sync: support mpsc send with `&self` (#2861)

Updates the mpsc channel to use the intrusive waker based sempahore.
This enables using `Sender` with `&self`.

Instead of using `Sender::poll_ready` to ensure capacity and updating
the `Sender` state, `async fn Sender::reserve()` is added. This function
returns a `Permit` value representing the reserved capacity.

Fixes: #2637
Refs: #2718 (intrusive waiters)

1 files changed, 2 insertions, 2 deletions

diff --git a/tokio/src/stream/stream_map.rs b/tokio/src/stream/stream_map.rs
index 2f60ea4d..a1c80f15 100644
--- a/tokio/src/stream/stream_map.rs
+++ b/tokio/src/stream/stream_map.rs
@@ -57,8 +57,8 @@ use std::task::{Context, Poll};
 ///
 /// #[tokio::main]
 /// async fn main() {
-///     let (mut tx1, rx1) = mpsc::channel(10);
-///     let (mut tx2, rx2) = mpsc::channel(10);
+///     let (tx1, rx1) = mpsc::channel(10);
+///     let (tx2, rx2) = mpsc::channel(10);
 ///
 ///     tokio::spawn(async move {
 ///         tx1.send(1).await.unwrap();