XCM WeightTrader: Swap Fee Asset for Native Asset (#1845)

Implements an XCM executor `WeightTrader`, facilitating fee payments in
any asset that can be exchanged for a native asset.

A few constraints need to be observed:
- `buy_weight` and `refund` operations must be atomic, as another weight
trader implementation might be attempted in case of failure.
- swap credit must be utilized since there isn’t an account to which an
asset of some class can be deposited with a guarantee to meet the
existential deposit requirement. Also, operating with credits enhances
the efficiency of the weight trader -
https://github.com/paritytech/polkadot-sdk/pull/1677

related PRs:
- (depends) https://github.com/paritytech/polkadot-sdk/pull/2031
- (depends) https://github.com/paritytech/polkadot-sdk/pull/1677
- (caused) https://github.com/paritytech/polkadot-sdk/pull/1847
- (caused) https://github.com/paritytech/polkadot-sdk/pull/1876

// DONE: impl `OnUnbalanced` for a `fungible/s` credit
// DONE: make the trader free from a concept of a native currency and
drop few fallible conversions. related issue -
https://github.com/paritytech/polkadot-sdk/issues/1842
// DONE: tests

---------

Co-authored-by: joe petrowski <25483142+joepetrowski@users.noreply.github.com>
Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>

Cargo.lock

@@ -1066,6 +1066,7 @@ dependencies = [
  "frame-support",
  "frame-system",
  "hex-literal",
+ "pallet-asset-conversion",
  "pallet-assets",
  "pallet-balances",
  "pallet-collator-selection",
@@ -1973,6 +1974,7 @@ dependencies = [
  "frame-support",
  "hex",
  "hex-literal",
+ "pallet-asset-conversion",
  "pallet-assets",
  "pallet-balances",
  "pallet-bridge-messages",
@@ -2165,6 +2167,7 @@ dependencies = [
  "cumulus-pallet-xcmp-queue",
  "emulated-integration-tests-common",
  "frame-support",
+ "pallet-asset-conversion",
  "pallet-assets",
  "pallet-balances",
  "pallet-bridge-messages",
@@ -4095,6 +4098,7 @@ dependencies = [
  "cumulus-primitives-core",
  "frame-support",
  "log",
+ "pallet-asset-conversion",
  "pallet-xcm-benchmarks",
  "parity-scale-codec",
  "polkadot-runtime-common",

cumulus/parachains/integration-tests/emulated/tests/assets/asset-hub-rococo/src/tests/send.rs

@@ -27,78 +27,3 @@ fn send_transact_as_superuser_from_relay_to_system_para_works() {
 		Some(Weight::from_parts(1_019_445_000, 200_000)),
 	)
 }
-
-/// Parachain should be able to send XCM paying its fee with sufficient asset
-/// in the System Parachain
-#[test]
-fn send_xcm_from_para_to_system_para_paying_fee_with_assets_works() {
-	let para_sovereign_account = AssetHubRococo::sovereign_account_id_of(
-		AssetHubRococo::sibling_location_of(PenpalA::para_id()),
-	);
-
-	// Force create and mint assets for Parachain's sovereign account
-	AssetHubRococo::force_create_and_mint_asset(
-		ASSET_ID,
-		ASSET_MIN_BALANCE,
-		true,
-		para_sovereign_account.clone(),
-		Some(Weight::from_parts(1_019_445_000, 200_000)),
-		ASSET_MIN_BALANCE * 1000000000,
-	);
-
-	// We just need a call that can pass the `SafeCallFilter`
-	// Call values are not relevant
-	let call = AssetHubRococo::force_create_asset_call(
-		ASSET_ID,
-		para_sovereign_account.clone(),
-		true,
-		ASSET_MIN_BALANCE,
-	);
-
-	let origin_kind = OriginKind::SovereignAccount;
-	let fee_amount = ASSET_MIN_BALANCE * 1000000;
-	let native_asset =
-		(X2(PalletInstance(ASSETS_PALLET_ID), GeneralIndex(ASSET_ID.into())), fee_amount).into();
-
-	let root_origin = <PenpalA as Chain>::RuntimeOrigin::root();
-	let system_para_destination = PenpalA::sibling_location_of(AssetHubRococo::para_id()).into();
-	let xcm = xcm_transact_paid_execution(
-		call,
-		origin_kind,
-		native_asset,
-		para_sovereign_account.clone(),
-	);
-
-	PenpalA::execute_with(|| {
-		assert_ok!(<PenpalA as PenpalAPallet>::PolkadotXcm::send(
-			root_origin,
-			bx!(system_para_destination),
-			bx!(xcm),
-		));
-
-		PenpalA::assert_xcm_pallet_sent();
-	});
-
-	AssetHubRococo::execute_with(|| {
-		type RuntimeEvent = <AssetHubRococo as Chain>::RuntimeEvent;
-
-		AssetHubRococo::assert_xcmp_queue_success(Some(Weight::from_parts(
-			15_594_564_000,
-			562_893,
-		)));
-
-		assert_expected_events!(
-			AssetHubRococo,
-			vec![
-				RuntimeEvent::Assets(pallet_assets::Event::Burned { asset_id, owner, balance }) => {
-					asset_id: *asset_id == ASSET_ID,
-					owner: *owner == para_sovereign_account,
-					balance: *balance == fee_amount,
-				},
-				RuntimeEvent::Assets(pallet_assets::Event::Issued { asset_id, .. }) => {
-					asset_id: *asset_id == ASSET_ID,
-				},
-			]
-		);
-	});
-}

cumulus/parachains/integration-tests/emulated/tests/assets/asset-hub-rococo/src/tests/swap.rs

