Add mutate_exists to StorageDoubleMap.

frame/support/src/lib.rs

@@ -629,6 +629,23 @@ mod tests {
 		});
 	}
 
+	#[test]
+	fn double_map_mutate_exists_should_work() {
+		new_test_ext().execute_with(|| {
+			type DoubleMap = DataDM;
+
+			let (key1, key2) = (11, 13);
+
+			// mutated
+			DoubleMap::mutate_exists(key1, key2, |v| *v = Some(1));
+			assert_eq!(DoubleMap::get(&key1, key2), 1);
+
+			// removed if mutated to `None`
+			DoubleMap::mutate_exists(key1, key2, |v| *v = None);
+			assert!(!DoubleMap::contains_key(&key1, key2));
+		});
+	}
+
 	#[test]
 	fn double_map_try_mutate_exists_should_work() {
 		new_test_ext().execute_with(|| {

frame/support/src/storage/generator/double_map.rs

@@ -228,6 +228,15 @@ impl<K1, K2, V, G> storage::StorageDoubleMap<K1, K2, V> for G where
 		Self::try_mutate(k1, k2, |v| Ok::<R, Never>(f(v))).expect("`Never` can not be constructed; qed")
 	}
 
+	fn mutate_exists<KArg1, KArg2, R, F>(k1: KArg1, k2: KArg2, f: F) -> R
+	where
+		KArg1: EncodeLike<K1>,
+		KArg2: EncodeLike<K2>,
+		F: FnOnce(&mut Option<V>) -> R,
+	{
+		Self::try_mutate_exists(k1, k2, |v| Ok::<R, Never>(f(v))).expect("`Never` can not be constructed; qed")
+	}
+
 	fn try_mutate<KArg1, KArg2, R, E, F>(k1: KArg1, k2: KArg2, f: F) -> Result<R, E> where
 		KArg1: EncodeLike<K1>,
 		KArg2: EncodeLike<K2>,

frame/support/src/storage/mod.rs

@@ -366,6 +366,13 @@ pub trait StorageDoubleMap<K1: FullEncode, K2: FullEncode, V: FullCodec> {
 		KArg2: EncodeLike<K2>,
 		F: FnOnce(&mut Self::Query) -> Result<R, E>;
 
+	/// Mutate the value under the given keys. Deletes the item if mutated to a `None`.
+	fn mutate_exists<KArg1, KArg2, R, F>(k1: KArg1, k2: KArg2, f: F) -> R
+	where
+		KArg1: EncodeLike<K1>,
+		KArg2: EncodeLike<K2>,
+		F: FnOnce(&mut Option<V>) -> R;
+
 	/// Mutate the item, only if an `Ok` value is returned. Deletes the item if mutated to a `None`.
 	fn try_mutate_exists<KArg1, KArg2, R, E, F>(k1: KArg1, k2: KArg2, f: F) -> Result<R, E>
 	where