Remove T: Debug bound on mpsc Debug impls (#866)

Following from https://github.com/tokio-rs/tokio/pull/865, this PR
removes `#[derive(Debug)]` on `mpsc` sender and receiver types in favor
of explicit `impl fmt::Debug` blocks that don't have a `T: fmt::Debug`
bound.

5 files changed, 102 insertions, 34 deletions

diff --git a/tokio-sync/src/mpsc/bounded.rs b/tokio-sync/src/mpsc/bounded.rs
index 879d063c..ad5d6eea 100644
--- a/tokio-sync/src/mpsc/bounded.rs
+++ b/tokio-sync/src/mpsc/bounded.rs
@@ -7,7 +7,6 @@ use std::fmt;
 /// Send values to the associated `Receiver`.
 ///
 /// Instances are created by the [`channel`](fn.channel.html) function.
-#[derive(Debug)]
 pub struct Sender<T> {
     chan: chan::Tx<T, Semaphore>,
 }
@@ -18,15 +17,30 @@ impl<T> Clone for Sender<T> {
     }
 }
 
+impl<T> fmt::Debug for Sender<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        fmt.debug_struct("Sender")
+            .field("chan", &self.chan)
+            .finish()
+    }
+}
+
 /// Receive values from the associated `Sender`.
 ///
 /// Instances are created by the [`channel`](fn.channel.html) function.
-#[derive(Debug)]
 pub struct Receiver<T> {
     /// The channel receiver
     chan: chan::Rx<T, Semaphore>,
 }
 
+impl<T> fmt::Debug for Receiver<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        fmt.debug_struct("Receiver")
+            .field("chan", &self.chan)
+            .finish()
+    }
+}
+
 /// Error returned by the `Sender`.
 #[derive(Debug)]
 pub struct SendError(());
diff --git a/tokio-sync/src/mpsc/chan.rs b/tokio-sync/src/mpsc/chan.rs
index f78aadfc..01d4bd75 100644
--- a/tokio-sync/src/mpsc/chan.rs
+++ b/tokio-sync/src/mpsc/chan.rs
@@ -4,23 +4,44 @@ use futures::task::AtomicTask;
 
 use std::cell::UnsafeCell;
 use std::process;
+use std::fmt;
 use std::sync::Arc;
 use std::sync::atomic::AtomicUsize;
 use std::sync::atomic::Ordering::{AcqRel, Relaxed};
 
 /// Channel sender
-#[derive(Debug)]
 pub(crate) struct Tx<T, S: Semaphore> {
     inner: Arc<Chan<T, S>>,
     permit: S::Permit,
 }
 
+impl<T, S: Semaphore> fmt::Debug for Tx<T, S>
+where S::Permit: fmt::Debug,
+      S: fmt::Debug
+{
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        fmt.debug_struct("Tx")
+            .field("inner", &self.inner)
+            .field("permit", &self.permit)
+            .finish()
+    }
+}
+
 /// Channel receiver
-#[derive(Debug)]
 pub(crate) struct Rx<T, S: Semaphore> {
     inner: Arc<Chan<T, S>>,
 }
 
+impl<T, S: Semaphore> fmt::Debug for Rx<T, S>
+where S: fmt::Debug
+{
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        fmt.debug_struct("Rx")
+            .field("inner", &self.inner)
+            .finish()
+    }
+}
+
 #[derive(Debug, Eq, PartialEq)]
 pub(crate) enum TrySendError {
     Closed,
@@ -53,7 +74,6 @@ pub(crate) trait Semaphore: Sync {
     fn close(&self);
 }
 
-#[derive(Debug)]
 struct Chan<T, S> {
     /// Handle to the push half of the lock-free list.
     tx: list::Tx<T>,
@@ -73,8 +93,20 @@ struct Chan<T, S> {
     rx_fields: UnsafeCell<RxFields<T>>,
 }
 
+impl<T, S> fmt::Debug for Chan<T, S> where S: fmt::Debug
+{
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        fmt.debug_struct("Chan")
+            .field("tx", &self.tx)
+            .field("semaphore", &self.semaphore)
+            .field("rx_task", &self.rx_task)
+            .field("tx_count", &self.tx_count)
+            .field("rx_fields", &self.rx_fields)
+            .finish()
+    }
+}
+
 /// Fields only accessed by `Rx` handle.
-#[derive(Debug)]
 struct RxFields<T> {
     /// Channel receiver. This field is only accessed by the `Receiver` type.
     list: list::Rx<T>,
@@ -83,6 +115,16 @@ struct RxFields<T> {
     rx_closed: bool,
 }
 