@@ -270,3 +270,130 @@ fn cannot_create_pool_from_pool_assets() {
 		);
 	});
 }
+
+#[test]
+fn pay_xcm_fee_with_some_asset_swapped_for_native() {
+	let asset_native = asset_hub_rococo_runtime::xcm_config::TokenLocation::get();
+	let asset_one = MultiLocation {
+		parents: 0,
+		interior: X2(PalletInstance(ASSETS_PALLET_ID), GeneralIndex(ASSET_ID.into())),
+	};
+	let penpal = AssetHubRococo::sovereign_account_id_of(AssetHubRococo::sibling_location_of(
+		PenpalA::para_id(),
+	));
+
+	AssetHubRococo::execute_with(|| {
+		type RuntimeEvent = <AssetHubRococo as Chain>::RuntimeEvent;
+
+		// set up pool with ASSET_ID <> NATIVE pair
+		assert_ok!(<AssetHubRococo as AssetHubRococoPallet>::Assets::create(
+			<AssetHubRococo as Chain>::RuntimeOrigin::signed(AssetHubRococoSender::get()),
+			ASSET_ID.into(),
+			AssetHubRococoSender::get().into(),
+			ASSET_MIN_BALANCE,
+		));
+		assert!(<AssetHubRococo as AssetHubRococoPallet>::Assets::asset_exists(ASSET_ID));
+
+		assert_ok!(<AssetHubRococo as AssetHubRococoPallet>::Assets::mint(
+			<AssetHubRococo as Chain>::RuntimeOrigin::signed(AssetHubRococoSender::get()),
+			ASSET_ID.into(),
+			AssetHubRococoSender::get().into(),
+			3_000_000_000_000,
+		));
+
+		assert_ok!(<AssetHubRococo as AssetHubRococoPallet>::AssetConversion::create_pool(
+			<AssetHubRococo as Chain>::RuntimeOrigin::signed(AssetHubRococoSender::get()),
+			Box::new(asset_native),
+			Box::new(asset_one),
+		));
+
+		assert_expected_events!(
+			AssetHubRococo,
+			vec![
+				RuntimeEvent::AssetConversion(pallet_asset_conversion::Event::PoolCreated { .. }) => {},
+			]
+		);
+
+		assert_ok!(<AssetHubRococo as AssetHubRococoPallet>::AssetConversion::add_liquidity(
+			<AssetHubRococo as Chain>::RuntimeOrigin::signed(AssetHubRococoSender::get()),
+			Box::new(asset_native),
+			Box::new(asset_one),
+			1_000_000_000_000,
+			2_000_000_000_000,
+			0,
+			0,
+			AssetHubRococoSender::get().into()
+		));
+
+		assert_expected_events!(
+			AssetHubRococo,
+			vec![
+				RuntimeEvent::AssetConversion(pallet_asset_conversion::Event::LiquidityAdded {lp_token_minted, .. }) => { lp_token_minted: *lp_token_minted == 1414213562273, },
+			]
+		);
+
+		// ensure `penpal` sovereign account has no native tokens and mint some `ASSET_ID`
+		assert_eq!(
+			<AssetHubRococo as AssetHubRococoPallet>::Balances::free_balance(penpal.clone()),
+			0
+		);
+
+		assert_ok!(<AssetHubRococo as AssetHubRococoPallet>::Assets::touch_other(
+			<AssetHubRococo as Chain>::RuntimeOrigin::signed(AssetHubRococoSender::get()),
+			ASSET_ID.into(),
+			penpal.clone().into(),
+		));
+
+		assert_ok!(<AssetHubRococo as AssetHubRococoPallet>::Assets::mint(
+			<AssetHubRococo as Chain>::RuntimeOrigin::signed(AssetHubRococoSender::get()),
+			ASSET_ID.into(),
+			penpal.clone().into(),
+			10_000_000_000_000,
+		));
+	});
+
+	PenpalA::execute_with(|| {
+		// send xcm transact from `penpal` account which as only `ASSET_ID` tokens on
+		// `AssetHubRococo`
+		let call = AssetHubRococo::force_create_asset_call(
+			ASSET_ID + 1000,
+			penpal.clone(),
+			true,
+			ASSET_MIN_BALANCE,
+		);
+
+		let penpal_root = <PenpalA as Chain>::RuntimeOrigin::root();
+		let fee_amount = 4_000_000_000_000u128;
+		let asset_one =
+			(X2(PalletInstance(ASSETS_PALLET_ID), GeneralIndex(ASSET_ID.into())), fee_amount)
+				.into();
+		let asset_hub_location = PenpalA::sibling_location_of(AssetHubRococo::para_id()).into();
+		let xcm = xcm_transact_paid_execution(
+			call,
+			OriginKind::SovereignAccount,
+			asset_one,
+			penpal.clone(),
+		);
+
+		assert_ok!(<PenpalA as PenpalAPallet>::PolkadotXcm::send(
+			penpal_root,
+			bx!(asset_hub_location),
+			bx!(xcm),
+		));
+
+		PenpalA::assert_xcm_pallet_sent();
+	});
+
+	AssetHubRococo::execute_with(|| {
+		type RuntimeEvent = <AssetHubRococo as Chain>::RuntimeEvent;
+
+		AssetHubRococo::assert_xcmp_queue_success(None);
+		assert_expected_events!(
+			AssetHubRococo,
+			vec![
+				RuntimeEvent::AssetConversion(pallet_asset_conversion::Event::SwapCreditExecuted { .. },) => {},
+				RuntimeEvent::MessageQueue(pallet_message_queue::Event::Processed { success: true,.. }) => {},
+			]
+		);
+	});
+}

cumulus/parachains/integration-tests/emulated/tests/assets/asset-hub-westend/src/tests/send.rs

@@ -27,78 +27,3 @@ fn send_transact_as_superuser_from_relay_to_system_para_works() {
 		Some(Weight::from_parts(1_019_445_000, 200_000)),
 	)
 }
-
-/// Parachain should be able to send XCM paying its fee with sufficient asset
-/// in the System Parachain
-#[test]
-fn send_xcm_from_para_to_system_para_paying_fee_with_assets_works() {
-	let para_sovereign_account = AssetHubWestend::sovereign_account_id_of(
-		AssetHubWestend::sibling_location_of(PenpalB::para_id()),
-	);
-
-	// Force create and mint assets for Parachain's sovereign account
-	AssetHubWestend::force_create_and_mint_asset(
-		ASSET_ID,
-		ASSET_MIN_BALANCE,
-		true,
-		para_sovereign_account.clone(),
-		Some(Weight::from_parts(1_019_445_000, 200_000)),
-		ASSET_MIN_BALANCE * 1000000000,
-	);
-
-	// We just need a call that can pass the `SafeCallFilter`
-	// Call values are not relevant
-	let call = AssetHubWestend::force_create_asset_call(
-		ASSET_ID,
-		para_sovereign_account.clone(),
-		true,
-		ASSET_MIN_BALANCE,
-	);
-
-	let origin_kind = OriginKind::SovereignAccount;
-	let fee_amount = ASSET_MIN_BALANCE * 1000000;
-	let native_asset =
-		(X2(PalletInstance(ASSETS_PALLET_ID), GeneralIndex(ASSET_ID.into())), fee_amount).into();
-
-	let root_origin = <PenpalB as Chain>::RuntimeOrigin::root();
-	let system_para_destination = PenpalB::sibling_location_of(AssetHubWestend::para_id()).into();
-	let xcm = xcm_transact_paid_execution(
-		call,
-		origin_kind,
-		native_asset,
-		para_sovereign_account.clone(),
-	);
-
-	PenpalB::execute_with(|| {
-		assert_ok!(<PenpalB as PenpalBPallet>::PolkadotXcm::send(
-			root_origin,
-			bx!(system_para_destination),
-			bx!(xcm),
-		));
-
-		PenpalB::assert_xcm_pallet_sent();
-	});
-
-	AssetHubWestend::execute_with(|| {
-		type RuntimeEvent = <AssetHubWestend as Chain>::RuntimeEvent;
-
-		AssetHubWestend::assert_xcmp_queue_success(Some(Weight::from_parts(
-			16_290_336_000,
-			562_893,
-		)));
-
-		assert_expected_events!(
-			AssetHubWestend,
-			vec![
-				RuntimeEvent::Assets(pallet_assets::Event::Burned { asset_id, owner, balance }) => {
-					asset_id: *asset_id == ASSET_ID,
-					owner: *owner == para_sovereign_account,
-					balance: *balance == fee_amount,
-				},
-				RuntimeEvent::Assets(pallet_assets::Event::Issued { asset_id, .. }) => {
-					asset_id: *asset_id == ASSET_ID,
-				},
-			]
-		);
-	});
-}

cumulus/parachains/integration-tests/emulated/tests/assets/asset-hub-westend/src/tests/swap.rs

