sync: add mem::forget to RwLockWriteGuard::downgrade.

tokio/src/sync/rwlock.rs

@@ -375,8 +375,6 @@ impl<'a, T: ?Sized> RwLockWriteGuard<'a, T> {
     /// let n = n.downgrade();
     /// assert_eq!(*n, 1, "downgrade is atomic");
     ///
-    /// assert_eq!(*lock.read().await, 1, "additional readers can obtain locks");
-    ///
     /// drop(n);
     /// handle.await.unwrap();
     /// assert_eq!(*lock.read().await, 2, "second writer obtained write lock");
@@ -389,7 +387,8 @@ impl<'a, T: ?Sized> RwLockWriteGuard<'a, T> {
 
         // Release all but one of the permits held by the write guard
         s.release(MAX_READS - 1);
-
+        // NB: Forget to avoid drop impl from being called.
+        mem::forget(self);
         RwLockReadGuard {
             s,
             data,

tokio/src/sync/tests/loom_rwlock.rs

@@ -76,3 +76,24 @@ fn concurrent_read_write() {
         assert_eq!(10, *guard);
     });
 }
+#[test]
+fn downgrade() {
+    loom::model(|| {
+        let lock = Arc::new(RwLock::new(1));
+
+        let n = block_on(lock.write());
+
+        let cloned_lock = lock.clone();
+        let handle = thread::spawn(move || {
+            let mut guard = block_on(cloned_lock.write());
+            *guard = 2;
+        });
+
+        let n = n.downgrade();
+        assert_eq!(*n, 1);
+
+        drop(n);
+        handle.join().unwrap();
+        assert_eq!(*block_on(lock.read()), 2);
+    });
+}