feat/overseer: introduce closure init (#3775)

* feat/overseer: introduce closure init

Enables removal of the connected/disconnected overseer state.

* feat/overseer: allow replacement logic to access the original

Allows to re-use init-once types, which would otherwise error.

* feat/overseer: introduce external connector

Preparation for removal of `AllSubsystems`
which is another prerequisite for removing
the connect/disconnect state.

* fix/test: replace needs closure

* fixup

* simplify

* mea culpa

* all-subsystems-gen test

polkadot/node/overseer/all-subsystems-gen/src/lib.rs

@@ -118,10 +118,11 @@ fn impl_subsystems_gen(item: TokenStream) -> Result<proc_macro2::TokenStream> {
 
 			// generate an impl of `fn replace_#name`
 			for NameTyTup { field: replacable_item, ty: replacable_item_ty } in replacable_items {
-				let keeper = all_fields
+				let keeper = &all_fields
 					.iter()
 					.filter(|ntt| ntt.field != replacable_item)
-					.map(|ntt| ntt.field.clone());
+					.map(|ntt| ntt.field.clone())
+					.collect::<Vec<_>>();
 				let strukt_ty = strukt_ty.clone();
 				let fname = Ident::new(&format!("replace_{}", replacable_item), span);
 				// adjust the generics such that the appropriate member type is replaced
@@ -154,11 +155,27 @@ fn impl_subsystems_gen(item: TokenStream) -> Result<proc_macro2::TokenStream> {
 				additive.extend(quote! {
 					impl #orig_generics #strukt_ty #orig_generics {
 						#[doc = #msg]
-						pub fn #fname < NEW > (self, replacement: NEW) -> #strukt_ty #modified_generics {
+						pub fn #fname < NEW, F > (self, gen_replacement_fn: F) -> #strukt_ty #modified_generics
+							where
+								F: FnOnce(#replacable_item_ty) -> NEW,
+						{
+							let Self {
+								// To be replaced field:
+								#replacable_item,
+								// Fields to keep:
+								#(
+									#keeper,
+								)*
+							} = self;
+
+							// Some cases require that parts of the original are copied
+							// over, since they include a one time initialization.
+							let replacement = gen_replacement_fn(#replacable_item);
+
 							#strukt_ty :: #modified_generics {
 								#replacable_item: replacement,
 								#(
-									#keeper: self.#keeper,
+									#keeper,
 								)*
 							}
 						}

polkadot/node/overseer/all-subsystems-gen/tests/ui/err-01-generic-used-twice.rs

@@ -12,5 +12,5 @@ fn main() {
 		a: 0_u16,
 		b: 1_u16,
 	};
-	let _all = all.replace_a(77u8);
+	let _all = all.replace_a(|_| 77u8);
 }

polkadot/node/overseer/all-subsystems-gen/tests/ui/err-01-generic-used-twice.stderr

@@ -10,5 +10,5 @@ error[E0599]: no method named `replace_a` found for struct `AllSubsystems<u16>`
 5  | struct AllSubsystems<X> {
    | ----------------------- method `replace_a` not found for this
 ...
-15 |     let _all = all.replace_a(77u8);
+15 |     let _all = all.replace_a(|_| 77u8);
    |                    ^^^^^^^^^ method not found in `AllSubsystems<u16>`

polkadot/node/overseer/all-subsystems-gen/tests/ui/err-01-no-generic.rs

@@ -13,5 +13,5 @@ fn main() {
 		a: 0_f32,
 		b: 1_u16,
 	};
-	let _all = all.replace_a(77u8);
+	let _all = all.replace_a(|_| 77u8);
 }

polkadot/node/overseer/all-subsystems-gen/tests/ui/err-01-no-generic.stderr

@@ -10,5 +10,5 @@ error[E0599]: no method named `replace_a` found for struct `AllSubsystems` in th
 6  | struct AllSubsystems {
    | -------------------- method `replace_a` not found for this
 ...
-16 |     let _all = all.replace_a(77u8);
+16 |     let _all = all.replace_a(|_| 77u8);
    |                    ^^^^^^^^^ method not found in `AllSubsystems`

polkadot/node/overseer/all-subsystems-gen/tests/ui/err-01-no-generics.stderr

@@ -10,5 +10,5 @@ error[E0599]: no method named `replace_a` found for struct `AllSubsystems<u16>`
 6  | struct AllSubsystems<X> {
    | ----------------------- method `replace_a` not found for this
 ...
-16 |     let _all = all.replace_a(77u8);
+16 |     let _all = all.replace_a(|_| 77u8);
    |                    ^^^^^^^^^ method not found in `AllSubsystems<u16>`

polkadot/node/overseer/all-subsystems-gen/tests/ui/ok-01-w-generics.rs

@@ -13,5 +13,5 @@ fn main() {
 		a: 0u8,
 		b: 1u16,
 	};
-	let _all: AllSubsystems<_,_> = all.replace_a::<u32>(777_777u32);
+	let _all: AllSubsystems<_,_> = all.replace_a::<u32,_>(|_| 777_777u32);
 }