feat(swarm): add ConnectionDenied::downcast_ref

Prior to this change it was only possible to downcast a `ConnectionDenied` error when you had ownership of it which isn't the case inside `NetworkBehaviour`s. This change allows downcasting by reference.

See #4018.

Pull-Request: #4020.

swarm/CHANGELOG.md

@@ -64,6 +64,8 @@
 
 - Remove deprecated items. See [PR 3956].
 
+- Add ability to `downcast_ref` ConnectionDenied errors. See [PR 4020].
+
 [PR 3292]: https://github.com/libp2p/rust-libp2p/pull/3292
 [PR 3605]: https://github.com/libp2p/rust-libp2p/pull/3605
 [PR 3651]: https://github.com/libp2p/rust-libp2p/pull/3651
@@ -80,6 +82,7 @@
 [PR 3927]: https://github.com/libp2p/rust-libp2p/pull/3927
 [PR 3955]: https://github.com/libp2p/rust-libp2p/pull/3955
 [PR 3956]: https://github.com/libp2p/rust-libp2p/pull/3956
+[PR 4020]: https://github.com/libp2p/rust-libp2p/pull/4020
 [PR 4037]: https://github.com/libp2p/rust-libp2p/pull/4037
 
 ## 0.42.2

swarm/src/lib.rs

@@ -1730,6 +1730,14 @@ impl ConnectionDenied {
 
         Ok(*inner)
     }
+
+    /// Attempt to downcast to a particular reason for why the connection was denied.
+    pub fn downcast_ref<E>(&self) -> Option<&E>
+    where
+        E: error::Error + Send + Sync + 'static,
+    {
+        self.inner.downcast_ref::<E>()
+    }
 }
 
 impl fmt::Display for ConnectionDenied {