Support fields with bundle effects in `Bundle` derive macro

[nfagerlund]

What problem does this solve or what need does it fill?

There's some nice new spawning ergonomics for spawning a hierarchy of entities all at once. However, I can't put a SpawnRelatedBundle into a field of a struct that I'm deriving Bundle for:

#[derive(Bundle)]
pub struct EnemyDingusBundle {
    pub identity: EnemyDingus,
    pub script_state: SpawnRelatedBundle<ChildOf, Spawn<ScriptState>>
}

impl EnemyDingusBundle {
    fn new(script: Handle<Script>) -> Self {
        Self {
            identity: EnemyDingus,
            script_state: Children::spawn(Spawn(ScriptState::root(script))),
        }
    }
}

This gives an error like the trait bound SpawnRelatedBundle<ChildOf, Spawn<SomeComponent>>: BundleFromComponents is not satisfied.

Putting the SpawnRelatedBundle beside a derived bundle struct and wrapping them in a tuple bundle works fine, though:

#[derive(Bundle)]
pub struct EnemyDingusBundle {
    pub identity: EnemyDingus,
}

impl EnemyDingusBundle {
  fn dingus_plus(script: ScriptState) -> impl Bundle {
    (
        Self { identity: EnemyDingus },
        Children::spawn(Spawn(ScriptState::root(script))),
    )
  }
}

It looks like bundles with bundle effects work as expected in tuple bundles because those have a manual/bespoke Bundle implementation, but the derive macro doesn't know what to do with them.

What solution would you like?

Support effectful bundles as fields of structs that derive Bundle, so that struct bundles have equal features to tuple bundles.

What alternative(s) have you considered?

• Make do with tuple bundle wrappers instead of well-defined bundle structs. (Which is what I'm going forward with for now.)