Create a more rigid overseer builder pattern that fails at compile time (#4753)

Introduces `Missing<Field>` and `Init<Field>` states, that are used in place of builder generics, and make this possible.

polkadot/node/overseer/overseer-gen/tests/ui/err-07-missing-spawner.rs

@@ -0,0 +1,61 @@
+#![allow(dead_code)]
+
+use polkadot_overseer_gen::*;
+
+#[derive(Default)]
+struct AwesomeSubSys;
+
+impl ::polkadot_overseer_gen::Subsystem<OverseerSubsystemContext<MsgStrukt>, OverseerError> for AwesomeSubSys {
+	fn start(self, _ctx: OverseerSubsystemContext<MsgStrukt>) -> SpawnedSubsystem<OverseerError> {
+		unimplemented!("starting yay!")
+	}
+}
+
+#[derive(Clone, Debug)]
+pub struct SigSigSig;
+
+pub struct Event;
+
+#[derive(Clone, Debug)]
+pub struct MsgStrukt(u8);
+
+#[overlord(signal=SigSigSig, error=OverseerError, event=Event, gen=AllMessages)]
+struct Overseer {
+	#[subsystem(no_dispatch, MsgStrukt)]
+	sub0: AwesomeSubSys,
+	i_like_pie: f64,
+}
+
+#[derive(Debug, Clone)]
+pub struct DummySpawner;
+
+impl SpawnNamed for DummySpawner {
+	fn spawn_blocking(
+		&self,
+		task_name: &'static str,
+		subsystem_name: Option<&'static str>,
+		_future: futures::future::BoxFuture<'static, ()>,
+	) {
+		unimplemented!("spawn blocking {} {}", task_name, subsystem_name.unwrap_or("default"))
+	}
+
+	fn spawn(
+		&self,
+		task_name: &'static str,
+		subsystem_name: Option<&'static str>,
+		_future: futures::future::BoxFuture<'static, ()>,
+	) {
+		unimplemented!("spawn {} {}", task_name, subsystem_name.unwrap_or("default"))
+	}
+}
+
+struct DummyCtx;
+
+fn main() {
+	let _ = Overseer::builder()
+		.sub0(AwesomeSubSys::default())
+		.i_like_pie(std::f64::consts::PI)
+		//.spawner(DummySpawner) // Spawner is missing
+		.build()
+		.unwrap();
+}