Add test test_weak_count_locked

src/liballoc/arc.rs

@@ -1501,6 +1501,25 @@ mod tests {
         assert!(Arc::ptr_eq(&five, &same_five));
         assert!(!Arc::ptr_eq(&five, &other_five));
     }
+
+    #[test]
+    #[cfg_attr(target_os = "emscripten", ignore)]
+    fn test_weak_count_locked() {
+        let mut a = Arc::new(atomic::AtomicBool::new(false));
+        let a2 = a.clone();
+        let t = thread::spawn(move || {
+            for _i in 0..1000000 {
+                Arc::get_mut(&mut a);
+            }
+            a.store(true, SeqCst);
+        });
+
+        while !a2.load(SeqCst) {
+            let n = Arc::weak_count(&a2);
+            assert!(n < 2, "bad weak count: {}", n);
+        }
+        t.join().unwrap();
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]