fix(swarm-derive): allow unreachable_code on uninhabited ToSwarm fields

swarm-derive/src/lib.rs

@@ -398,7 +398,16 @@ fn build_struct(ast: &DeriveInput, data_struct: &DataStruct) -> syn::Result<Toke
 
             quote! {
                 match #trait_to_impl::poll(&mut self.#field, cx) {
-                    std::task::Poll::Ready(e) => return std::task::Poll::Ready(e.map_out(#map_out_event).map_in(#map_in_event)),
+                    std::task::Poll::Ready(e) => {
+                        // For a field whose `ToSwarm` is uninhabited (e.g. `Infallible`, as with
+                        // `connection_limits::Behaviour`), no `e` can ever reach this arm, so the
+                        // call below is unreachable in practice. Recent nightly lints that as
+                        // `unreachable_code` under `-D warnings`; the arm itself must still be
+                        // generated so this compiles for every other field whose `ToSwarm` is
+                        // inhabited.
+                        #[allow(unreachable_code)]
+                        return std::task::Poll::Ready(e.map_out(#map_out_event).map_in(#map_in_event));
+                    }
                     std::task::Poll::Pending => {},
                 }
             }

swarm/tests/swarm_derive.rs

@@ -613,6 +613,23 @@ fn custom_out_event_no_type_parameters() {
     require_net_behaviour::<Behaviour<()>>();
 }
 
+// Regression test for https://github.com/libp2p/rust-libp2p/issues/6600: a field whose
+// `NetworkBehaviour::ToSwarm` is uninhabited (`dummy::Behaviour`'s is `Infallible`) must not
+// make the generated `poll()` fail to compile under `-D warnings` on toolchains that lint the
+// resulting unreachable `Poll::Ready(e) => ...` arm as `unreachable_code`.
+#[test]
+fn uninhabited_to_swarm_field_compiles() {
+    #[allow(dead_code)]
+    #[derive(NetworkBehaviour)]
+    #[behaviour(prelude = "libp2p_swarm::derive_prelude")]
+    struct Foo {
+        ping: ping::Behaviour,
+        limits: dummy::Behaviour,
+    }
+
+    require_net_behaviour::<Foo>();
+}
+
 #[test]
 fn ui() {
     let t = trybuild::TestCases::new();