feat(swarm): add ConnectionDenied::downcast_ref

Prior to this change it was possible to only have a reference to a ConnectionDenied error and not be
able to downcast it to check if the connection-limits produced the error.

swarm/CHANGELOG.md

@@ -58,6 +58,8 @@
 
 - Remove deprecated items. See [PR 3956].
 
+- Add ability to `downcast_ref` ConnectionDenied errors. See [PR 4020].
+
 [PR 3605]: https://github.com/libp2p/rust-libp2p/pull/3605
 [PR 3651]: https://github.com/libp2p/rust-libp2p/pull/3651
 [PR 3715]: https://github.com/libp2p/rust-libp2p/pull/3715
@@ -73,6 +75,7 @@
 [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
 
 ## 0.42.2
 

swarm/src/lib.rs

@@ -1710,6 +1710,14 @@ 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 {