std: move `Once` implementations to `sys`

1 files changed, 32 insertions, 0 deletions

diff --git a/library/std/src/sys/sync/mutex/no_threads.rs b/library/std/src/sys/sync/mutex/no_threads.rs
new file mode 100644
index 00000000000..4a13c55fb8b
--- /dev/null
+++ b/library/std/src/sys/sync/mutex/no_threads.rs
@@ -0,0 +1,32 @@
+use crate::cell::Cell;
+
+pub struct Mutex {
+    // This platform has no threads, so we can use a Cell here.
+    locked: Cell<bool>,
+}
+
+unsafe impl Send for Mutex {}
+unsafe impl Sync for Mutex {} // no threads on this platform
+
+impl Mutex {
+    #[inline]
+    #[rustc_const_stable(feature = "const_locks", since = "1.63.0")]
+    pub const fn new() -> Mutex {
+        Mutex { locked: Cell::new(false) }
+    }
+
+    #[inline]
+    pub fn lock(&self) {
+        assert_eq!(self.locked.replace(true), false, "cannot recursively acquire mutex");
+    }
+
+    #[inline]
+    pub unsafe fn unlock(&self) {
+        self.locked.set(false);
+    }
+
+    #[inline]
+    pub fn try_lock(&self) -> bool {
+        self.locked.replace(true) == false
+    }
+}