use associated iterator types for InspectEnumerable (#12389)

* use associated iterator types for InspectEnumerable

* Update frame/uniques/src/impl_nonfungibles.rs

Co-authored-by: parity-processbot <>
Co-authored-by: Bastian Köcher <git@kchr.de>

substrate/frame/support/src/traits/tokens/nonfungible.rs

@@ -65,11 +65,16 @@ pub trait Inspect<AccountId> {
 /// Interface for enumerating items in existence or owned by a given account over a collection
 /// of NFTs.
 pub trait InspectEnumerable<AccountId>: Inspect<AccountId> {
+	/// The iterator type for [`Self::items`].
+	type ItemsIterator: Iterator<Item = Self::ItemId>;
+	/// The iterator type for [`Self::owned`].
+	type OwnedIterator: Iterator<Item = Self::ItemId>;
+
 	/// Returns an iterator of the items within a `collection` in existence.
-	fn items() -> Box<dyn Iterator<Item = Self::ItemId>>;
+	fn items() -> Self::ItemsIterator;
 
 	/// Returns an iterator of the items of all collections owned by `who`.
-	fn owned(who: &AccountId) -> Box<dyn Iterator<Item = Self::ItemId>>;
+	fn owned(who: &AccountId) -> Self::OwnedIterator;
 }
 
 /// Trait for providing an interface for NFT-like items which may be minted, burned and/or have
@@ -149,10 +154,15 @@ impl<
 		AccountId,
 	> InspectEnumerable<AccountId> for ItemOf<F, A, AccountId>
 {
-	fn items() -> Box<dyn Iterator<Item = Self::ItemId>> {
+	type ItemsIterator = <F as nonfungibles::InspectEnumerable<AccountId>>::ItemsIterator;
+	type OwnedIterator =
+		<F as nonfungibles::InspectEnumerable<AccountId>>::OwnedInCollectionIterator;
+
+	fn items() -> Self::ItemsIterator {
 		<F as nonfungibles::InspectEnumerable<AccountId>>::items(&A::get())
 	}
-	fn owned(who: &AccountId) -> Box<dyn Iterator<Item = Self::ItemId>> {
+
+	fn owned(who: &AccountId) -> Self::OwnedIterator {
 		<F as nonfungibles::InspectEnumerable<AccountId>>::owned_in_collection(&A::get(), who)
 	}
 }

substrate/frame/support/src/traits/tokens/nonfungibles.rs

@@ -105,20 +105,29 @@ pub trait Inspect<AccountId> {
 /// Interface for enumerating items in existence or owned by a given account over many collections
 /// of NFTs.
 pub trait InspectEnumerable<AccountId>: Inspect<AccountId> {
+	/// The iterator type for [`Self::collections`].
+	type CollectionsIterator: Iterator<Item = Self::CollectionId>;
+	/// The iterator type for [`Self::items`].
+	type ItemsIterator: Iterator<Item = Self::ItemId>;
+	/// The iterator type for [`Self::owned`].
+	type OwnedIterator: Iterator<Item = (Self::CollectionId, Self::ItemId)>;
+	/// The iterator type for [`Self::owned_in_collection`].
+	type OwnedInCollectionIterator: Iterator<Item = Self::ItemId>;
+
 	/// Returns an iterator of the collections in existence.
-	fn collections() -> Box<dyn Iterator<Item = Self::CollectionId>>;
+	fn collections() -> Self::CollectionsIterator;
 
 	/// Returns an iterator of the items of a `collection` in existence.
-	fn items(collection: &Self::CollectionId) -> Box<dyn Iterator<Item = Self::ItemId>>;
+	fn items(collection: &Self::CollectionId) -> Self::ItemsIterator;
 
 	/// Returns an iterator of the items of all collections owned by `who`.
-	fn owned(who: &AccountId) -> Box<dyn Iterator<Item = (Self::CollectionId, Self::ItemId)>>;
+	fn owned(who: &AccountId) -> Self::OwnedIterator;
 
 	/// Returns an iterator of the items of `collection` owned by `who`.
 	fn owned_in_collection(
 		collection: &Self::CollectionId,
 		who: &AccountId,
-	) -> Box<dyn Iterator<Item = Self::ItemId>>;
+	) -> Self::OwnedInCollectionIterator;
 }
 
 /// Trait for providing the ability to create collections of nonfungible items.