Allow other pallets to check asset ids. (#12666)

* Make it easier for other pallets to check asset ids.

* Avoid boxing

* cargo fmt

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

@@ -27,6 +27,8 @@ use sp_std::vec::Vec;
 
 pub mod approvals;
 mod balanced;
+pub mod enumerable;
+pub use enumerable::InspectEnumerable;
 pub mod metadata;
 pub use balanced::{Balanced, Unbalanced};
 mod imbalance;

substrate/frame/support/src/traits/tokens/fungibles/enumerable.rs

@@ -0,0 +1,26 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2019-2022 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use crate::traits::fungibles::Inspect;
+
+/// Interface for enumerating assets in existence or owned by a given account.
+pub trait InspectEnumerable<AccountId>: Inspect<AccountId> {
+	type AssetsIterator;
+
+	/// Returns an iterator of the collections in existence.
+	fn asset_ids() -> Self::AssetsIterator;
+}