Rollup merge of #57703 - m-ou-se:mutexguard-debug, r=cramertj

Make MutexGuard's Debug implementation more useful.

Fixes #57702.

src/libstd/sync/mutex.rs

@@ -450,9 +450,7 @@ impl<'a, T: ?Sized> Drop for MutexGuard<'a, T> {
 #[stable(feature = "std_debug", since = "1.16.0")]
 impl<'a, T: ?Sized + fmt::Debug> fmt::Debug for MutexGuard<'a, T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        f.debug_struct("MutexGuard")
-            .field("lock", &self.__lock)
-            .finish()
+        fmt::Debug::fmt(&**self, f)
     }
 }