feat: add downcast_ref support to ConnectionDenied type

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.

See #4018

misc/connection-limits/src/lib.rs

@@ -466,6 +466,57 @@ mod tests {
         quickcheck(prop as fn(_));
     }
 
+    #[test]
+    fn max_established_incoming_downcast_ref() {
+        fn prop(Limit(limit): Limit) {
+            let mut swarm1 = Swarm::new_ephemeral(|_| {
+                Behaviour::new(
+                    ConnectionLimits::default().with_max_established_incoming(Some(limit)),
+                )
+            });
+            let mut swarm2 = Swarm::new_ephemeral(|_| {
+                Behaviour::new(
+                    ConnectionLimits::default().with_max_established_incoming(Some(limit)),
+                )
+            });
+
+            async_std::task::block_on(async {
+                let (listen_addr, _) = swarm1.listen().await;
+
+                for _ in 0..limit {
+                    swarm2.connect(&mut swarm1).await;
+                }
+
+                swarm2.dial(listen_addr).unwrap();
+
+                async_std::task::spawn(swarm2.loop_on_next());
+
+                let cause = swarm1
+                    .wait(|event| match event {
+                        SwarmEvent::IncomingConnectionError {
+                            error: ListenError::Denied { cause },
+                            ..
+                        } => Some(cause),
+                        _ => None,
+                    })
+                    .await;
+
+                assert_eq!(cause.downcast_ref::<Exceeded>().unwrap().limit, limit);
+            });
+        }
+
+        #[derive(Debug, Clone)]
+        struct Limit(u32);
+
+        impl Arbitrary for Limit {
+            fn arbitrary(g: &mut Gen) -> Self {
+                Self(g.gen_range(1..10))
+            }
+        }
+
+        quickcheck(prop as fn(_));
+    }
+
     #[derive(libp2p_swarm_derive::NetworkBehaviour)]
     #[behaviour(prelude = "libp2p_swarm::derive_prelude")]
     struct Behaviour {

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 {