+impl<T> fmt::Debug for RxFields<T>
+{
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        fmt.debug_struct("RxFields")
+            .field("list", &self.list)
+            .field("rx_closed", &self.rx_closed)
+            .finish()
+    }
+}
+
 unsafe impl<T: Send, S: Send> Send for Chan<T, S> {}
 unsafe impl<T: Send, S: Sync> Sync for Chan<T, S> {}
 
diff --git a/tokio-sync/src/mpsc/list.rs b/tokio-sync/src/mpsc/list.rs
index dde380bf..aedd9954 100644
--- a/tokio-sync/src/mpsc/list.rs
+++ b/tokio-sync/src/mpsc/list.rs
@@ -212,7 +212,7 @@ impl<T> Tx<T> {
     }
 }
 
-impl<T: fmt::Debug> fmt::Debug for Tx<T> {
+impl<T> fmt::Debug for Tx<T> {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         use std::sync::atomic::Ordering::Relaxed;
 
@@ -318,7 +318,7 @@ impl<T> Rx<T> {
     }
 }
 
-impl<T: fmt::Debug> fmt::Debug for Rx<T> {
+impl<T> fmt::Debug for Rx<T> {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         fmt.debug_struct("Rx")
             .field("head", &self.head)
diff --git a/tokio-sync/src/mpsc/unbounded.rs b/tokio-sync/src/mpsc/unbounded.rs
index 16bb659e..1473c1b9 100644
--- a/tokio-sync/src/mpsc/unbounded.rs
+++ b/tokio-sync/src/mpsc/unbounded.rs
@@ -9,7 +9,6 @@ use std::fmt;
 ///
 /// Instances are created by the
 /// [`unbounded_channel`](fn.unbounded_channel.html) function.
-#[derive(Debug)]
 pub struct UnboundedSender<T> {
     chan: chan::Tx<T, Semaphore>,
 }
@@ -20,16 +19,31 @@ impl<T> Clone for UnboundedSender<T> {
     }
 }
 
+impl<T> fmt::Debug for UnboundedSender<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        fmt.debug_struct("UnboundedSender")
+            .field("chan", &self.chan)
+            .finish()
+    }
+}
+
 /// Receive values from the associated `UnboundedSender`.
 ///
 /// Instances are created by the
 /// [`unbounded_channel`](fn.unbounded_channel.html) function.
-#[derive(Debug)]
 pub struct UnboundedReceiver<T> {
     /// The channel receiver
     chan: chan::Rx<T, Semaphore>,
 }
 
+impl<T> fmt::Debug for UnboundedReceiver<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        fmt.debug_struct("UnboundedReceiver")
+            .field("chan", &self.chan)
+            .finish()
+    }
+}
+
 /// Error returned by the `UnboundedSender`.
 #[derive(Debug)]
 pub struct UnboundedSendError(());
diff --git a/tokio-sync/tests/mpsc.rs b/tokio-sync/tests/mpsc.rs
index 4f2ea4f9..ce3fafc1 100644
--- a/tokio-sync/tests/mpsc.rs
+++ b/tokio-sync/tests/mpsc.rs
@@ -92,33 +92,31 @@ fn send_recv_unbounded() {
 }
 
 #[test]
-fn clone_sender_no_t_clone_buffer() {
-    #[derive(Debug, PartialEq, Eq)]
-    struct NotClone;
-    let (mut tx, mut rx) = mpsc::channel(100);
-    tx.try_send(NotClone).unwrap();
-    tx.clone().try_send(NotClone).unwrap();
-
-    let val = assert_ready!(rx.poll());
-    assert_eq!(val, Some(NotClone));
-
-    let val = assert_ready!(rx.poll());
-    assert_eq!(val, Some(NotClone));
+fn no_t_bounds_buffer() {
+    struct NoImpls;
+    let (tx, mut rx) = mpsc::channel(100);
+
+    // sender should be Debug even though T isn't Debug
+    println!("{:?}", tx);
+    // same with Receiver
+    println!("{:?}", rx);
+    // and sender should be Clone even though T isn't Clone
+    assert!(tx.clone().try_send(NoImpls).is_ok());
+    assert!(assert_ready!(rx.poll()).is_some());
 }
 
 #[test]
-fn clone_sender_no_t_clone_unbounded() {
-    #[derive(Debug, PartialEq, Eq)]
-    struct NotClone;
-    let (mut tx, mut rx) = mpsc::unbounded_channel();
-    tx.try_send(NotClone).unwrap();
-    tx.clone().try_send(NotClone).unwrap();
-
-    let val = assert_ready!(rx.poll());
-    assert_eq!(val, Some(NotClone));
-
-    let val = assert_ready!(rx.poll());
-    assert_eq!(val, Some(NotClone));
+fn no_t_bounds_unbounded() {
+    struct NoImpls;
+    let (tx, mut rx) = mpsc::unbounded_channel();
+
+    // sender should be Debug even though T isn't Debug
+    println!("{:?}", tx);
+    // same with Receiver
+    println!("{:?}", rx);
+    // and sender should be Clone even though T isn't Clone
+    assert!(tx.clone().try_send(NoImpls).is_ok());
+    assert!(assert_ready!(rx.poll()).is_some());
 }
 
 #[test]