core/muxing: Drop Sync requirement for StreamMuxer on `StreamMuxe…

…rBox` (#2775)

`StreamMuxerBox` is never shared across threads but owned by a single
connection. Restricting it to be `Sync` unnecessarily limits the design
space around the `StreamMuxer` trait and its implementations.

core/CHANGELOG.md

@@ -3,9 +3,11 @@
 - Remove `StreamMuxer::poll_event` in favor of individual functions: `poll_inbound`, `poll_outbound`
   and `poll_address_change`. Consequently, `StreamMuxerEvent` is also removed. See [PR 2724].
 - Drop `Unpin` requirement from `SubstreamBox`. See [PR 2762] and [PR 2776].
+- Drop `Sync` requirement on `StreamMuxer` for constructing `StreamMuxerBox`. See [PR 2775].
 
 [PR 2724]: https://github.com/libp2p/rust-libp2p/pull/2724
 [PR 2762]: https://github.com/libp2p/rust-libp2p/pull/2762
+[PR 2775]: https://github.com/libp2p/rust-libp2p/pull/2775
 [PR 2776]: https://github.com/libp2p/rust-libp2p/pull/2776
 
 # 0.34.0

core/src/muxing/boxed.rs

@@ -10,7 +10,7 @@ use std::task::{Context, Poll};
 
 /// Abstract `StreamMuxer`.
 pub struct StreamMuxerBox {
-    inner: Box<dyn StreamMuxer<Substream = SubstreamBox, Error = io::Error> + Send + Sync>,
+    inner: Box<dyn StreamMuxer<Substream = SubstreamBox, Error = io::Error> + Send>,
 }
 
 /// Abstract type for asynchronous reading and writing.
@@ -70,7 +70,7 @@ impl StreamMuxerBox {
     /// Turns a stream muxer into a `StreamMuxerBox`.
     pub fn new<T>(muxer: T) -> StreamMuxerBox
     where
-        T: StreamMuxer + Send + Sync + 'static,
+        T: StreamMuxer + Send + 'static,
         T::Substream: Send + 'static,
         T::Error: Send + Sync + 'static,
     {

core/src/transport/upgrade.rs

@@ -299,7 +299,7 @@ impl<T> Multiplexed<T> {
         T::Dial: Send + 'static,
         T::ListenerUpgrade: Send + 'static,
         T::Error: Send + Sync,
-        M: StreamMuxer + Send + Sync + 'static,
+        M: StreamMuxer + Send + 'static,
         M::Substream: Send + 'static,
         M::Error: Send + Sync + 'static,
     {