sync: improve Debug impl for RwLock (#5647)

tokio/src/sync/rwlock.rs

@@ -85,7 +85,6 @@ const MAX_READS: u32 = 10;
 /// [`RwLockWriteGuard`]: struct@RwLockWriteGuard
 /// [`Send`]: trait@std::marker::Send
 /// [_write-preferring_]: https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock#Priority_policies
-#[derive(Debug)]
 pub struct RwLock<T: ?Sized> {
     #[cfg(all(tokio_unstable, feature = "tracing"))]
     resource_span: tracing::Span,
@@ -1095,3 +1094,17 @@ where
         Self::new(T::default())
     }
 }
+
+impl<T: ?Sized> std::fmt::Debug for RwLock<T>
+where
+    T: std::fmt::Debug,
+{
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let mut d = f.debug_struct("RwLock");
+        match self.try_read() {
+            Ok(inner) => d.field("data", &&*inner),
+            Err(_) => d.field("data", &format_args!("<locked>")),
+        };
+        d.finish()
+    }
+}