Override Debug for oneshot::{Sender,Receiver}.

Prior to this diff, the Debug impl for oneshot channels printed something like:
```
Sender { inner: Inner { complete: false, data: Lock { locked: false, data: UnsafeCell }, rx_task: Lock { locked: false, data: UnsafeCell }, tx_task: Lock { locked: false, data: UnsafeCell } } }
```
which isn't very helpful. Instead, just print
`Sender { complete: false }` (or `true`, if the other end has dropped).

Note that the `T: Debug` bound is retained to allow for the possibility
of allowing slightly more interesting debug output in the future.

Author: goffrie

futures-channel/src/oneshot.rs

@@ -16,15 +16,13 @@ use crate::lock::Lock;
 ///
 /// This is created by the [`channel`](channel) function.
 #[must_use = "futures do nothing unless you `.await` or poll them"]
-#[derive(Debug)]
 pub struct Receiver<T> {
     inner: Arc<Inner<T>>,
 }
 
 /// A means of transmitting a single value to another task.
 ///
 /// This is created by the [`channel`](channel) function.
-#[derive(Debug)]
 pub struct Sender<T> {
     inner: Arc<Inner<T>>,
 }
@@ -35,7 +33,6 @@ impl<T> Unpin for Sender<T> {}
 
 /// Internal state of the `Receiver`/`Sender` pair above. This is all used as
 /// the internal synchronization between the two for send/recv operations.
-#[derive(Debug)]
 struct Inner<T> {
     /// Indicates whether this oneshot is complete yet. This is filled in both
     /// by `Sender::drop` and by `Receiver::drop`, and both sides interpret it
@@ -394,6 +391,12 @@ impl<T> Drop for Sender<T> {
     }
 }
 
+impl<T: fmt::Debug> fmt::Debug for Sender<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Sender").field("complete", &self.inner.complete).finish()
+    }
+}
+
 /// A future that resolves when the receiving end of a channel has hung up.
 ///
 /// This is an `.await`-friendly interface around [`poll_canceled`](Sender::poll_canceled).
@@ -481,3 +484,9 @@ impl<T> Drop for Receiver<T> {
         self.inner.drop_rx()
     }
 }
+
+impl<T: fmt::Debug> fmt::Debug for Receiver<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Receiver").field("complete", &self.inner.complete).finish()
+    }
+}

futures/tests/oneshot.rs

@@ -64,3 +64,15 @@ fn oneshot_drop_rx() {
     drop(rx);
     assert_eq!(Err(2), tx.send(2));
 }
+
+#[test]
+fn oneshot_debug() {
+    let (tx, rx) = oneshot::channel::<i32>();
+    assert_eq!(format!("{:?}", tx), "Sender { complete: false }");
+    assert_eq!(format!("{:?}", rx), "Receiver { complete: false }");
+    drop(rx);
+    assert_eq!(format!("{:?}", tx), "Sender { complete: true }");
+    let (tx, rx) = oneshot::channel::<i32>();
+    drop(tx);
+    assert_eq!(format!("{:?}", rx), "Receiver { complete: true }");
+}