1 files changed, 65 insertions, 0 deletions
diff --git a/library/std/src/sys/sync/rwlock/no_threads.rs b/library/std/src/sys/sync/rwlock/no_threads.rs
new file mode 100644
index 00000000000..789ef9b29e5
--- /dev/null
+++ b/library/std/src/sys/sync/rwlock/no_threads.rs
@@ -0,0 +1,65 @@
+use crate::cell::Cell;
+
+pub struct RwLock {
+    // This platform has no threads, so we can use a Cell here.
+    mode: Cell<isize>,
+}
+
+unsafe impl Send for RwLock {}
+unsafe impl Sync for RwLock {} // no threads on this platform
+
+impl RwLock {
+    #[inline]
+    #[rustc_const_stable(feature = "const_locks", since = "1.63.0")]
+    pub const fn new() -> RwLock {
+        RwLock { mode: Cell::new(0) }
+    }
+
+    #[inline]
+    pub fn read(&self) {
+        let m = self.mode.get();
+        if m >= 0 {
+            self.mode.set(m + 1);
+        } else {
+            rtabort!("rwlock locked for writing");
+        }
+    }
+
+    #[inline]
+    pub fn try_read(&self) -> bool {
+        let m = self.mode.get();
+        if m >= 0 {
+            self.mode.set(m + 1);
+            true
+        } else {
+            false
+        }
+    }
+
+    #[inline]
+    pub fn write(&self) {
+        if self.mode.replace(-1) != 0 {
+            rtabort!("rwlock locked for reading")
+        }
+    }
+
+    #[inline]
+    pub fn try_write(&self) -> bool {
+        if self.mode.get() == 0 {
+            self.mode.set(-1);
+            true
+        } else {
+            false
+        }
+    }
+
+    #[inline]
+    pub unsafe fn read_unlock(&self) {
+        self.mode.set(self.mode.get() - 1);
+    }
+
+    #[inline]
+    pub unsafe fn write_unlock(&self) {
+        assert_eq!(self.mode.replace(0), -1);
+    }
+}