Use (empty) as debug representation of PollOpt::empty(). (tokio-rs#690)

Fixes tokio-rs#669.

src/event_imp.rs

@@ -504,10 +504,22 @@ impl fmt::Debug for PollOpt {
             }
         }
 
+        if !one {
+            fmt.write_str("(empty)")?;
+        }
+
         Ok(())
     }
 }
 
+#[test]
+fn test_debug_pollopt() {
+    assert_eq!("(empty)", format!("{:?}", PollOpt::empty()));
+    assert_eq!("Edge-Triggered", format!("{:?}", PollOpt::edge()));
+    assert_eq!("Level-Triggered", format!("{:?}", PollOpt::level()));
+    assert_eq!("OneShot", format!("{:?}", PollOpt::oneshot()));
+}
+
 /// A set of readiness event kinds
 ///
 /// `Ready` is a set of operation descriptors indicating which kind of an
@@ -914,6 +926,13 @@ impl fmt::Debug for Ready {
     }
 }
 
+#[test]
+fn test_debug_ready() {
+    assert_eq!("(empty)", format!("{:?}", Ready::empty()));
+    assert_eq!("Readable", format!("{:?}", Ready::readable()));
+    assert_eq!("Writable", format!("{:?}", Ready::writable()));
+}
+
 /// An readiness event returned by [`Poll::poll`].
 ///
 /// `Event` is a [readiness state] paired with a [`Token`]. It is returned by