fix(derive): restore support for inline generic type constraints

[aakoshh]

Description

Fixes the #[NetworkBehaviour] macro to support generic constraints on behaviours without a where clause, which was the case before v0.51.

Notes & open questions

We encountered the following problem when upgrading from v0.49 to v0.53:

We have a behaviour with generic type constraint inherited from a wrapped behaviour, which we then try to bundle up with all the behaviours in our application and use derive on the bunch:

#[derive(NetworkBehaviour)]
pub struct Behaviour<P: StoreParams> {
...
    content: content::Behaviour<P>,
}
...
mod content {
  pub struct Behaviour<P: StoreParams> {
      inner: Bitswap<P>,
  ...
}

After the update, the #[NetworkBehaviour] macro produced the following error:

error[E0277]: the trait bound `P: StoreParams` is not satisfied
    |
439 |     #[derive(NetworkBehaviour)]
    |              ^^^^^^^^^^^^^^^^ the trait `StoreParams` is not implemented for `P`
    |

With cargo expand we were able to see that the reason was the missing type constraints:

    #[doc = "`NetworkBehaviour::ToSwarm` produced by Behaviour."]
    pub enum BehaviourEvent<P>
    where
...
        content::Behaviour<P>: ::libp2p::swarm::derive_prelude::NetworkBehaviour,
    {
...
        Content(
            <content::Behaviour<P> as ::libp2p::swarm::derive_prelude::NetworkBehaviour>::ToSwarm,
        ),
    }

Comparing that to the output generated by v0.49:

    pub enum BehaviourEvent<P: StoreParams> {
...
        Content(
            <content::Behaviour<P> as ::libp2p::swarm::derive_prelude::NetworkBehaviour>::OutEvent,
        ),
    }

Note that BehaviourEvent<P: StoreParams> allows content::Behaviour<P> as ::libp2p::swarm::derive_prelude::NetworkBehaviour, but it's missing in the newer version.

During testing I noticed that this can be solved by rearranging the generics like so:

#[derive(NetworkBehaviour)]
pub struct Behaviour<P>
where 
  P: StoreParams 
{
    ...
    content: content::Behaviour<P>,
}

I'm not sure if this was a deliberate change and this intended to be the only way to declare generic constraints on behaviours, but with the restoring of the original technique the code compiles as it used to be, and no other tests seem to complain, which to me seems better since both definitions are valid Rust code.

Test

Without the fix, the test fails to compile:

❯ cargo  +nightly -Zlints -q test -p libp2p-swarm --features=macros --test swarm_derive
error[E0277]: the trait bound `A: Mark` is not satisfied
   --> swarm/tests/swarm_derive.rs:439:14
    |
439 |     #[derive(NetworkBehaviour)]
    |              ^^^^^^^^^^^^^^^^ the trait `Mark` is not implemented for `A`
    |
note: required by a bound in `with_generics_constrained::Bar`
   --> swarm/tests/swarm_derive.rs:382:19
    |
382 |     struct Bar<A: Mark> {
    |                   ^^^^ required by this bound in `Bar`
    = 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`.
error: could not compile `libp2p-swarm` (test "swarm_derive") due to previous error

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[thomaseizinger]

Given the simplicity of the fix, I am okay with this :)