@@ -264,3 +264,130 @@ fn cannot_create_pool_from_pool_assets() {
 		);
 	});
 }
+
+#[test]
+fn pay_xcm_fee_with_some_asset_swapped_for_native() {
+	let asset_native = asset_hub_westend_runtime::xcm_config::WestendLocation::get();
+	let asset_one = MultiLocation {
+		parents: 0,
+		interior: X2(PalletInstance(ASSETS_PALLET_ID), GeneralIndex(ASSET_ID.into())),
+	};
+	let penpal = AssetHubWestend::sovereign_account_id_of(AssetHubWestend::sibling_location_of(
+		PenpalB::para_id(),
+	));
+
+	AssetHubWestend::execute_with(|| {
+		type RuntimeEvent = <AssetHubWestend as Chain>::RuntimeEvent;
+
+		// set up pool with ASSET_ID <> NATIVE pair
+		assert_ok!(<AssetHubWestend as AssetHubWestendPallet>::Assets::create(
+			<AssetHubWestend as Chain>::RuntimeOrigin::signed(AssetHubWestendSender::get()),
+			ASSET_ID.into(),
+			AssetHubWestendSender::get().into(),
+			ASSET_MIN_BALANCE,
+		));
+		assert!(<AssetHubWestend as AssetHubWestendPallet>::Assets::asset_exists(ASSET_ID));
+
+		assert_ok!(<AssetHubWestend as AssetHubWestendPallet>::Assets::mint(
+			<AssetHubWestend as Chain>::RuntimeOrigin::signed(AssetHubWestendSender::get()),
+			ASSET_ID.into(),
+			AssetHubWestendSender::get().into(),
+			3_000_000_000_000,
+		));
+
+		assert_ok!(<AssetHubWestend as AssetHubWestendPallet>::AssetConversion::create_pool(
+			<AssetHubWestend as Chain>::RuntimeOrigin::signed(AssetHubWestendSender::get()),
+			Box::new(asset_native),
+			Box::new(asset_one),
+		));
+
+		assert_expected_events!(
+			AssetHubWestend,
+			vec![
+				RuntimeEvent::AssetConversion(pallet_asset_conversion::Event::PoolCreated { .. }) => {},
+			]
+		);
+
+		assert_ok!(<AssetHubWestend as AssetHubWestendPallet>::AssetConversion::add_liquidity(
+			<AssetHubWestend as Chain>::RuntimeOrigin::signed(AssetHubWestendSender::get()),
+			Box::new(asset_native),
+			Box::new(asset_one),
+			1_000_000_000_000,
+			2_000_000_000_000,
+			0,
+			0,
+			AssetHubWestendSender::get().into()
+		));
+
+		assert_expected_events!(
+			AssetHubWestend,
+			vec![
+				RuntimeEvent::AssetConversion(pallet_asset_conversion::Event::LiquidityAdded {lp_token_minted, .. }) => { lp_token_minted: *lp_token_minted == 1414213562273, },
+			]
+		);
+
+		// ensure `penpal` sovereign account has no native tokens and mint some `ASSET_ID`
+		assert_eq!(
+			<AssetHubWestend as AssetHubWestendPallet>::Balances::free_balance(penpal.clone()),
+			0
+		);
+
+		assert_ok!(<AssetHubWestend as AssetHubWestendPallet>::Assets::touch_other(
+			<AssetHubWestend as Chain>::RuntimeOrigin::signed(AssetHubWestendSender::get()),
+			ASSET_ID.into(),
+			penpal.clone().into(),
+		));
+
+		assert_ok!(<AssetHubWestend as AssetHubWestendPallet>::Assets::mint(
+			<AssetHubWestend as Chain>::RuntimeOrigin::signed(AssetHubWestendSender::get()),
+			ASSET_ID.into(),
+			penpal.clone().into(),
+			10_000_000_000_000,
+		));
+	});
+
+	PenpalB::execute_with(|| {
+		// send xcm transact from `penpal` account which as only `ASSET_ID` tokens on
+		// `AssetHubWestend`
+		let call = AssetHubWestend::force_create_asset_call(
+			ASSET_ID + 1000,
+			penpal.clone(),
+			true,
+			ASSET_MIN_BALANCE,
+		);
+
+		let penpal_root = <PenpalB as Chain>::RuntimeOrigin::root();
+		let fee_amount = 4_000_000_000_000u128;
+		let asset_one =
+			(X2(PalletInstance(ASSETS_PALLET_ID), GeneralIndex(ASSET_ID.into())), fee_amount)
+				.into();
+		let asset_hub_location = PenpalB::sibling_location_of(AssetHubWestend::para_id()).into();
+		let xcm = xcm_transact_paid_execution(
+			call,
+			OriginKind::SovereignAccount,
+			asset_one,
+			penpal.clone(),
+		);
+
+		assert_ok!(<PenpalB as PenpalBPallet>::PolkadotXcm::send(
+			penpal_root,
+			bx!(asset_hub_location),
+			bx!(xcm),
+		));
+
+		PenpalB::assert_xcm_pallet_sent();
+	});
+
+	AssetHubWestend::execute_with(|| {
+		type RuntimeEvent = <AssetHubWestend as Chain>::RuntimeEvent;
+
+		AssetHubWestend::assert_xcmp_queue_success(None);
+		assert_expected_events!(
+			AssetHubWestend,
+			vec![
+				RuntimeEvent::AssetConversion(pallet_asset_conversion::Event::SwapCreditExecuted { .. },) => {},
+				RuntimeEvent::MessageQueue(pallet_message_queue::Event::Processed { success: true,.. }) => {},
+			]
+		);
+	});
+}

cumulus/parachains/integration-tests/emulated/tests/bridges/bridge-hub-rococo/Cargo.toml

@@ -20,6 +20,7 @@ hex-literal = "0.4.1"
 sp-core = { path = "../../../../../../../substrate/primitives/core", default-features = false }
 frame-support = { path = "../../../../../../../substrate/frame/support", default-features = false }
 pallet-assets = { path = "../../../../../../../substrate/frame/assets", default-features = false }
+pallet-asset-conversion = { path = "../../../../../../../substrate/frame/asset-conversion", default-features = false }
 pallet-balances = { path = "../../../../../../../substrate/frame/balances", default-features = false }
 pallet-message-queue = { path = "../../../../../../../substrate/frame/message-queue" }
 sp-runtime = { path = "../../../../../../../substrate/primitives/runtime", default-features = false }

cumulus/parachains/integration-tests/emulated/tests/bridges/bridge-hub-rococo/src/lib.rs

