Reason for not reporting error in FromSwarm::ConnectionClosed

[dariusc93]

I do have a branch with the changes but before making the PR, I decided to look around since I thought there was a discussion on this but I could not find the reasoning on why an error was supplied for SwarmEvent::ConnectionClosed but not FromSwarm::ConnectionClosed. Supplying an error, if any, could be helpful in figuring out the cause and allow the behaviour to act accordingly, assuming its not just for logging purposes. The following patch should work in this case but wanting to know the reasoning why it was excluded since the search came up empty.

diff --git a/protocols/gossipsub/src/behaviour/tests.rs b/protocols/gossipsub/src/behaviour/tests.rs
index 570cdf43f..90d486035 100644
--- a/protocols/gossipsub/src/behaviour/tests.rs
+++ b/protocols/gossipsub/src/behaviour/tests.rs
@@ -273,6 +273,7 @@ where
                 connection_id,
                 endpoint: &fake_endpoint,
                 remaining_established: active_connections,
+                cause: None,
             }));
         }
     }
diff --git a/protocols/perf/src/client/behaviour.rs b/protocols/perf/src/client/behaviour.rs
index 880bcdd9c..5e430f8f0 100644
--- a/protocols/perf/src/client/behaviour.rs
+++ b/protocols/perf/src/client/behaviour.rs
@@ -116,6 +116,7 @@ impl NetworkBehaviour for Behaviour {
                 connection_id: _,
                 endpoint: _,
                 remaining_established,
+                ..
             }) => {
                 if remaining_established == 0 {
                     assert!(self.connected.remove(&peer_id));
diff --git a/swarm/src/behaviour.rs b/swarm/src/behaviour.rs
index 4be129a4e..51138e36e 100644
--- a/swarm/src/behaviour.rs
+++ b/swarm/src/behaviour.rs
@@ -30,8 +30,8 @@ use crate::connection::ConnectionId;
 use crate::dial_opts::DialOpts;
 use crate::listen_opts::ListenOpts;
 use crate::{
-    ConnectionDenied, ConnectionHandler, DialError, ListenError, THandler, THandlerInEvent,
-    THandlerOutEvent,
+    ConnectionDenied, ConnectionError, ConnectionHandler, DialError, ListenError, THandler,
+    THandlerInEvent, THandlerOutEvent,
 };
 use libp2p_core::{transport::ListenerId, ConnectedPoint, Endpoint, Multiaddr};
 use libp2p_identity::PeerId;
@@ -454,6 +454,7 @@ pub struct ConnectionClosed<'a> {
     pub peer_id: PeerId,
     pub connection_id: ConnectionId,
     pub endpoint: &'a ConnectedPoint,
+    pub cause: Option<&'a ConnectionError>,
     pub remaining_established: usize,
 }
 
diff --git a/swarm/src/lib.rs b/swarm/src/lib.rs
index 0354f39cf..fc910aa7c 100644
--- a/swarm/src/lib.rs
+++ b/swarm/src/lib.rs
@@ -894,6 +894,7 @@ where
                         peer_id,
                         connection_id: id,
                         endpoint: &endpoint,
+                        cause: error.as_ref(),
                         remaining_established: num_established as usize,
                     }));
                 self.pending_swarm_events
diff --git a/swarm/src/test.rs b/swarm/src/test.rs
index 547277550..d49b50439 100644
--- a/swarm/src/test.rs
+++ b/swarm/src/test.rs
@@ -301,6 +301,7 @@ where
             connection_id,
             endpoint,
             remaining_established,
+            cause,
         }: ConnectionClosed,
     ) {
         let mut other_closed_connections = self
@@ -350,6 +351,7 @@ where
                 connection_id,
                 endpoint,
                 remaining_established,
+                cause,
             }));
     }
 }

[thomaseizinger]

There is an issue discussing the duplication between these two in general: #4268