Companion for #13352 (#2180)

* Assets balances

* Update docs

* AssetsApi with MultiLocation as preparation for multi pallet_assets instances (#2187)

* AssetsApi with MultiLocation for Westmint + assets-common

* AssetsApi with MultiLocation for Statemine/t

* typo

* typo for check-docs job

* WIP: AssetsApi return MultiAsset instead of (MultiLocation, Balance)

* WIP: assets_api + conversion refactor

* WIP: assets_api + conversion refactor

* Finished asset runtimes

* Refactor AssetsApi to FungiblesApi

* Refactor

* Fix check-rust-docs

* Removed todo

* Fix check-rust-doc

* Update parachains/runtimes/assets/common/Cargo.toml

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

* update lockfile for {"substrate", "polkadot"}

---------

Co-authored-by: Branislav Kontur <bkontur@gmail.com>
Co-authored-by: parity-processbot <>
Co-authored-by: Bastian Köcher <git@kchr.de>

parachains/runtimes/assets/common/src/runtime_api.rs

@@ -0,0 +1,41 @@
+// Copyright (C) 2023 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.
+
+//! Runtime API definition for fungibles.
+
+use codec::{Codec, Decode, Encode};
+use frame_support::RuntimeDebug;
+use sp_std::vec::Vec;
+use xcm::latest::MultiAsset;
+
+/// The possible errors that can happen querying the storage of assets.
+#[derive(Eq, PartialEq, Encode, Decode, RuntimeDebug)]
+pub enum FungiblesAccessError {
+	/// `MultiLocation` to `AssetId`/`ClassId` conversion failed.
+	AssetIdConversionFailed,
+	/// `u128` amount to currency `Balance` conversion failed.
+	AmountToBalanceConversionFailed,
+}
+
+sp_api::decl_runtime_apis! {
+	/// The API for querying account's balances from runtime.
+	pub trait FungiblesApi<AccountId>
+	where
+		AccountId: Codec,
+	{
+		/// Returns the list of all [`MultiAsset`] that an `AccountId` has.
+		fn query_account_balances(account: AccountId) -> Result<Vec<MultiAsset>, FungiblesAccessError>;
+	}
+}