std: move `Once` implementations to `sys`

1 files changed, 54 insertions, 0 deletions

diff --git a/library/std/src/sys/sync/mutex/windows7.rs b/library/std/src/sys/sync/mutex/windows7.rs
new file mode 100644
index 00000000000..ef2f84082cd
--- /dev/null
+++ b/library/std/src/sys/sync/mutex/windows7.rs
@@ -0,0 +1,54 @@
+//! System Mutexes
+//!
+//! The Windows implementation of mutexes is a little odd and it might not be
+//! immediately obvious what's going on. The primary oddness is that SRWLock is
+//! used instead of CriticalSection, and this is done because:
+//!
+//! 1. SRWLock is several times faster than CriticalSection according to
+//!    benchmarks performed on both Windows 8 and Windows 7.
+//!
+//! 2. CriticalSection allows recursive locking while SRWLock deadlocks. The
+//!    Unix implementation deadlocks so consistency is preferred. See #19962 for
+//!    more details.
+//!
+//! 3. While CriticalSection is fair and SRWLock is not, the current Rust policy
+//!    is that there are no guarantees of fairness.
+
+use crate::cell::UnsafeCell;
+use crate::sys::c;
+
+pub struct Mutex {
+    srwlock: UnsafeCell<c::SRWLOCK>,
+}
+
+unsafe impl Send for Mutex {}
+unsafe impl Sync for Mutex {}
+
+#[inline]
+pub unsafe fn raw(m: &Mutex) -> c::PSRWLOCK {
+    m.srwlock.get()
+}
+
+impl Mutex {
+    #[inline]
+    pub const fn new() -> Mutex {
+        Mutex { srwlock: UnsafeCell::new(c::SRWLOCK_INIT) }
+    }
+
+    #[inline]
+    pub fn lock(&self) {
+        unsafe {
+            c::AcquireSRWLockExclusive(raw(self));
+        }
+    }
+
+    #[inline]
+    pub fn try_lock(&self) -> bool {
+        unsafe { c::TryAcquireSRWLockExclusive(raw(self)) != 0 }
+    }
+
+    #[inline]
+    pub unsafe fn unlock(&self) {
+        c::ReleaseSRWLockExclusive(raw(self));
+    }
+}