fix(swarm-derive): add bounds to OutEvent on generic fields (#3393)

This PR isolates the bugfix for the `NetworkBehaviour` derive implementation for structures with generic fields. When `out_event` was not provided, the generated enum was missing the `NetworkBehaviour` impl constraint for the generic variants whilst using the generics for `<Generic>::OutEvent`.

Meanwhile I also found that the generated `poll` function `loop`s the sub behaviours and either `return`'s when `Poll::Ready` or `break`'s when `Poll::Pending`. This is a relict from when we still had `NetworkBehaviourEventProcess` which had added a branch within this loop that did not `return` but consume the event and `continue`. This trait was removed a while ago meaning this `loop` is no longer needed.

swarm/tests/swarm_derive.rs

@@ -308,6 +308,44 @@ fn with_either() {
     }
 }
 
+#[test]
+fn with_generics() {
+    #[allow(dead_code)]
+    #[derive(NetworkBehaviour)]
+    #[behaviour(prelude = "libp2p_swarm::derive_prelude")]
+    struct Foo<A, B> {
+        a: A,
+        b: B,
+    }
+
+    #[allow(dead_code)]
+    fn foo() {
+        require_net_behaviour::<
+            Foo<
+                libp2p_kad::Kademlia<libp2p_kad::record::store::MemoryStore>,
+                libp2p_ping::Behaviour,
+            >,
+        >();
+    }
+}
+
+#[test]
+fn with_generics_mixed() {
+    #[allow(dead_code)]
+    #[derive(NetworkBehaviour)]
+    #[behaviour(prelude = "libp2p_swarm::derive_prelude")]
+    struct Foo<A> {
+        a: A,
+        ping: libp2p_ping::Behaviour,
+    }
+
+    #[allow(dead_code)]
+    fn foo() {
+        require_net_behaviour::<Foo<libp2p_kad::Kademlia<libp2p_kad::record::store::MemoryStore>>>(
+        );
+    }
+}
+
 #[test]
 fn custom_event_with_either() {
     use either::Either;