Add a method to the mpsc::Receiver for producing a non-blocking iterator

src/libstd/sync/mpsc/mod.rs

@@ -311,6 +311,17 @@ pub struct Iter<'a, T: 'a> {
     rx: &'a Receiver<T>
 }
 
+/// An iterator that attempts to yield all pending values for a receiver.
+/// `None` will be returned when there are no pending values remaining or
+/// if the corresponding channel has hung up.
+///
+/// This Iterator will never block the caller in order to wait for data to
+/// become available. Instead, it will return `None`.
+#[unstable(feature = "receiver_try_iter", issue = "34931")]
+pub struct TryIter<'a, T: 'a> {
+    rx: &'a Receiver<T>
+}
+
 /// An owning iterator over messages on a receiver, this iterator will block
 /// whenever `next` is called, waiting for a new message, and `None` will be
 /// returned when the corresponding channel has hung up.
@@ -982,6 +993,16 @@ impl<T> Receiver<T> {
     pub fn iter(&self) -> Iter<T> {
         Iter { rx: self }
     }
+
+    /// Returns an iterator that will attempt to yield all pending values.
+    /// It will return `None` if there are no more pending values or if the
+    /// channel has hung up. The iterator will never `panic!` or block the
+    /// user by waiting for values.
+    #[unstable(feature = "receiver_try_iter", issue = "34931")]
+    pub fn try_iter(&self) -> TryIter<T> {
+        TryIter { rx: self }
+    }
+
 }
 
 impl<T> select::Packet for Receiver<T> {
@@ -1077,6 +1098,13 @@ impl<'a, T> Iterator for Iter<'a, T> {
     fn next(&mut self) -> Option<T> { self.rx.recv().ok() }
 }
 
+#[unstable(feature = "receiver_try_iter", issue = "34931")]
+impl<'a, T> Iterator for TryIter<'a, T> {
+    type Item = T;
+
+    fn next(&mut self) -> Option<T> { self.rx.try_recv().ok() }
+}
+
 #[stable(feature = "receiver_into_iter", since = "1.1.0")]
 impl<'a, T> IntoIterator for &'a Receiver<T> {
     type Item = T;
@@ -1814,6 +1842,34 @@ mod tests {
         assert_eq!(count_rx.recv().unwrap(), 4);
     }
 
+    #[test]
+    fn test_recv_try_iter() {
+        let (request_tx, request_rx) = channel();
+        let (response_tx, response_rx) = channel();
+
+        // Request `x`s until we have `6`.
+        let t = thread::spawn(move|| {
+            let mut count = 0;
+            loop {
+                for x in response_rx.try_iter() {
+                    count += x;
+                    if count == 6 {
+                        return count;
+                    }
+                }
+                request_tx.send(()).unwrap();
+            }
+        });
+
+        for _ in request_rx.iter() {
+            if response_tx.send(2).is_err() {
+                break;
+            }
+        }
+
+        assert_eq!(t.join().unwrap(), 6);
+    }
+
     #[test]
     fn test_recv_into_iter_owned() {
         let mut iter = {