@@ -61,7 +61,8 @@ pub use rococo_westend_system_emulated_network::{
 	rococo_emulated_chain::{genesis::ED as ROCOCO_ED, RococoRelayPallet as RococoPallet},
 	AssetHubRococoPara as AssetHubRococo, AssetHubRococoParaReceiver as AssetHubRococoReceiver,
 	AssetHubRococoParaSender as AssetHubRococoSender, AssetHubWestendPara as AssetHubWestend,
-	AssetHubWestendParaReceiver as AssetHubWestendReceiver, BridgeHubRococoPara as BridgeHubRococo,
+	AssetHubWestendParaReceiver as AssetHubWestendReceiver,
+	AssetHubWestendParaSender as AssetHubWestendSender, BridgeHubRococoPara as BridgeHubRococo,
 	BridgeHubRococoParaSender as BridgeHubRococoSender, BridgeHubWestendPara as BridgeHubWestend,
 	RococoRelay as Rococo, RococoRelayReceiver as RococoReceiver,
 	RococoRelaySender as RococoSender,

cumulus/parachains/integration-tests/emulated/tests/bridges/bridge-hub-rococo/src/tests/asset_transfers.rs

@@ -49,6 +49,49 @@ fn send_rocs_from_asset_hub_rococo_to_asset_hub_westend() {
 		AssetHubWestend::para_id(),
 	);
 
+	AssetHubWestend::execute_with(|| {
+		type RuntimeEvent = <AssetHubWestend as Chain>::RuntimeEvent;
+
+		// setup a pool to pay xcm fees with `roc_at_asset_hub_westend` tokens
+		assert_ok!(<AssetHubWestend as AssetHubWestendPallet>::ForeignAssets::mint(
+			<AssetHubWestend as Chain>::RuntimeOrigin::signed(AssetHubWestendSender::get()),
+			roc_at_asset_hub_westend.into(),
+			AssetHubWestendSender::get().into(),
+			3_000_000_000_000,
+		));
+
+		assert_ok!(<AssetHubWestend as AssetHubWestendPallet>::AssetConversion::create_pool(
+			<AssetHubWestend as Chain>::RuntimeOrigin::signed(AssetHubWestendSender::get()),
+			Box::new(Parent.into()),
+			Box::new(roc_at_asset_hub_westend),
+		));
+
+		assert_expected_events!(
+			AssetHubWestend,
+			vec![
+				RuntimeEvent::AssetConversion(pallet_asset_conversion::Event::PoolCreated { .. }) => {},
+			]
+		);
+
+		assert_ok!(<AssetHubWestend as AssetHubWestendPallet>::AssetConversion::add_liquidity(
+			<AssetHubWestend as Chain>::RuntimeOrigin::signed(AssetHubWestendSender::get()),
+			Box::new(Parent.into()),
+			Box::new(roc_at_asset_hub_westend),
+			1_000_000_000_000,
+			2_000_000_000_000,
+			1,
+			1,
+			AssetHubWestendSender::get().into()
+		));
+
+		assert_expected_events!(
+			AssetHubWestend,
+			vec![
+				RuntimeEvent::AssetConversion(pallet_asset_conversion::Event::LiquidityAdded {..}) => {},
+			]
+		);
+	});
+
 	let rocs_in_reserve_on_ahr_before =
 		<AssetHubRococo as Chain>::account_data_of(sov_ahw_on_ahr.clone()).free;
 	let sender_rocs_before =
@@ -58,7 +101,7 @@ fn send_rocs_from_asset_hub_rococo_to_asset_hub_westend() {
 		<Assets as Inspect<_>>::balance(roc_at_asset_hub_westend, &AssetHubWestendReceiver::get())
 	});
 
-	let amount = ASSET_HUB_ROCOCO_ED * 1_000;
+	let amount = ASSET_HUB_ROCOCO_ED * 1_000_000;
 	send_asset_from_asset_hub_rococo_to_asset_hub_westend(roc_at_asset_hub_rococo, amount);
 	AssetHubWestend::execute_with(|| {
 		type RuntimeEvent = <AssetHubWestend as Chain>::RuntimeEvent;

cumulus/parachains/integration-tests/emulated/tests/bridges/bridge-hub-westend/Cargo.toml

@@ -16,6 +16,7 @@ codec = { package = "parity-scale-codec", version = "3.4.0", default-features =
 # Substrate
 frame-support = { path = "../../../../../../../substrate/frame/support", default-features = false }
 pallet-assets = { path = "../../../../../../../substrate/frame/assets", default-features = false }
+pallet-asset-conversion = { path = "../../../../../../../substrate/frame/asset-conversion", default-features = false }
 pallet-balances = { path = "../../../../../../../substrate/frame/balances", default-features = false }
 pallet-message-queue = { path = "../../../../../../../substrate/frame/message-queue" }
 sp-runtime = { path = "../../../../../../../substrate/primitives/runtime", default-features = false }

cumulus/parachains/integration-tests/emulated/tests/bridges/bridge-hub-westend/src/lib.rs

@@ -55,7 +55,8 @@ pub use rococo_westend_system_emulated_network::{
 	},
 	westend_emulated_chain::WestendRelayPallet as WestendPallet,
 	AssetHubRococoPara as AssetHubRococo, AssetHubRococoParaReceiver as AssetHubRococoReceiver,
-	AssetHubWestendPara as AssetHubWestend, AssetHubWestendParaReceiver as AssetHubWestendReceiver,
+	AssetHubRococoParaSender as AssetHubRococoSender, AssetHubWestendPara as AssetHubWestend,
+	AssetHubWestendParaReceiver as AssetHubWestendReceiver,
 	AssetHubWestendParaSender as AssetHubWestendSender, BridgeHubRococoPara as BridgeHubRococo,
 	BridgeHubWestendPara as BridgeHubWestend, BridgeHubWestendParaSender as BridgeHubWestendSender,
 	WestendRelay as Westend,

cumulus/parachains/integration-tests/emulated/tests/bridges/bridge-hub-westend/src/tests/asset_transfers.rs

@@ -48,6 +48,49 @@ fn send_wnds_from_asset_hub_westend_to_asset_hub_rococo() {
 		AssetHubRococo::para_id(),
 	);
 
