derive(NetworkBehaviour) triggers unreachable_code on nightly when ToSwarm is uninhabited

Summary

With -D warnings #[derive(NetworkBehaviour)] fails to compile if any field's ToSwarm is uninhabited (e.g. connection_limits::Behaviour where ToSwarm = Infallible). Recent nightly now lints unreachable call.

Expected behavior

Generated unreachable code should have #[allow(unreachable_code)]

Actual behavior

error: unreachable call
  --> src/behaviour.rs:12:10
   | #[derive(NetworkBehaviour)]
   |          ^^^^^^^^^^^^^^^^ unreachable call
   = note: `-D unreachable-code` implied by `-D warnings`
   = note: this error originates in the derive macro `NetworkBehaviour`

Relevant log output

Possible Solution

Suggested fix: have libp2p-swarm-derive emit #[allow(unreachable_code)] on the generated conversion/poll code.

Workaround: #[allow(unreachable_code)] at the derive site downstream.

Version

libp2p 0.56, rustc 1.100.0-nightly

Would you like to work on fixing this bug?

Maybe

Start in the libp2p-swarm-derive crate and inspect the generated conversion and poll code for NetworkBehaviour. Reproduce the derive with connection_limits::Behaviour, whose ToSwarm is Infallible, under -D warnings; done means the generated code compiles without an unreachable-code error.