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.

swarm/src/behaviour.rs

@@ -77,14 +77,14 @@ use std::{task::Context, task::Poll};
 /// [`NetworkBehaviour::poll`] it will first poll the first `struct` member until it returns
 /// [`Poll::Pending`] before moving on to later members.
 ///
-/// Events ([`NetworkBehaviour::OutEvent`]) returned by each `struct` member are wrapped in a new
+/// Events ([`NetworkBehaviour::ToSwarm`]) returned by each `struct` member are wrapped in a new
 /// `enum` event, with an `enum` variant for each `struct` member. Users can define this event
-/// `enum` themselves and provide the name to the derive macro via `#[behaviour(out_event =
-/// "MyCustomOutEvent")]`. If the user does not specify an `out_event`, the derive macro generates
+/// `enum` themselves and provide the name to the derive macro via `#[behaviour(to_swarm =
+/// "MyCustomOutEvent")]`. If the user does not specify an `to_swarm`, the derive macro generates
 /// the event definition itself, naming it `<STRUCT_NAME>Event`.
 ///
 /// The aforementioned conversion of each of the event types generated by the struct members to the
-/// custom `out_event` is handled by [`From`] implementations which the user needs to define in
+/// custom `to_swarm` is handled by [`From`] implementations which the user needs to define in
 /// addition to the event `enum` itself.
 ///
 /// ``` rust
@@ -92,7 +92,7 @@ use std::{task::Context, task::Poll};
 /// # use libp2p_ping as ping;
 /// # use libp2p_swarm_derive::NetworkBehaviour;
 /// #[derive(NetworkBehaviour)]
-/// #[behaviour(out_event = "Event")]
+/// #[behaviour(to_swarm = "Event")]
 /// # #[behaviour(prelude = "libp2p_swarm::derive_prelude")]
 /// struct MyBehaviour {
 ///   identify: identify::Behaviour,
@@ -121,7 +121,7 @@ pub trait NetworkBehaviour: 'static {
     type ConnectionHandler: ConnectionHandler;
 
     /// Event generated by the `NetworkBehaviour` and that the swarm will report back.
-    type OutEvent: Send + 'static;
+    type ToSwarm: Send + 'static;
 
     /// Callback that is invoked for every new inbound connection.
     ///
@@ -212,7 +212,7 @@ pub trait NetworkBehaviour: 'static {
         &mut self,
         cx: &mut Context<'_>,
         params: &mut impl PollParameters,
-    ) -> Poll<ToSwarm<Self::OutEvent, THandlerInEvent<Self>>>;
+    ) -> Poll<ToSwarm<Self::ToSwarm, THandlerInEvent<Self>>>;
 }
 
 /// Parameters passed to `poll()`, that the `NetworkBehaviour` has access to.