+	AssetHubRococo::execute_with(|| {
+		type RuntimeEvent = <AssetHubRococo as Chain>::RuntimeEvent;
+
+		// setup a pool to pay xcm fees with `wnd_at_asset_hub_rococo` tokens
+		assert_ok!(<AssetHubRococo as AssetHubRococoPallet>::ForeignAssets::mint(
+			<AssetHubRococo as Chain>::RuntimeOrigin::signed(AssetHubRococoSender::get()),
+			wnd_at_asset_hub_rococo.into(),
+			AssetHubRococoSender::get().into(),
+			3_000_000_000_000,
+		));
+
+		assert_ok!(<AssetHubRococo as AssetHubRococoPallet>::AssetConversion::create_pool(
+			<AssetHubRococo as Chain>::RuntimeOrigin::signed(AssetHubRococoSender::get()),
+			Box::new(Parent.into()),
+			Box::new(wnd_at_asset_hub_rococo),
+		));
+
+		assert_expected_events!(
+			AssetHubRococo,
+			vec![
+				RuntimeEvent::AssetConversion(pallet_asset_conversion::Event::PoolCreated { .. }) => {},
+			]
+		);
+
+		assert_ok!(<AssetHubRococo as AssetHubRococoPallet>::AssetConversion::add_liquidity(
+			<AssetHubRococo as Chain>::RuntimeOrigin::signed(AssetHubRococoSender::get()),
+			Box::new(Parent.into()),
+			Box::new(wnd_at_asset_hub_rococo),
+			1_000_000_000_000,
+			2_000_000_000_000,
+			1,
+			1,
+			AssetHubRococoSender::get().into()
+		));
+
+		assert_expected_events!(
+			AssetHubRococo,
+			vec![
+				RuntimeEvent::AssetConversion(pallet_asset_conversion::Event::LiquidityAdded {..}) => {},
+			]
+		);
+	});
+
 	let wnds_in_reserve_on_ahw_before =
 		<AssetHubWestend as Chain>::account_data_of(sov_ahr_on_ahw.clone()).free;
 	let sender_wnds_before =

cumulus/parachains/runtimes/assets/asset-hub-rococo/src/lib.rs

@@ -312,7 +312,7 @@ impl pallet_assets::Config<PoolAssetsInstance> for Runtime {
 	type BenchmarkHelper = ();
 }
 
-/// Union fungibles implementation for `Assets`` and `ForeignAssets`.
+/// Union fungibles implementation for `Assets` and `ForeignAssets`.
 pub type LocalAndForeignAssets = fungibles::UnionOf<
 	Assets,
 	ForeignAssets,
@@ -324,18 +324,21 @@ pub type LocalAndForeignAssets = fungibles::UnionOf<
 	AccountId,
 >;
 
+/// Union fungibles implementation for [`LocalAndForeignAssets`] and `Balances`.
+pub type NativeAndAssets = fungible::UnionOf<
+	Balances,
+	LocalAndForeignAssets,
+	TargetFromLeft<TokenLocation>,
+	MultiLocation,
+	AccountId,
+>;
+
 impl pallet_asset_conversion::Config for Runtime {
 	type RuntimeEvent = RuntimeEvent;
 	type Balance = Balance;
 	type HigherPrecisionBalance = sp_core::U256;
 	type AssetKind = MultiLocation;
-	type Assets = fungible::UnionOf<
-		Balances,
-		LocalAndForeignAssets,
-		TargetFromLeft<TokenLocation>,
-		Self::AssetKind,
-		Self::AccountId,
-	>;
+	type Assets = NativeAndAssets;
 	type PoolId = (Self::AssetKind, Self::AssetKind);
 	type PoolLocator =
 		pallet_asset_conversion::WithFirstAsset<TokenLocation, AccountId, Self::AssetKind>;

cumulus/parachains/runtimes/assets/asset-hub-rococo/src/xcm_config.rs

@@ -15,17 +15,21 @@
 
 use super::{
 	AccountId, AllPalletsWithSystem, Assets, Authorship, Balance, Balances, BaseDeliveryFee,
-	FeeAssetId, ForeignAssets, ForeignAssetsInstance, ParachainInfo, ParachainSystem, PolkadotXcm,
-	PoolAssets, Runtime, RuntimeCall, RuntimeEvent, RuntimeOrigin, ToWestendXcmRouter,
-	TransactionByteFee, TrustBackedAssetsInstance, WeightToFee, XcmpQueue,
+	CollatorSelection, FeeAssetId, ForeignAssets, ForeignAssetsInstance, ParachainInfo,
+	ParachainSystem, PolkadotXcm, PoolAssets, Runtime, RuntimeCall, RuntimeEvent, RuntimeOrigin,
+	ToWestendXcmRouter, TransactionByteFee, TrustBackedAssetsInstance, WeightToFee, XcmpQueue,
 };
 use assets_common::{
 	local_and_foreign_assets::MatchesLocalAndForeignAssetsMultiLocation,
 	matching::{FromNetwork, FromSiblingParachain, IsForeignConcreteAsset},
+	TrustBackedAssetsAsMultiLocation,
 };
 use frame_support::{
 	match_types, parameter_types,
-	traits::{ConstU32, Contains, Equals, Everything, Nothing, PalletInfoAccess},
+	traits::{
+		tokens::imbalance::ResolveAssetTo, ConstU32, Contains, Equals, Everything, Nothing,
+		PalletInfoAccess,
+	},
 };
 use frame_system::EnsureRoot;
 use pallet_xcm::XcmPassthrough;
@@ -73,6 +77,7 @@ parameter_types! {
 	pub PoolAssetsPalletLocation: MultiLocation =
 		PalletInstance(<PoolAssets as PalletInfoAccess>::index() as u8).into();
 	pub CheckingAccount: AccountId = PolkadotXcm::check_account();
+	pub StakingPot: AccountId = CollatorSelection::account_id();
 	pub const GovernanceLocation: MultiLocation = MultiLocation::parent();
 	pub TreasuryAccount: AccountId = TREASURY_PALLET_ID.into_account_truncating();
 	pub RelayTreasuryLocation: MultiLocation = (Parent, PalletInstance(rococo_runtime_constants::TREASURY_PALLET_ID)).into();
@@ -550,31 +555,17 @@ impl xcm_executor::Config for XcmConfig {
 	>;
 	type Trader = (
 		UsingComponents<WeightToFee, TokenLocation, AccountId, Balances, ToStakingPot<Runtime>>,
-		// This trader allows to pay with `is_sufficient=true` "Trust Backed" assets from dedicated
-		// `pallet_assets` instance - `Assets`.
-		cumulus_primitives_utility::TakeFirstAssetTrader<
-			AccountId,
-			AssetFeeAsExistentialDepositMultiplierFeeCharger,
-			TrustBackedAssetsConvertedConcreteId,
-			Assets,
-			cumulus_primitives_utility::XcmFeesTo32ByteAccount<
-				FungiblesTransactor,
-				AccountId,
-				XcmAssetFeesReceiver,
-			>,
-		>,
-		// This trader allows to pay with `is_sufficient=true` "Foreign" assets from dedicated
-		// `pallet_assets` instance - `ForeignAssets`.
-		cumulus_primitives_utility::TakeFirstAssetTrader<
-			AccountId,
-			ForeignAssetFeeAsExistentialDepositMultiplierFeeCharger,
+		cumulus_primitives_utility::SwapFirstAssetTrader<
+			TokenLocation,
+			crate::AssetConversion,
+			WeightToFee,
+			crate::NativeAndAssets,
+			(
+				TrustBackedAssetsAsMultiLocation<TrustBackedAssetsPalletLocation, Balance>,
 				ForeignAssetsConvertedConcreteId,
-			ForeignAssets,
-			cumulus_primitives_utility::XcmFeesTo32ByteAccount<
-				ForeignFungiblesTransactor,
+			),
+			ResolveAssetTo<StakingPot, crate::NativeAndAssets>,
 			AccountId,
-				XcmAssetFeesReceiver,
-			>,
 		>,
 	);
 	type ResponseHandler = PolkadotXcm;

