swarm-derive/: Derive Debug for generated OutEvent (#2821)

When generating an `OutEvent` `enum` definition for a user, derive `Debug`
for that `enum`.

Why not derive `Clone`, `PartialEq` and `Eq` for the generated `enum`
definition?

While it is fine to require all sub-`OutEvent`s to implement
`Debug`, the same does not apply to traits like `Clone`. I
suggest users that need `Clone` to define their own `OutEvent`.

swarm-derive/src/lib.rs

@@ -159,6 +159,7 @@ fn build_struct(ast: &DeriveInput, data_struct: &DataStruct) -> TokenStream {
                     let visibility = &ast.vis;
 
                     Some(quote! {
+                        #[derive(::std::fmt::Debug)]
                         #visibility enum #name #impl_generics
                             #where_clause
                         {

swarm-derive/tests/test.rs

@@ -21,6 +21,7 @@
 use futures::prelude::*;
 use libp2p::swarm::{NetworkBehaviour, SwarmEvent};
 use libp2p_swarm_derive::*;
+use std::fmt::Debug;
 
 /// Small utility to check that a type implements `NetworkBehaviour`.
 #[allow(dead_code)]
@@ -275,7 +276,10 @@ fn custom_event_mismatching_field_names() {
 fn bound() {
     #[allow(dead_code)]
     #[derive(NetworkBehaviour)]
-    struct Foo<T: Copy + NetworkBehaviour> {
+    struct Foo<T: Copy + NetworkBehaviour>
+    where
+        <T as NetworkBehaviour>::OutEvent: Debug,
+    {
         ping: libp2p::ping::Ping,
         bar: T,
     }
@@ -288,6 +292,7 @@ fn where_clause() {
     struct Foo<T>
     where
         T: Copy + NetworkBehaviour,
+        <T as NetworkBehaviour>::OutEvent: Debug,
     {
         ping: libp2p::ping::Ping,
         bar: T,
@@ -489,3 +494,21 @@ fn event_process() {
         };
     }
 }
+
+#[test]
+fn generated_out_event_derive_debug() {
+    #[allow(dead_code)]
+    #[derive(NetworkBehaviour)]
+    struct Foo {
+        ping: libp2p::ping::Ping,
+    }
+
+    fn require_debug<T>()
+    where
+        T: NetworkBehaviour,
+        <T as NetworkBehaviour>::OutEvent: Debug,
+    {
+    }
+
+    require_debug::<Foo>();
+}