refactor+feat: allow subsystems to send only declared messages, generate graphviz (#5314)

Closes #3774
Closes #3826

polkadot/node/overseer/overseer-gen/examples/solo.rs

@@ -0,0 +1,54 @@
+#![allow(dead_code)] // overseer events are not used
+
+//! A minimal demo to be used with cargo expand.
+
+use polkadot_overseer_gen::{self as overseer, SpawnNamed, *};
+mod misc;
+
+pub use self::misc::*;
+
+#[overlord(signal=SigSigSig, event=EvX, error=Yikes, gen=AllMessages)]
+struct Solo<T> {
+	#[subsystem(consumes: Plinko, sends: [MsgStrukt])]
+	goblin_tower: GoblinTower,
+}
+
+#[derive(Default)]
+pub struct Fortified;
+
+#[overseer::subsystem(GoblinTower, error=Yikes)]
+impl<Context> Fortified {
+	fn start(self, mut ctx: Context) -> SpawnedSubsystem<Yikes> {
+		let mut sender = ctx.sender().clone();
+		ctx.spawn(
+			"GoblinTower",
+			Box::pin(async move {
+				sender.send_message(MsgStrukt(8u8)).await;
+			}),
+		)
+		.unwrap();
+		unimplemented!("welcum")
+	}
+}
+
+fn main() {
+	use futures::{executor, pin_mut};
+
+	executor::block_on(async move {
+		let (overseer, _handle): (Solo<_>, _) = Solo::builder()
+			.goblin_tower(Fortified::default())
+			.spawner(DummySpawner)
+			.build()
+			.unwrap();
+
+		let overseer_fut = overseer
+			.running_subsystems
+			.into_future()
+			.timeout(std::time::Duration::from_millis(300))
+			.fuse();
+
+		pin_mut!(overseer_fut);
+
+		overseer_fut.await
+	});
+}