cumulus/parachains/runtimes/assets/asset-hub-rococo/tests/tests.rs

@@ -17,34 +17,38 @@
 
 //! Tests for the Rococo Assets Hub chain.
 
-use asset_hub_rococo_runtime::xcm_config::{
-	AssetFeeAsExistentialDepositMultiplierFeeCharger, TokenLocation,
-	TrustBackedAssetsPalletLocation,
+use asset_hub_rococo_runtime::{
+	xcm_config,
+	xcm_config::{bridging, ForeignCreatorsSovereignAccountOf, LocationToAccountId, TokenLocation},
+	AllPalletsWithoutSystem, MetadataDepositBase, MetadataDepositPerByte, RuntimeCall,
+	RuntimeEvent, ToWestendXcmRouterInstance, XcmpQueue,
 };
 pub use asset_hub_rococo_runtime::{
-	xcm_config::{
-		self, bridging, CheckingAccount, ForeignCreatorsSovereignAccountOf, LocationToAccountId,
-		XcmConfig,
-	},
-	AllPalletsWithoutSystem, AssetDeposit, Assets, Balances, ExistentialDeposit, ForeignAssets,
-	ForeignAssetsInstance, MetadataDepositBase, MetadataDepositPerByte, ParachainSystem, Runtime,
-	RuntimeCall, RuntimeEvent, SessionKeys, System, ToWestendXcmRouterInstance,
-	TrustBackedAssetsInstance, XcmpQueue,
+	xcm_config::{CheckingAccount, TrustBackedAssetsPalletLocation, XcmConfig},
+	AssetConversion, AssetDeposit, Assets, Balances, CollatorSelection, ExistentialDeposit,
+	ForeignAssets, ForeignAssetsInstance, ParachainSystem, Runtime, SessionKeys, System,
+	TrustBackedAssetsInstance,
 };
 use asset_test_utils::{
 	test_cases_over_bridge::TestBridgingConfig, CollatorSessionKey, CollatorSessionKeys, ExtBuilder,
 };
 use codec::{Decode, Encode};
-use cumulus_primitives_utility::ChargeWeightInFungibles;
 use frame_support::{
-	assert_noop, assert_ok,
-	traits::fungibles::InspectEnumerable,
+	assert_ok,
+	traits::{
+		fungible::{Inspect, Mutate},
+		fungibles::{
+			Create, Inspect as FungiblesInspect, InspectEnumerable, Mutate as FungiblesMutate,
+		},
+	},
 	weights::{Weight, WeightToFee as WeightToFeeT},
 };
 use parachains_common::{
-	rococo::fee::WeightToFee, AccountId, AssetIdForTrustBackedAssets, AuraId, Balance,
+	rococo::{currency::UNITS, fee::WeightToFee},
+	AccountId, AssetIdForTrustBackedAssets, AuraId, Balance,
 };
 use sp_runtime::traits::MaybeEquivalence;
+use std::convert::Into;
 use xcm::latest::prelude::*;
 use xcm_executor::traits::{Identity, JustTry, WeightTrader};
 
@@ -69,7 +73,7 @@ fn collator_session_keys() -> CollatorSessionKeys<Runtime> {
 }
 
 #[test]
-fn test_asset_xcm_trader() {
+fn test_buy_and_refund_weight_in_native() {
 	ExtBuilder::<Runtime>::default()
 		.with_collators(vec![AccountId::from(ALICE)])
 		.with_session_keys(vec![(
@@ -79,77 +83,55 @@ fn test_asset_xcm_trader() {
 		)])
 		.build()
 		.execute_with(|| {
-			// We need root origin to create a sufficient asset
-			let minimum_asset_balance = 3333333_u128;
-			let local_asset_id = 1;
-			assert_ok!(Assets::force_create(
-				RuntimeHelper::root_origin(),
-				local_asset_id.into(),
-				AccountId::from(ALICE).into(),
-				true,
-				minimum_asset_balance
-			));
+			let bob: AccountId = SOME_ASSET_ADMIN.into();
+			let staking_pot = CollatorSelection::account_id();
+			let native_location = TokenLocation::get();
+			let initial_balance = 200 * UNITS;
 
-			// We first mint enough asset for the account to exist for assets
-			assert_ok!(Assets::mint(
-				RuntimeHelper::origin_of(AccountId::from(ALICE)),
-				local_asset_id.into(),
-				AccountId::from(ALICE).into(),
-				minimum_asset_balance
-			));
+			assert_ok!(Balances::mint_into(&bob, initial_balance));
+			assert_ok!(Balances::mint_into(&staking_pot, initial_balance));
 
-			// get asset id as multilocation
-			let asset_multilocation =
-				AssetIdForTrustBackedAssetsConvert::convert_back(&local_asset_id).unwrap();
+			// keep initial total issuance to assert later.
+			let total_issuance = Balances::total_issuance();
 
-			// Set Alice as block author, who will receive fees
-			RuntimeHelper::run_to_block(2, AccountId::from(ALICE));
-
-			// We are going to buy 4e9 weight
-			let bought = Weight::from_parts(4_000_000_000u64, 0);
-
-			// Lets calculate amount needed
-			let asset_amount_needed =
-				AssetFeeAsExistentialDepositMultiplierFeeCharger::charge_weight_in_fungibles(
-					local_asset_id,
-					bought,
-				)
-				.expect("failed to compute");
-
-			// Lets pay with: asset_amount_needed + asset_amount_extra
-			let asset_amount_extra = 100_u128;
-			let asset: MultiAsset =
-				(asset_multilocation, asset_amount_needed + asset_amount_extra).into();
-
-			let mut trader = <XcmConfig as xcm_executor::Config>::Trader::new();
+			// prepare input to buy weight.
+			let weight = Weight::from_parts(4_000_000_000, 0);
+			let fee = WeightToFee::weight_to_fee(&weight);
+			let extra_amount = 100;
 			let ctx = XcmContext { origin: None, message_id: XcmHash::default(), topic: None };
+			let payment: MultiAsset = (native_location, fee + extra_amount).into();
 
