#[derive(NetworkBehaviour)] doesn't work as documented when out_event is set

[jacklund]

Summary

When using #[derive(NetworkBehaviour)] as documented here, I get a compiler error.

Example code:

use libp2p::{
    floodsub::{Floodsub, FloodsubEvent},
    mdns::{Mdns, MdnsEvent},
    NetworkBehaviour,
};
use std::error::Error;

#[tokio::main]
async fn main() -> Result<(), Box<dyn Error>> {
    #[derive(NetworkBehaviour)]
    #[behaviour(out_event = Event)]
    struct MyBehaviour {
        floodsub: Floodsub,
        mdns: Mdns,
    }

    enum Event {
        FloodsubEvent(FloodsubEvent),
        MdnsEvent(MdnsEvent),
    }

    impl From<FloodsubEvent> for Event {
        fn from(event: FloodsubEvent) -> Self {
            Self::FloodsubEvent(event)
        }
    }

    impl From<MdnsEvent> for Event {
        fn from(event: MdnsEvent) -> Self {
            Self::MdnsEvent(event)
        }
    }
}

Expected behaviour

The code compiles

Actual behaviour

I get the following:

error[E0277]: the trait bound `(): From<MdnsEvent>` is not satisfied
  --> src/bin.rs:52:14
   |
52 |     #[derive(NetworkBehaviour)]
   |              ^^^^^^^^^^^^^^^^ the trait `From<MdnsEvent>` is not implemented for `()`
   |
   = help: the following implementations were found:
             <(Vec<u8>, std::net::SocketAddr) as From<trust_dns_proto::xfer::serial_message::SerialMessage>>
             <(netlink_packet_core::message::NetlinkMessage<T>, netlink_sys::addr::SocketAddr, M) as From<netlink_proto::protocol::request::Request<T, M>>>
   = help: see issue #48214
   = note: this error originates in the derive macro `NetworkBehaviour` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0277]: the trait bound `(): From<FloodsubEvent>` is not satisfied
  --> src/bin.rs:52:14
   |
52 |     #[derive(NetworkBehaviour)]
   |              ^^^^^^^^^^^^^^^^ the trait `From<FloodsubEvent>` is not implemented for `()`
   |
   = help: the following implementations were found:
             <(Vec<u8>, std::net::SocketAddr) as From<trust_dns_proto::xfer::serial_message::SerialMessage>>
             <(netlink_packet_core::message::NetlinkMessage<T>, netlink_sys::addr::SocketAddr, M) as From<netlink_proto::protocol::request::Request<T, M>>>
   = help: see issue #48214
   = note: this error originates in the derive macro `NetworkBehaviour` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0277`.

Note: This may be because of #2373

Possible Solution

Not sure, but I'm guessing that somehow the default out_event of () is being used, somehow.

Version

• libp2p version (version number, commit, or branch): 0.44.0

Would you like to work on fixing this bug?

Sure, but I'm not very familiar with the code, having just started playing with your very awesome library.

[elenaf9]

Hi @jacklund, I think the reason for your error is that #[behaviour(out_event = Event)] is incorrect syntax. The attribute macro requires that the name of your custom event is provided as a String literal (see syn::MetaNameValue).

So it would have to be

    #[derive(NetworkBehaviour)]
    #[behaviour(out_event = "Event")] // Note the quotation marks around "Event".
    struct MyBehaviour {

If the syntax is incorrect, the NetworkBehaviour macro just sets the out_event to (), which caused your error. Lmk if this fixes it.

---

Imo it would be easier to debug this if we just panic within the macro if the syntax is incorrect. I would propose to do that in an Err case here https://github.com/libp2p/rust-libp2p/blob/ef2afcd41eac1459c0408a7a6eb599bfc2035938/swarm-derive/src/lib.rs#L113-L116

What do folks think? @jacklund Would you be interested in doing a PR for this?

[jacklund]

:man_facepalming:

Amazing how I saw the way to do it, but my brain put it without the quotes, I think because I'm used to that mode with Rust.

I'll be happy to take a look at PR-ing that, thank you soooo much @elenaf9 for clearing that up!!

[jacklund]

https://github.com/libp2p/rust-libp2p/pull/2672

[thomaseizinger]

Closing as stale.