feat(swarm): rename associated types for message passing

Previously, the associated types on `NetworkBehaviour` and `ConnectionHandler` carried generic names like `InEvent` and `OutEvent`. These names are _correct_ in that `OutEvent`s are passed out and `InEvent`s are passed in but they don't help users understand how these types are used.

In theory, a `ConnectionHandler` could be used separately from `NetworkBehaviour`s but that is highly unlikely. Thus, we rename these associated types to indicate, where the message is going to be sent to:

- `NetworkBehaviour::OutEvent` is renamed to `ToSwarm`: It describes the message(s) a `NetworkBehaviour` can emit to the `Swarm`. The user is going to receive those in `SwarmEvent::Behaviour`.
- `ConnectionHandler::InEvent` is renamed to `FromBehaviour`: It describes the message(s) a `ConnectionHandler` can receive from its behaviour via `ConnectionHandler::on_swarm_event`. The `NetworkBehaviour` can send it via the `ToSwarm::NotifyHandler` command.
- `ConnectionHandler::OutEvent` is renamed to `ToBehaviour`: It describes the message(s) a `ConnectionHandler` can send back to the behaviour via the now also renamed `ConnectionHandlerEvent::NotifyBehaviour` (previously `ConnectionHandlerEvent::Custom`)

Resolves: #2854.

Pull-Request: #3848.

protocols/rendezvous/src/client.rs

@@ -163,7 +163,7 @@ pub enum Event {
 impl NetworkBehaviour for Behaviour {
     type ConnectionHandler =
         SubstreamConnectionHandler<void::Void, outbound::Stream, outbound::OpenInfo>;
-    type OutEvent = Event;
+    type ToSwarm = Event;
 
     fn handle_established_inbound_connection(
         &mut self,
@@ -244,7 +244,7 @@ impl NetworkBehaviour for Behaviour {
         &mut self,
         cx: &mut Context<'_>,
         _: &mut impl PollParameters,
-    ) -> Poll<ToSwarm<Self::OutEvent, THandlerInEvent<Self>>> {
+    ) -> Poll<ToSwarm<Self::ToSwarm, THandlerInEvent<Self>>> {
         if let Some(event) = self.events.pop_front() {
             return Poll::Ready(event);
         }

protocols/rendezvous/src/server.rs

@@ -110,7 +110,7 @@ pub enum Event {
 
 impl NetworkBehaviour for Behaviour {
     type ConnectionHandler = SubstreamConnectionHandler<inbound::Stream, Void, ()>;
-    type OutEvent = Event;
+    type ToSwarm = Event;
 
     fn handle_established_inbound_connection(
         &mut self,
@@ -165,7 +165,7 @@ impl NetworkBehaviour for Behaviour {
         &mut self,
         cx: &mut Context<'_>,
         _: &mut impl PollParameters,
-    ) -> Poll<ToSwarm<Self::OutEvent, THandlerInEvent<Self>>> {
+    ) -> Poll<ToSwarm<Self::ToSwarm, THandlerInEvent<Self>>> {
         if let Poll::Ready(ExpiredRegistration(registration)) = self.registrations.poll(cx) {
             return Poll::Ready(ToSwarm::GenerateEvent(Event::RegistrationExpired(
                 registration,

protocols/rendezvous/src/substream_handler.rs

@@ -354,8 +354,8 @@ where
     TInboundSubstreamHandler: Send + 'static,
     TOutboundSubstreamHandler: Send + 'static,
 {
-    type InEvent = InEvent<TOutboundOpenInfo, TInboundInEvent, TOutboundInEvent>;
-    type OutEvent = OutEvent<TInboundOutEvent, TOutboundOutEvent, TInboundError, TOutboundError>;
+    type FromBehaviour = InEvent<TOutboundOpenInfo, TInboundInEvent, TOutboundInEvent>;
+    type ToBehaviour = OutEvent<TInboundOutEvent, TOutboundOutEvent, TInboundError, TOutboundError>;
     type Error = Void;
     type InboundProtocol = PassthroughProtocol;
     type OutboundProtocol = PassthroughProtocol;
@@ -402,7 +402,7 @@ where
         }
     }
 
-    fn on_behaviour_event(&mut self, event: Self::InEvent) {
+    fn on_behaviour_event(&mut self, event: Self::FromBehaviour) {
         match event {
             InEvent::NewSubstream { open_info } => self.new_substreams.push_back(open_info),
             InEvent::NotifyInboundSubstream { id, message } => {
@@ -456,7 +456,7 @@ where
         ConnectionHandlerEvent<
             Self::OutboundProtocol,
             Self::OutboundOpenInfo,
-            Self::OutEvent,
+            Self::ToBehaviour,
             Self::Error,
         >,
     > {