Speed up storage iteration from within the runtime (#13479)

* Speed up storage iteration from within the runtime

* Move the cached iterator into an `Option`

* Use `RefCell` in no_std

* Simplify the code slightly

* Use `Option::replace`

* Update doc comment for `next_storage_key_slow`

substrate/frame/benchmarking/pov/src/benchmarking.rs

@@ -316,6 +316,19 @@ frame_benchmarking::benchmarks! {
 		call.dispatch_bypass_filter(RawOrigin::Root.into()).unwrap();
 	}
 
+	storage_iteration {
+		for i in 0..65000 {
+			UnboundedMapTwox::<T>::insert(i, sp_std::vec![0; 64]);
+		}
+	}: {
+		for (key, value) in UnboundedMapTwox::<T>::iter() {
+			unsafe {
+				core::ptr::read_volatile(&key);
+				core::ptr::read_volatile(value.as_ptr());
+			}
+		}
+	}
+
 	impl_benchmark_test_suite!(
 		Pallet,
 		mock::new_test_ext(),

substrate/frame/benchmarking/pov/src/lib.rs

@@ -107,6 +107,11 @@ pub mod pallet {
 	pub(crate) type UnboundedMap2<T: Config> =
 		StorageMap<Hasher = Blake2_256, Key = u32, Value = Vec<u32>, QueryKind = OptionQuery>;
 
+	#[pallet::storage]
+	#[pallet::unbounded]
+	pub(crate) type UnboundedMapTwox<T: Config> =
+		StorageMap<Hasher = Twox64Concat, Key = u32, Value = Vec<u32>, QueryKind = OptionQuery>;
+
 	#[pallet::event]
 	#[pallet::generate_deposit(pub(super) fn deposit_event)]
 	pub enum Event<T: Config> {