Make reason for try_send errors clearer (#864)

Author: jonhoo

tokio-sync/src/mpsc/bounded.rs

@@ -239,6 +239,24 @@ impl<T> TrySendError<T> {
     pub fn into_inner(self) -> T {
         self.value
     }
+
+    /// Did the send fail because the channel has been closed?
+    pub fn is_closed(&self) -> bool {
+        if let ErrorKind::Closed = self.kind {
+            true
+        } else {
+            false
+        }
+    }
+
+    /// Did the send fail because the channel was at capacity?
+    pub fn is_full(&self) -> bool {
+        if let ErrorKind::NoCapacity = self.kind {
+            true
+        } else {
+            false
+        }
+    }
 }
 
 impl<T: fmt::Debug> fmt::Display for TrySendError<T> {

tokio-sync/src/mpsc/unbounded.rs

@@ -48,7 +48,7 @@ impl<T> fmt::Debug for UnboundedReceiver<T> {
 #[derive(Debug)]
 pub struct UnboundedSendError(());
 
-/// Error returned by `UnboundedSender::try_send`.
+/// Returned by `UnboundedSender::try_send` when the channel has been closed.
 #[derive(Debug)]
 pub struct UnboundedTrySendError<T>(T);
 

tokio-sync/tests/mpsc.rs

@@ -255,7 +255,7 @@ fn try_send_fail() {
     tx.try_send("hello").unwrap();
 
     // This should fail
-    assert!(tx.try_send("fail").is_err());
+    assert!(tx.try_send("fail").unwrap_err().is_full());
 
     assert_eq!(rx.next().unwrap().unwrap(), "hello");
 
@@ -300,6 +300,19 @@ fn dropping_rx_closes_channel() {
     assert_eq!(1, Arc::strong_count(&msg));
 }
 
+#[test]
+fn dropping_rx_closes_channel_for_try() {
+    let (mut tx, rx) = mpsc::channel(100);
+
+    let msg = Arc::new(());
+    tx.try_send(msg.clone()).unwrap();
+
+    drop(rx);
+    assert!(tx.try_send(msg.clone()).unwrap_err().is_closed());
+
+    assert_eq!(1, Arc::strong_count(&msg));
+}
+
 #[test]
 fn unconsumed_messagers_are_dropped() {
     let msg = Arc::new(());