbsd",
+        target_os = "dragonfly",
+        target_os = "fuchsia",
+        target_os = "hermit",
+    ))] {
+        mod futex;
+        pub use futex::{Once, OnceState};
+    } else if #[cfg(any(
+        windows,
+        target_family = "unix",
+        all(target_vendor = "fortanix", target_env = "sgx"),
+        target_os = "solid_asp3",
+        target_os = "xous",
+    ))] {
+        mod queue;
+        pub use queue::{Once, OnceState};
+    } else {
+        mod no_threads;
+        pub use no_threads::{Once, OnceState};
+    }
+}