Cancel on_empty on drop of FutureSender

Fixes an assertion that otherwise gets tripped.

Closes rust-lang#184

src/stream/channel.rs

@@ -187,3 +187,13 @@ impl<T, E> Future for FutureSender<T, E> {
         }
     }
 }
+
+impl<T, E> Drop for FutureSender<T, E> {
+    fn drop(&mut self) {
+        if let Some(token) = self.on_empty_token.take() {
+            if let Some(sender) = self.sender.take() {
+                sender.inner.slot.cancel(token);
+            }
+        }
+    }
+}

tests/channel.rs

@@ -57,15 +57,15 @@ impl task::Unpark for Unpark {
 
 #[test]
 fn poll_future_then_drop() {
-    let (tx, _) = channel::<u32, u32>();
+    let (tx, _rx) = channel::<u32, u32>();
 
     let tx = tx.send(Ok(1));
     let mut t = task::spawn(tx);
 
     // First poll succeeds
     let tx = match t.poll_future(Arc::new(Unpark)) {
         Ok(Async::Ready(tx)) => tx,
-        _ => unreachable!(),
+        _ => panic!(),
     };
 
     // Send another value
@@ -75,7 +75,7 @@ fn poll_future_then_drop() {
     // Second poll doesn't
     match t.poll_future(Arc::new(Unpark)) {
         Ok(Async::NotReady) => {},
-        _ => unreachable!(),
+        _ => panic!(),
     };
 
     drop(t);