std::sync::mpsc: Add fmt::Debug stubs

Minimal fix for https://github.com/rust-lang/rust/issues/30563

This covers all the public structs I think; except for Iter and
IntoIter, which I don't know if or how they should be handled.

src/libstd/sync/mpsc/mod.rs

@@ -635,6 +635,13 @@ impl<T> Drop for Sender<T> {
     }
 }
 
+#[stable(feature = "mpsc_debug", since = "1.7.0")]
+impl<T> fmt::Debug for Sender<T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "Sender {{ .. }}")
+    }
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // SyncSender
 ////////////////////////////////////////////////////////////////////////////////
@@ -693,6 +700,13 @@ impl<T> Drop for SyncSender<T> {
     }
 }
 
+#[stable(feature = "mpsc_debug", since = "1.7.0")]
+impl<T> fmt::Debug for SyncSender<T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "SyncSender {{ .. }}")
+    }
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // Receiver
 ////////////////////////////////////////////////////////////////////////////////
@@ -987,6 +1001,13 @@ impl<T> Drop for Receiver<T> {
     }
 }
 
+#[stable(feature = "mpsc_debug", since = "1.7.0")]
+impl<T> fmt::Debug for Receiver<T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "Receiver {{ .. }}")
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> fmt::Debug for SendError<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
@@ -2199,4 +2220,22 @@ mod sync_tests {
             repro()
         }
     }
+
+    #[test]
+    fn fmt_debug_sender() {
+        let (tx, _) = channel::<i32>();
+        assert_eq!(format!("{:?}", tx), "Sender { .. }");
+    }
+
+    #[test]
+    fn fmt_debug_recv() {
+        let (_, rx) = channel::<i32>();
+        assert_eq!(format!("{:?}", rx), "Receiver { .. }");
+    }
+
+    #[test]
+    fn fmt_debug_sync_sender() {
+        let (tx, _) = sync_channel::<i32>(1);
+        assert_eq!(format!("{:?}", tx), "SyncSender { .. }");
+    }
 }

src/libstd/sync/mpsc/select.rs

@@ -58,6 +58,8 @@
             issue = "27800")]
 
 
+use fmt;
+
 use core::cell::{Cell, UnsafeCell};
 use core::marker;
 use core::ptr;
@@ -350,6 +352,20 @@ impl Iterator for Packets {
     }
 }
 
+#[stable(feature = "mpsc_debug", since = "1.7.0")]
+impl fmt::Debug for Select {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "Select {{ .. }}")
+    }
+}
+
+#[stable(feature = "mpsc_debug", since = "1.7.0")]
+impl<'rx, T:Send+'rx> fmt::Debug for Handle<'rx, T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "Handle {{ .. }}")
+    }
+}
+
 #[cfg(test)]
 #[allow(unused_imports)]
 mod tests {
@@ -762,4 +778,18 @@ mod tests {
             }
         }
     }
+
+    #[test]
+    fn fmt_debug_select() {
+        let sel = Select::new();
+        assert_eq!(format!("{:?}", sel), "Select { .. }");
+    }
+
+    #[test]
+    fn fmt_debug_handle() {
+        let (_, rx) = channel::<i32>();
+        let sel = Select::new();
+        let mut handle = sel.handle(&rx);
+        assert_eq!(format!("{:?}", handle), "Handle { .. }");
+    }
 }