diff --git a/src/libsync/sync/mutex.rs b/src/libsync/sync/mutex.rs
index b37e2f3a45dc..2bd85526e686 100644
--- a/src/libsync/sync/mutex.rs
+++ b/src/libsync/sync/mutex.rs
@@ -143,6 +143,7 @@ pub struct StaticMutex {
 
 /// An RAII implementation of a "scoped lock" of a mutex. When this structure is
 /// dropped (falls out of scope), the lock will be unlocked.
+#[must_use]
 pub struct Guard<'a> {
     priv lock: &'a mut StaticMutex,
 }