sync: add #[must_use] to lock guards (#4886)

benches/sync_rwlock.rs

@@ -14,7 +14,7 @@ fn read_uncontended(b: &mut Bencher) {
         rt.block_on(async move {
             for _ in 0..6 {
                 let read = lock.read().await;
-                black_box(read);
+                let _read = black_box(read);
             }
         })
     });
@@ -28,7 +28,7 @@ fn read_concurrent_uncontended_multi(b: &mut Bencher) {
 
     async fn task(lock: Arc<RwLock<()>>) {
         let read = lock.read().await;
-        black_box(read);
+        let _read = black_box(read);
     }
 
     let lock = Arc::new(RwLock::new(()));
@@ -55,7 +55,7 @@ fn read_concurrent_uncontended(b: &mut Bencher) {
 
     async fn task(lock: Arc<RwLock<()>>) {
         let read = lock.read().await;
-        black_box(read);
+        let _read = black_box(read);
     }
 
     let lock = Arc::new(RwLock::new(()));
@@ -82,7 +82,7 @@ fn read_concurrent_contended_multi(b: &mut Bencher) {
 
     async fn task(lock: Arc<RwLock<()>>) {
         let read = lock.read().await;
-        black_box(read);
+        let _read = black_box(read);
     }
 
     let lock = Arc::new(RwLock::new(()));
@@ -110,7 +110,7 @@ fn read_concurrent_contended(b: &mut Bencher) {
 
     async fn task(lock: Arc<RwLock<()>>) {
         let read = lock.read().await;
-        black_box(read);
+        let _read = black_box(read);
     }
 
     let lock = Arc::new(RwLock::new(()));

tokio/src/sync/mutex.rs

@@ -141,6 +141,7 @@ pub struct Mutex<T: ?Sized> {
 ///
 /// The lock is automatically released whenever the guard is dropped, at which
 /// point `lock` will succeed yet again.
+#[must_use = "if unused the Mutex will immediately unlock"]
 pub struct MutexGuard<'a, T: ?Sized> {
     #[cfg(all(tokio_unstable, feature = "tracing"))]
     resource_span: tracing::Span,
@@ -766,7 +767,7 @@ impl<'a, T: ?Sized> MutexGuard<'a, T> {
     /// # async fn main() {
     /// #     let mutex = Mutex::new(0u32);
     /// #     let guard = mutex.lock().await;
-    /// #     unlock_and_relock(guard).await;
+    /// #     let _guard = unlock_and_relock(guard).await;
     /// # }
     /// ```
     #[inline]

tokio/src/sync/rwlock/read_guard.rs

@@ -12,6 +12,7 @@ use std::ops;
 ///
 /// [`read`]: method@crate::sync::RwLock::read
 /// [`RwLock`]: struct@crate::sync::RwLock
+#[must_use = "if unused the RwLock will immediately unlock"]
 pub struct RwLockReadGuard<'a, T: ?Sized> {
     #[cfg(all(tokio_unstable, feature = "tracing"))]
     pub(super) resource_span: tracing::Span,

tokio/src/sync/rwlock/write_guard.rs

@@ -14,6 +14,7 @@ use std::ops;
 ///
 /// [`write`]: method@crate::sync::RwLock::write
 /// [`RwLock`]: struct@crate::sync::RwLock
+#[must_use = "if unused the RwLock will immediately unlock"]
 pub struct RwLockWriteGuard<'a, T: ?Sized> {
     #[cfg(all(tokio_unstable, feature = "tracing"))]
     pub(super) resource_span: tracing::Span,

tokio/tests/sync_mutex.rs

@@ -53,7 +53,7 @@ fn readiness() {
     // But once g unlocks, we can acquire it
     drop(g);
     assert!(t2.is_woken());
-    assert_ready!(t2.poll());
+    let _t2 = assert_ready!(t2.poll());
 }
 
 /*
@@ -103,7 +103,7 @@ async fn aborted_future_1() {
         timeout(Duration::from_millis(1u64), async move {
             let iv = interval(Duration::from_millis(1000));
             tokio::pin!(iv);
-            m2.lock().await;
+            let _g = m2.lock().await;
             iv.as_mut().tick().await;
             iv.as_mut().tick().await;
         })
@@ -112,7 +112,7 @@ async fn aborted_future_1() {
     }
     // This should succeed as there is no lock left for the mutex.
     timeout(Duration::from_millis(1u64), async move {
-        m1.lock().await;
+        let _g = m1.lock().await;
     })
     .await
     .expect("Mutex is locked");
@@ -134,15 +134,15 @@ async fn aborted_future_2() {
             let m2 = m1.clone();
             // Try to lock mutex in a future that is aborted prematurely
             timeout(Duration::from_millis(1u64), async move {
-                m2.lock().await;
+                let _g = m2.lock().await;
             })
             .await
             .unwrap_err();
         }
     }
     // This should succeed as there is no lock left for the mutex.
     timeout(Duration::from_millis(1u64), async move {
-        m1.lock().await;
+        let _g = m1.lock().await;
     })
     .await
     .expect("Mutex is locked");

tokio/tests/sync_panic.rs

@@ -30,7 +30,7 @@ fn mutex_blocking_lock_panic_caller() -> Result<(), Box<dyn Error>> {
         let rt = basic();
         rt.block_on(async {
             let mutex = Mutex::new(5_u32);
-            mutex.blocking_lock();
+            let _g = mutex.blocking_lock();
         });
     });
 

tokio/tests/sync_rwlock.rs

@@ -31,7 +31,7 @@ fn read_shared() {
     let mut t1 = spawn(rwlock.read());
     let _g1 = assert_ready!(t1.poll());
     let mut t2 = spawn(rwlock.read());
-    assert_ready!(t2.poll());
+    let _g2 = assert_ready!(t2.poll());
 }
 
 // When there is an active shared owner, exclusive access should not be possible
@@ -75,7 +75,7 @@ fn exhaust_reading() {
     let g2 = reads.pop().unwrap();
     drop(g2);
     assert!(t1.is_woken());
-    assert_ready!(t1.poll());
+    let _g1 = assert_ready!(t1.poll());
 }
 
 // When there is an active exclusive owner, subsequent exclusive access should not be possible
@@ -100,7 +100,7 @@ fn write_shared_drop() {
     assert_pending!(t2.poll());
     drop(g1);
     assert!(t2.is_woken());
-    assert_ready!(t2.poll());
+    let _g2 = assert_ready!(t2.poll());
 }
 
 // when there is an active shared owner, and exclusive access is triggered,
@@ -112,7 +112,7 @@ fn write_read_shared_pending() {
     let _g1 = assert_ready!(t1.poll());
 
     let mut t2 = spawn(rwlock.read());
-    assert_ready!(t2.poll());
+    let _g2 = assert_ready!(t2.poll());
 
     let mut t3 = spawn(rwlock.write());
     assert_pending!(t3.poll());
@@ -137,7 +137,7 @@ fn write_read_shared_drop_pending() {
     drop(t2);
 
     assert!(t3.is_woken());
-    assert_ready!(t3.poll());
+    let _t3 = assert_ready!(t3.poll());
 }
 
 // Acquire an RwLock nonexclusively by a single task