Expose SendFuture and RecvFuture types

Having these types available makes it easier to store futures for later
use as the named types can be embedded in other types at compile time.

embassy/src/util/mpsc.rs

@@ -156,18 +156,10 @@ where
     /// closed by `recv` until they are all consumed.
     ///
     /// [`close`]: Self::close
-    pub async fn recv(&mut self) -> Option<T> {
-        futures::future::poll_fn(|cx| self.recv_poll(cx)).await
-    }
-
-    fn recv_poll(&mut self, cx: &mut Context<'_>) -> Poll<Option<T>> {
-        Channel::lock(self.channel_cell, |c| {
-            match c.try_recv_with_context(Some(cx)) {
-                Ok(v) => Poll::Ready(Some(v)),
-                Err(TryRecvError::Closed) => Poll::Ready(None),
-                Err(TryRecvError::Empty) => Poll::Pending,
-            }
-        })
+    pub fn recv(&mut self) -> RecvFuture<'ch, M, T, N> {
+        RecvFuture {
+            channel_cell: self.channel_cell,
+        }
     }
 
     /// Attempts to immediately receive a message on this `Receiver`
@@ -202,6 +194,30 @@ where
     }
 }
 
+pub struct RecvFuture<'ch, M, T, const N: usize>
+where
+    M: Mutex<Data = ()>,
+{
+    channel_cell: &'ch UnsafeCell<ChannelCell<M, T, N>>,
+}
+
+impl<'ch, M, T, const N: usize> Future for RecvFuture<'ch, M, T, N>
+where
+    M: Mutex<Data = ()>,
+{
+    type Output = Option<T>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T>> {
+        Channel::lock(self.channel_cell, |c| {
+            match c.try_recv_with_context(Some(cx)) {
+                Ok(v) => Poll::Ready(Some(v)),
+                Err(TryRecvError::Closed) => Poll::Ready(None),
+                Err(TryRecvError::Empty) => Poll::Pending,
+            }
+        })
+    }
+}
+
 impl<'ch, M, T, const N: usize> Sender<'ch, M, T, N>
 where
     M: Mutex<Data = ()>,
@@ -224,12 +240,11 @@ where
     ///
     /// [`close`]: Receiver::close
     /// [`Receiver`]: Receiver
-    pub async fn send(&self, message: T) -> Result<(), SendError<T>> {
+    pub fn send(&self, message: T) -> SendFuture<'ch, M, T, N> {
         SendFuture {
             sender: self.clone(),
             message: Some(message),
         }
-        .await
     }
 
     /// Attempts to immediately send a message on this `Sender`
@@ -278,7 +293,7 @@ where
     }
 }
 
-struct SendFuture<'ch, M, T, const N: usize>
+pub struct SendFuture<'ch, M, T, const N: usize>
 where
     M: Mutex<Data = ()>,
 {