-			// Lets buy_weight and make sure buy_weight does not return an error
-			let unused_assets = trader.buy_weight(bought, asset.into(), &ctx).expect("Expected Ok");
-			// Check whether a correct amount of unused assets is returned
-			assert_ok!(
-				unused_assets.ensure_contains(&(asset_multilocation, asset_amount_extra).into())
-			);
+			// init trader and buy weight.
+			let mut trader = <XcmConfig as xcm_executor::Config>::Trader::new();
+			let unused_asset =
+				trader.buy_weight(weight, payment.into(), &ctx).expect("Expected Ok");
 
-			// Drop trader
+			// assert.
+			let unused_amount =
+				unused_asset.fungible.get(&native_location.into()).map_or(0, |a| *a);
+			assert_eq!(unused_amount, extra_amount);
+			assert_eq!(Balances::total_issuance(), total_issuance);
+
+			// prepare input to refund weight.
+			let refund_weight = Weight::from_parts(1_000_000_000, 0);
+			let refund = WeightToFee::weight_to_fee(&refund_weight);
+
+			// refund.
+			let actual_refund = trader.refund_weight(refund_weight, &ctx).unwrap();
+			assert_eq!(actual_refund, (native_location, refund).into());
+
+			// assert.
+			assert_eq!(Balances::balance(&staking_pot), initial_balance);
+			// only after `trader` is dropped we expect the fee to be resolved into the treasury
+			// account.
 			drop(trader);
-
-			// Make sure author(Alice) has received the amount
-			assert_eq!(
-				Assets::balance(local_asset_id, AccountId::from(ALICE)),
-				minimum_asset_balance + asset_amount_needed
-			);
-
-			// We also need to ensure the total supply increased
-			assert_eq!(
-				Assets::total_supply(local_asset_id),
-				minimum_asset_balance + asset_amount_needed
-			);
-		});
+			assert_eq!(Balances::balance(&staking_pot), initial_balance + fee - refund);
+			assert_eq!(Balances::total_issuance(), total_issuance + fee - refund);
+		})
 }
 
 #[test]
-fn test_asset_xcm_trader_with_refund() {
+fn test_buy_and_refund_weight_with_swap_local_asset_xcm_trader() {
 	ExtBuilder::<Runtime>::default()
 		.with_collators(vec![AccountId::from(ALICE)])
 		.with_session_keys(vec![(
@@ -159,249 +141,192 @@ fn test_asset_xcm_trader_with_refund() {
 		)])
 		.build()
 		.execute_with(|| {
-			// We need root origin to create a sufficient asset
-			// We set existential deposit to be identical to the one for Balances first
-			assert_ok!(Assets::force_create(
-				RuntimeHelper::root_origin(),
-				1.into(),
-				AccountId::from(ALICE).into(),
-				true,
-				ExistentialDeposit::get()
+			let bob: AccountId = SOME_ASSET_ADMIN.into();
+			let staking_pot = CollatorSelection::account_id();
+			let asset_1: u32 = 1;
+			let native_location = TokenLocation::get();
+			let asset_1_location =
+				AssetIdForTrustBackedAssetsConvert::convert_back(&asset_1).unwrap();
+			// bob's initial balance for native and `asset1` assets.
+			let initial_balance = 200 * UNITS;
+			// liquidity for both arms of (native, asset1) pool.
+			let pool_liquidity = 100 * UNITS;
+
+			// init asset, balances and pool.
+			assert_ok!(<Assets as Create<_>>::create(asset_1, bob.clone(), true, 10));
+
+			assert_ok!(Assets::mint_into(asset_1, &bob, initial_balance));
+			assert_ok!(Balances::mint_into(&bob, initial_balance));
+			assert_ok!(Balances::mint_into(&staking_pot, initial_balance));
+
+			assert_ok!(AssetConversion::create_pool(
+				RuntimeHelper::origin_of(bob.clone()),
+				Box::new(native_location),
+				Box::new(asset_1_location)
 			));
 
-			// We first mint enough asset for the account to exist for assets
-			assert_ok!(Assets::mint(
-				RuntimeHelper::origin_of(AccountId::from(ALICE)),
-				1.into(),
-				AccountId::from(ALICE).into(),
-				ExistentialDeposit::get()
+			assert_ok!(AssetConversion::add_liquidity(
+				RuntimeHelper::origin_of(bob.clone()),
+				Box::new(native_location),
+				Box::new(asset_1_location),
+				pool_liquidity,
+				pool_liquidity,
+				1,
+				1,
+				bob,
 			));
 
-			let mut trader = <XcmConfig as xcm_executor::Config>::Trader::new();
+			// keep initial total issuance to assert later.
+			let asset_total_issuance = Assets::total_issuance(asset_1);
+			let native_total_issuance = Balances::total_issuance();
+
+			// prepare input to buy weight.
+			let weight = Weight::from_parts(4_000_000_000, 0);
+			let fee = WeightToFee::weight_to_fee(&weight);
+			let asset_fee =
+				AssetConversion::get_amount_in(&fee, &pool_liquidity, &pool_liquidity).unwrap();
+			let extra_amount = 100;
 			let ctx = XcmContext { origin: None, message_id: XcmHash::default(), topic: None };
+			let payment: MultiAsset = (asset_1_location, asset_fee + extra_amount).into();
 
-			// Set Alice as block author, who will receive fees
-			RuntimeHelper::run_to_block(2, AccountId::from(ALICE));
+			// init trader and buy weight.
+			let mut trader = <XcmConfig as xcm_executor::Config>::Trader::new();
+			let unused_asset =
+				trader.buy_weight(weight, payment.into(), &ctx).expect("Expected Ok");
 
-			// We are going to buy 4e9 weight
-			let bought = Weight::from_parts(4_000_000_000u64, 0);
+			// assert.
+			let unused_amount =
+				unused_asset.fungible.get(&asset_1_location.into()).map_or(0, |a| *a);
+			assert_eq!(unused_amount, extra_amount);
+			assert_eq!(Assets::total_issuance(asset_1), asset_total_issuance + asset_fee);
 
-			let asset_multilocation = AssetIdForTrustBackedAssetsConvert::convert_back(&1).unwrap();
+			// prepare input to refund weight.
+			let refund_weight = Weight::from_parts(1_000_000_000, 0);
+			let refund = WeightToFee::weight_to_fee(&refund_weight);
+			let (reserve1, reserve2) =
+				AssetConversion::get_reserves(native_location, asset_1_location).unwrap();
+			let asset_refund =
+				AssetConversion::get_amount_out(&refund, &reserve1, &reserve2).unwrap();
 
