[xcm] Initial of runtime RPC for querying Fungibles from runtime

xcm/xcm-builder/Cargo.toml

@@ -12,6 +12,7 @@ scale-info = { version = "2.1.2", default-features = false, features = ["derive"
 xcm = { path = "..", default-features = false }
 xcm-executor = { path = "../xcm-executor", default-features = false }
 sp-std = { git = "/~https://github.com/paritytech/substrate", branch = "master", default-features = false }
+sp-api = { git = "/~https://github.com/paritytech/substrate", branch = "master", default-features = false }
 sp-arithmetic = { git = "/~https://github.com/paritytech/substrate", branch = "master", default-features = false }
 sp-io = { git = "/~https://github.com/paritytech/substrate", branch = "master", default-features = false }
 sp-runtime = { git = "/~https://github.com/paritytech/substrate", branch = "master", default-features = false }
@@ -43,6 +44,7 @@ std = [
 	"xcm/std",
 	"xcm-executor/std",
 	"sp-std/std",
+	"sp-api/std",
 	"sp-arithmetic/std",
 	"sp-io/std",
 	"sp-runtime/std",

xcm/xcm-builder/src/lib.rs

@@ -83,3 +83,6 @@ pub use universal_exports::{
 	HaulBlobError, HaulBlobExporter, NetworkExportTable, SovereignPaidRemoteExporter,
 	UnpaidLocalExporter, UnpaidRemoteExporter,
 };
+
+mod runtime_api;
+pub use runtime_api::*;

xcm/xcm-builder/src/runtime_api.rs

@@ -0,0 +1,44 @@
+// Copyright Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Polkadot is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Runtime API definition for fungibles.
+//!
+//! (Initial version: /~https://github.com/paritytech/cumulus/pull/2180#issuecomment-1442952274)
+
+use frame_support::RuntimeDebug;
+use parity_scale_codec::{Codec, Decode, Encode};
+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>;
+	}
+}