implement ConversionToAssetBalance in asset-rate (#2903)

Implements the `ConversionToAssetBalance` trait to asset-rate by doing
`1/rate*(balance)`.

prdoc/pr_2903.prdoc

@@ -0,0 +1,15 @@
+# Schema: Polkadot SDK PRDoc Schema (prdoc) v1.0.0
+# See doc at https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/prdoc/schema_user.json
+
+title: "Implement `ConversionToAssetBalance` in asset-rate"
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      Implements the `ConversionToAssetBalance` trait to the asset-rate pallet.
+
+      Previously only the `ConversionFromAssetBalance` trait was implemented, which would allow to convert an asset balance into the corresponding native balance.
+
+      The `ConversionToAssetBalance` allows to use pallet-asset-rate, e.g., as a mechanism to charge XCM fees in an asset that is not the native.
+crates: [ ]
+

substrate/frame/asset-rate/src/lib.rs

@@ -59,8 +59,14 @@
 
 #![cfg_attr(not(feature = "std"), no_std)]
 
-use frame_support::traits::{fungible::Inspect, tokens::ConversionFromAssetBalance};
-use sp_runtime::{traits::Zero, FixedPointNumber, FixedU128};
+use frame_support::traits::{
+	fungible::Inspect,
+	tokens::{ConversionFromAssetBalance, ConversionToAssetBalance},
+};
+use sp_runtime::{
+	traits::{CheckedDiv, Zero},
+	FixedPointNumber, FixedU128,
+};
 use sp_std::boxed::Box;
 
 pub use pallet::*;
@@ -144,6 +150,8 @@ pub mod pallet {
 		UnknownAssetKind,
 		/// The given asset ID already has an assigned conversion rate and cannot be re-created.
 		AlreadyExists,
+		/// Overflow ocurred when calculating the inverse rate.
+		Overflow,
 	}
 
 	#[pallet::call]
@@ -246,3 +254,25 @@ where
 		pallet::ConversionRateToNative::<T>::set(asset_id.clone(), Some(1.into()));
 	}
 }
+
+/// Exposes conversion of a native balance to an asset balance.
+impl<T> ConversionToAssetBalance<BalanceOf<T>, AssetKindOf<T>, BalanceOf<T>> for Pallet<T>
+where
+	T: Config,
+{
+	type Error = pallet::Error<T>;
+
+	fn to_asset_balance(
+		balance: BalanceOf<T>,
+		asset_kind: AssetKindOf<T>,
+	) -> Result<BalanceOf<T>, pallet::Error<T>> {
+		let rate = pallet::ConversionRateToNative::<T>::get(asset_kind)
+			.ok_or(pallet::Error::<T>::UnknownAssetKind.into())?;
+
+		// We cannot use `saturating_div` here so we use `checked_div`.
+		Ok(FixedU128::from_u32(1)
+			.checked_div(&rate)
+			.ok_or(pallet::Error::<T>::Overflow.into())?
+			.saturating_mul_int(balance))
+	}
+}

substrate/frame/asset-rate/src/tests.rs

@@ -131,12 +131,19 @@ fn convert_works() {
 			FixedU128::from_float(2.51)
 		));
 
-		let conversion = <AssetRate as ConversionFromAssetBalance<
+		let conversion_from_asset = <AssetRate as ConversionFromAssetBalance<
 			BalanceOf<Test>,
 			<Test as pallet_asset_rate::Config>::AssetKind,
 			BalanceOf<Test>,
 		>>::from_asset_balance(10, ASSET_ID);
-		assert_eq!(conversion.expect("Conversion rate exists for asset"), 25);
+		assert_eq!(conversion_from_asset.expect("Conversion rate exists for asset"), 25);
+
+		let conversion_to_asset = <AssetRate as ConversionToAssetBalance<
+			BalanceOf<Test>,
+			<Test as pallet_asset_rate::Config>::AssetKind,
+			BalanceOf<Test>,
+		>>::to_asset_balance(25, ASSET_ID);
+		assert_eq!(conversion_to_asset.expect("Conversion rate exists for asset"), 9);
 	});
 }
 
@@ -151,3 +158,21 @@ fn convert_unknown_throws() {
 		assert!(conversion.is_err());
 	});
 }
+
+#[test]
+fn convert_overflow_throws() {
+	new_test_ext().execute_with(|| {
+		assert_ok!(AssetRate::create(
+			RuntimeOrigin::root(),
+			Box::new(ASSET_ID),
+			FixedU128::from_u32(0)
+		));
+
+		let conversion = <AssetRate as ConversionToAssetBalance<
+			BalanceOf<Test>,
+			<Test as pallet_asset_rate::Config>::AssetKind,
+			BalanceOf<Test>,
+		>>::to_asset_balance(10, ASSET_ID);
+		assert!(conversion.is_err());
+	});
+}