Add Sender::make_receiver and Receiver::make_sender

src/lib.rs

@@ -838,6 +838,19 @@ impl<T> Sender<T> {
     pub fn same_channel(&self, other: &Sender<T>) -> bool {
         Arc::ptr_eq(&self.shared, &other.shared)
     }
+
+    /// Create a new [`Receiver`] for the same channel as this sender.
+    ///
+    /// This is handy when only one half of the channel is stored somewhere (such as inside a
+    /// data structure) and the other half needs to be recovered without threading it through
+    /// separately. If every receiver has already been dropped, the returned receiver will
+    /// observe the channel as disconnected, just as the sender does.
+    pub fn make_receiver(&self) -> Receiver<T> {
+        self.shared.receiver_count.fetch_add(1, Ordering::Relaxed);
+        Receiver {
+            shared: self.shared.clone(),
+        }
+    }
 }
 
 impl<T> Clone for Sender<T> {
@@ -1040,6 +1053,19 @@ impl<T> Receiver<T> {
     pub fn same_channel(&self, other: &Receiver<T>) -> bool {
         Arc::ptr_eq(&self.shared, &other.shared)
     }
+
+    /// Create a new [`Sender`] for the same channel as this receiver.
+    ///
+    /// This is handy when only one half of the channel is stored somewhere (such as inside a
+    /// data structure) and the other half needs to be recovered without threading it through
+    /// separately. If every sender has already been dropped, the returned sender will observe
+    /// the channel as disconnected, just as the receiver does.
+    pub fn make_sender(&self) -> Sender<T> {
+        self.shared.sender_count.fetch_add(1, Ordering::Relaxed);
+        Sender {
+            shared: self.shared.clone(),
+        }
+    }
 }
 
 impl<T> Clone for Receiver<T> {

tests/basic.rs

@@ -454,3 +454,40 @@ fn weak_upgrade() {
     tx2.send(()).unwrap();
     assert!(rx.try_recv().is_ok());
 }
+
+#[test]
+fn make_receiver() {
+    let (tx, rx) = unbounded();
+    let rx2 = tx.make_receiver();
+    assert!(rx.same_channel(&rx2));
+    assert_eq!(tx.receiver_count(), 2);
+
+    tx.send(42).unwrap();
+    // A message is only received by a single receiver, so exactly one of the two gets it.
+    let a = rx.try_recv().ok();
+    let b = rx2.try_recv().ok();
+    assert_eq!(a.or(b), Some(42));
+
+    // The channel stays open as long as the recovered receiver lives.
+    drop(rx);
+    assert!(!tx.is_disconnected());
+    drop(rx2);
+    assert!(tx.is_disconnected());
+}
+
+#[test]
+fn make_sender() {
+    let (tx, rx) = unbounded();
+    let tx2 = rx.make_sender();
+    assert!(tx.same_channel(&tx2));
+    assert_eq!(rx.sender_count(), 2);
+
+    tx2.send(1).unwrap();
+    assert_eq!(rx.try_recv().unwrap(), 1);
+
+    // The channel stays open as long as the recovered sender lives.
+    drop(tx);
+    assert!(!rx.is_disconnected());
+    drop(tx2);
+    assert!(rx.is_disconnected());
+}