sync: better Debug for Mutex (#2725)

tokio/src/sync/mutex.rs

@@ -115,7 +115,6 @@ use std::sync::Arc;
 /// [`std::sync::Mutex`]: struct@std::sync::Mutex
 /// [`Send`]: trait@std::marker::Send
 /// [`lock`]: method@Mutex::lock
-#[derive(Debug)]
 pub struct Mutex<T: ?Sized> {
     s: semaphore::Semaphore,
     c: UnsafeCell<T>,
@@ -373,6 +372,20 @@ where
     }
 }
 
+impl<T> std::fmt::Debug for Mutex<T>
+where
+    T: std::fmt::Debug,
+{
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let mut d = f.debug_struct("Mutex");
+        match self.try_lock() {
+            Ok(inner) => d.field("data", &*inner),
+            Err(_) => d.field("data", &format_args!("<locked>")),
+        };
+        d.finish()
+    }
+}
+
 // === impl MutexGuard ===
 
 impl<T: ?Sized> Drop for MutexGuard<'_, T> {

tokio/tests/sync_mutex.rs

@@ -152,3 +152,12 @@ async fn debug_format() {
     let m = Mutex::new(s.to_string());
     assert_eq!(format!("{:?}", s), format!("{:?}", m.lock().await));
 }
+
+#[tokio::test]
+async fn mutex_debug() {
+    let s = "data";
+    let m = Mutex::new(s.to_string());
+    assert_eq!(format!("{:?}", m), r#"Mutex { data: "data" }"#);
+    let _guard = m.lock().await;
+    assert_eq!(format!("{:?}", m), r#"Mutex { data: <locked> }"#)
+}