Unable to use QUIC Transport with SwarmBuilder

[Firaenix]

Summary

Following the example at https://docs.rs/libp2p/latest/libp2p/quic/index.html, I tried to use the QUIC transport to create a Swarm object. However
Example code:

Expected behaviour

I would expect the quic::Connection struct (Which implements StreamMuxer) to be boxed up and accepted as a transport for the SwarmBuilder parameter.

Actual behaviour

I get the following compile-time error:

Version

libp2p 0.50

[thomaseizinger]

For the time being, you need to box up the Connection object yourself. See the tests on how to:

rust-libp2p/transports/quic/tests/smoke.rs

Lines 310 to 319 in be0b62a

	fn create_transport<P: Provider>() -> (PeerId, Boxed<(PeerId, StreamMuxerBox)>) {
	let keypair = generate_tls_keypair();
	let peer_id = keypair.public().to_peer_id();
	let transport = quic::GenTransport::<P>::new(quic::Config::new(&keypair))
	.map(|(p, c), _| (p, StreamMuxerBox::new(c)))
	.boxed();
	(peer_id, transport)
	}

[Firaenix]

Great! Thats very helpful, thank you. I was wondering if there was something like that.

[thomaseizinger]

I think we should rework the API of Swarm to do the boxing internally so this is not necessary.

[thomaseizinger]

Related #3186. Adding a T: Transport type parameter and doing the boxing internally would solve this issue but creates a lot of type parameter and trait bound noise because we have SwarmBuilder and Swarm today. We should streamline the creation of a Swarm first to minimise the impact of this type parameter change.

@libp2p/rust-libp2p-maintainers Thoughts?

[mxinden]

No opinion here. Doing the boxing in Swarm or SwarmBuilder sounds fine to me.

[thomaseizinger]

Using QUIC and WebRTC together with other transports is a bit clunky at the moment:

https://github.com/libp2p/test-plans/blob/c916d4b247d40d20fee995ea070779365426814a/ping/rust/src/bin/testplan_0510.rs#L24-L40

[thomaseizinger]

With #3588 merged, there is now only a single place to (SwarmBuilder) where we are accepting a transport::Boxed. I'd like to see if we can solve this issue by accepting a T: Transport instead, therefore doing the boxing internally and allow every Transport to be passed to SwarmBuilder.