Feature gate do_task in frame_system (#2707)

https://github.com/paritytech/polkadot-sdk/pull/1343/ introduced Tasks
API. This one moves `do_task` call in frame_system under the
experimental flag, till the previous one is audited.

---------

Co-authored-by: command-bot <>

substrate/frame/system/Cargo.toml

@@ -56,6 +56,7 @@ runtime-benchmarks = [
 	"sp-runtime/runtime-benchmarks",
 ]
 try-runtime = ["frame-support/try-runtime", "sp-runtime/try-runtime"]
+experimental = ["frame-support/experimental"]
 
 [[bench]]
 name = "bench"

substrate/frame/system/src/lib.rs

@@ -323,9 +323,11 @@ pub mod pallet {
 			#[inject_runtime_type]
 			type RuntimeCall = ();
 
-			/// Converts a module to the index of the module, injected by `construct_runtime!`.
+			/// The aggregated Task type, injected by `construct_runtime!`.
 			#[inject_runtime_type]
 			type RuntimeTask = ();
+
+			/// Converts a module to the index of the module, injected by `construct_runtime!`.
 			#[inject_runtime_type]
 			type PalletInfo = ();
 
@@ -637,6 +639,7 @@ pub mod pallet {
 			Ok(().into())
 		}
 
+		#[cfg(feature = "experimental")]
 		#[pallet::call_index(8)]
 		#[pallet::weight(task.weight())]
 		pub fn do_task(origin: OriginFor<T>, task: T::RuntimeTask) -> DispatchResultWithPostInfo {
@@ -675,10 +678,13 @@ pub mod pallet {
 		KilledAccount { account: T::AccountId },
 		/// On on-chain remark happened.
 		Remarked { sender: T::AccountId, hash: T::Hash },
+		#[cfg(feature = "experimental")]
 		/// A [`Task`] has started executing
 		TaskStarted { task: T::RuntimeTask },
+		#[cfg(feature = "experimental")]
 		/// A [`Task`] has finished executing.
 		TaskCompleted { task: T::RuntimeTask },
+		#[cfg(feature = "experimental")]
 		/// A [`Task`] failed during execution.
 		TaskFailed { task: T::RuntimeTask, err: DispatchError },
 	}
@@ -702,8 +708,10 @@ pub mod pallet {
 		NonZeroRefCount,
 		/// The origin filter prevent the call to be dispatched.
 		CallFiltered,
+		#[cfg(feature = "experimental")]
 		/// The specified [`Task`] is not valid.
 		InvalidTask,
+		#[cfg(feature = "experimental")]
 		/// The specified [`Task`] failed during execution.
 		FailedTask,
 	}