[FRAME] Auto-bound RuntimeTask: From<Task<Self>>

prdoc/pr_12227.prdoc

@@ -0,0 +1,32 @@
+# Schema: Polkadot SDK PRDoc Schema (prdoc) v1.0.0
+# See doc at https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/prdoc/schema_user.json
+
+title: "[FRAME] Auto-bound `RuntimeTask: From<Task<Self>>` in pallet Config when using tasks"
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      Pallets using `#[pallet::tasks_experimental]` now automatically get
+      `frame_system::Config<RuntimeTask: From<Task<Self>>>` added as a supertrait
+      on their `Config` trait by the macro — mirroring the existing automatic
+      `RuntimeEvent: From<Event<Self>>` bound added for pallets with events.
+
+      Previously, pallet authors had to manually declare a `RuntimeTask` associated
+      type with the correct `From<Task<Self>>` bound, or omit it entirely with no
+      compile-time enforcement. Now the compiler enforces that any runtime including
+      a task-bearing pallet must have a `RuntimeTask` type that can represent that
+      pallet's tasks.
+
+      This change is backward compatible: existing pallets that already declare
+      `RuntimeTask` manually continue to compile unchanged. Runtimes built with
+      `construct_runtime!` already satisfy the new bound since the macro generates
+      the required `From` impl for each pallet with tasks.
+
+      The system pallet itself is exempt via the existing
+      `#[pallet::config(frame_system_config)]` attribute.
+
+crates:
+  - name: frame-support-procedural
+    bump: minor
+  - name: frame-support
+    bump: minor

substrate/frame/support/procedural/src/pallet/expand/config.rs

@@ -72,6 +72,25 @@ Consequently, a runtime that wants to include this pallet must implement this tr
 		}
 	}
 
+	// insert `frame_system::Config` supertrait with `RuntimeTask: From<Task<Self>>` if tasks are
+	// defined for this pallet.
+	if def.tasks.is_some() && !def.is_frame_system {
+		let frame_system = &def.frame_system;
+
+		let task_use_gen = if def.config.has_instance {
+			quote! { Self, I }
+		} else {
+			quote! { Self }
+		};
+
+		let supertrait_with_task_bound = syn::parse2::<syn::TypeParamBound>(
+			quote! { #frame_system::Config<RuntimeTask: From<Task<#task_use_gen>>> },
+		)
+		.expect("Parsing super trait doesn't fail; qed");
+
+		config_item.supertraits.push(supertrait_with_task_bound.into());
+	}
+
 	// we only emit `DefaultConfig` if there are trait items, so an empty `DefaultConfig` is
 	// impossible consequently.
 	match &config.default_sub_trait {