NetworkBehaviour derive macro now requires From<Void> for corresponding OutEvent

[drHuangMHT]

Why From<Void>? Void means something never happens. Is it intentional? Errors start popping up when using latest libp2p in this repo.

[thomaseizinger]

It depends which NetworkBehaviours you are composing. Some of them never emit an event and thus have their OutEvent set to Void.

[drHuangMHT]

Before I upgraded. It's not an issue in 0.50 or 0.51 when I started using libp2p. The code compiles just fine with keep_alive without deriving From<Void>. Confirmed with code on 0.50 with feature full.

[thomaseizinger]

To answer your initial question, yes it is intentional. The custom-derive doesn't know the semantics of the code so it cannot "skip" this implementation despite it being not possible at runtime. The implementation is very simple though:

impl From<Void> for Event {
	fn(v: Void) -> Event {
		void::unreachable(v)
	}
}

[drHuangMHT]

But libp2p does not re-export Void, is that also intentional?

[thomaseizinger]

void is already 1.0.0 so we don't export it as we consider it stable enough to just depend on it.

[thomaseizinger]

Also see #3977.