feat(mpsc): add methods to errors

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

src/mpsc.rs

@@ -20,7 +20,6 @@ use core::{fmt, ops::Index, task::Poll};
 
 /// Error returned by the [`Sender::try_send`] (and [`StaticSender::try_send`])
 /// methods.
-#[derive(Debug)]
 #[non_exhaustive]
 pub enum TrySendError<T = ()> {
     /// The data could not be sent on the channel because the channel is
@@ -33,7 +32,6 @@ pub enum TrySendError<T = ()> {
 
 /// Error returned by [`Sender::send`] and [`Sender::send_ref`], if the
 /// [`Receiver`] half of the channel has been dropped.
-#[derive(Debug)]
 pub struct Closed<T = ()>(T);
 
 #[derive(Debug)]
@@ -67,7 +65,31 @@ struct SendRefInner<'a, T, N: Notify> {
 struct NotifyRx<'a, N: Notify>(&'a WaitCell<N>);
 struct NotifyTx<'a, N: Notify + Unpin>(&'a WaitQueue<N>);
 
-// ==== impl TrySendError ===
+// === impl Closed ===
+
+impl<T> Closed<T> {
+    /// Unwraps the inner `T` value held by this error.
+    ///
+    /// This method allows recovering the original message when sending to a
+    /// channel has failed.
+    pub fn into_inner(self) -> T {
+        self.0
+    }
+}
+
+impl<T> fmt::Debug for Closed<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str("Closed(..)")
+    }
+}
+
+impl<T> fmt::Display for Closed<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str("channel closed")
+    }
+}
+
+// === impl TrySendError ===
 
 impl TrySendError {
     fn with_value<T>(self, value: T) -> TrySendError<T> {
@@ -78,7 +100,54 @@ impl TrySendError {
     }
 }
 
+impl<T> TrySendError<T> {
+    /// Returns `true` if this error was returned because the channel was at
+    /// capacity.
+    pub fn is_full(&self) -> bool {
+        matches!(self, Self::Full(_))
+    }
+
+    /// Returns `true` if this error was returned because the channel has closed
+    /// (e.g. the `Receiver` end has been dropped).
+    ///
+    /// If this returns `true`, no future `try_send` or `send` operation on this
+    /// channel will succeed.
+    pub fn is_closed(&self) -> bool {
+        matches!(self, Self::Full(_))
+    }
+
+    /// Unwraps the inner `T` value held by this error.
+    ///
+    /// This method allows recovering the original message when sending to a
+    /// channel has failed.
+    pub fn into_inner(self) -> T {
+        match self {
+            Self::Full(val) => val,
+            Self::Closed(val) => val,
+        }
+    }
+}
+
+impl<T> fmt::Debug for TrySendError<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(match self {
+            Self::Full(_) => "TrySendError::Full(..)",
+            Self::Closed(_) => "TrySendError::Closed(..)",
+        })
+    }
+}
+
+impl<T> fmt::Display for TrySendError<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(match self {
+            Self::Full(_) => "no available capacity",
+            Self::Closed(_) => "channel closed",
+        })
+    }
+}
+
 // ==== impl Inner ====
+
 impl<N> ChannelCore<N> {
     #[cfg(not(loom))]
     const fn new(capacity: usize) -> Self {