-			// lets calculate amount needed
-			let amount_bought = WeightToFee::weight_to_fee(&bought);
-
-			let asset: MultiAsset = (asset_multilocation, amount_bought).into();
-
-			// Make sure buy_weight does not return an error
-			assert_ok!(trader.buy_weight(bought, asset.clone().into(), &ctx));
-
-			// Make sure again buy_weight does return an error
-			// This assert relies on the fact, that we use `TakeFirstAssetTrader` in `WeightTrader`
-			// tuple chain, which cannot be called twice
-			assert_noop!(trader.buy_weight(bought, asset.into(), &ctx), XcmError::TooExpensive);
-
-			// We actually use half of the weight
-			let weight_used = bought / 2;
-
-			// Make sure refurnd works.
-			let amount_refunded = WeightToFee::weight_to_fee(&(bought - weight_used));
+			// refund.
+			let actual_refund = trader.refund_weight(refund_weight, &ctx).unwrap();
+			assert_eq!(actual_refund, (asset_1_location, asset_refund).into());
 
+			// assert.
+			assert_eq!(Balances::balance(&staking_pot), initial_balance);
+			// only after `trader` is dropped we expect the fee to be resolved into the treasury
+			// account.
+			drop(trader);
+			assert_eq!(Balances::balance(&staking_pot), initial_balance + fee - refund);
 			assert_eq!(
-				trader.refund_weight(bought - weight_used, &ctx),
-				Some((asset_multilocation, amount_refunded).into())
+				Assets::total_issuance(asset_1),
+				asset_total_issuance + asset_fee - asset_refund
 			);
+			assert_eq!(Balances::total_issuance(), native_total_issuance);
+		})
+}
 
-			// Drop trader
-			drop(trader);
+#[test]
+fn test_buy_and_refund_weight_with_swap_foreign_asset_xcm_trader() {
+	ExtBuilder::<Runtime>::default()
+		.with_collators(vec![AccountId::from(ALICE)])
+		.with_session_keys(vec![(
+			AccountId::from(ALICE),
+			AccountId::from(ALICE),
+			SessionKeys { aura: AuraId::from(sp_core::sr25519::Public::from_raw(ALICE)) },
+		)])
+		.build()
+		.execute_with(|| {
+			let bob: AccountId = SOME_ASSET_ADMIN.into();
+			let staking_pot = CollatorSelection::account_id();
+			let native_location = TokenLocation::get();
+			let foreign_location =
+				MultiLocation { parents: 1, interior: X2(Parachain(1234), GeneralIndex(12345)) };
+			// bob's initial balance for native and `asset1` assets.
+			let initial_balance = 200 * UNITS;
+			// liquidity for both arms of (native, asset1) pool.
+			let pool_liquidity = 100 * UNITS;
 
-			// We only should have paid for half of the bought weight
-			let fees_paid = WeightToFee::weight_to_fee(&weight_used);
+			// init asset, balances and pool.
+			assert_ok!(<ForeignAssets as Create<_>>::create(
+				foreign_location,
+				bob.clone(),
+				true,
+				10
+			));
 
+			assert_ok!(ForeignAssets::mint_into(foreign_location, &bob, initial_balance));
+			assert_ok!(Balances::mint_into(&bob, initial_balance));
+			assert_ok!(Balances::mint_into(&staking_pot, initial_balance));
+
+			assert_ok!(AssetConversion::create_pool(
+				RuntimeHelper::origin_of(bob.clone()),
+				Box::new(native_location),
+				Box::new(foreign_location)
+			));
+
+			assert_ok!(AssetConversion::add_liquidity(
+				RuntimeHelper::origin_of(bob.clone()),
+				Box::new(native_location),
+				Box::new(foreign_location),
+				pool_liquidity,
+				pool_liquidity,
+				1,
+				1,
+				bob,
+			));
+
+			// keep initial total issuance to assert later.
+			let asset_total_issuance = ForeignAssets::total_issuance(foreign_location);
+			let native_total_issuance = Balances::total_issuance();
+
+			// prepare input to buy weight.
+			let weight = Weight::from_parts(4_000_000_000, 0);
+			let fee = WeightToFee::weight_to_fee(&weight);
+			let asset_fee =
+				AssetConversion::get_amount_in(&fee, &pool_liquidity, &pool_liquidity).unwrap();
+			let extra_amount = 100;
+			let ctx = XcmContext { origin: None, message_id: XcmHash::default(), topic: None };
+			let payment: MultiAsset = (foreign_location, asset_fee + extra_amount).into();
+
+			// init trader and buy weight.
+			let mut trader = <XcmConfig as xcm_executor::Config>::Trader::new();
+			let unused_asset =
+				trader.buy_weight(weight, payment.into(), &ctx).expect("Expected Ok");
+
+			// assert.
+			let unused_amount =
+				unused_asset.fungible.get(&foreign_location.into()).map_or(0, |a| *a);
+			assert_eq!(unused_amount, extra_amount);
 			assert_eq!(
-				Assets::balance(1, AccountId::from(ALICE)),
-				ExistentialDeposit::get() + fees_paid
+				ForeignAssets::total_issuance(foreign_location),
+				asset_total_issuance + asset_fee
 			);
 
-			// We also need to ensure the total supply increased
-			assert_eq!(Assets::total_supply(1), ExistentialDeposit::get() + fees_paid);
-		});
-}
+			// prepare input to refund weight.
+			let refund_weight = Weight::from_parts(1_000_000_000, 0);
+			let refund = WeightToFee::weight_to_fee(&refund_weight);
+			let (reserve1, reserve2) =
+				AssetConversion::get_reserves(native_location, foreign_location).unwrap();
+			let asset_refund =
+				AssetConversion::get_amount_out(&refund, &reserve1, &reserve2).unwrap();
 
-#[test]
-fn test_asset_xcm_trader_refund_not_possible_since_amount_less_than_ed() {
-	ExtBuilder::<Runtime>::default()
-		.with_collators(vec![AccountId::from(ALICE)])
-		.with_session_keys(vec![(
-			AccountId::from(ALICE),
-			AccountId::from(ALICE),
-			SessionKeys { aura: AuraId::from(sp_core::sr25519::Public::from_raw(ALICE)) },
-		)])
-		.build()
-		.execute_with(|| {
-			// We need root origin to create a sufficient asset
-			// We set existential deposit to be identical to the one for Balances first
-			assert_ok!(Assets::force_create(
-				RuntimeHelper::root_origin(),
-				1.into(),
-				AccountId::from(ALICE).into(),
-				true,
-				ExistentialDeposit::get()
-			));
+			// refund.
+			let actual_refund = trader.refund_weight(refund_weight, &ctx).unwrap();
+			assert_eq!(actual_refund, (foreign_location, asset_refund).into());
 
-			let mut trader = <XcmConfig as xcm_executor::Config>::Trader::new();
-			let ctx = XcmContext { origin: None, message_id: XcmHash::default(), topic: None };
-
-			// Set Alice as block author, who will receive fees
-			RuntimeHelper::run_to_block(2, AccountId::from(ALICE));
-
-			// We are going to buy small amount
-			let bought = Weight::from_parts(500_000_000u64, 0);
-
-			let asset_multilocation = AssetIdForTrustBackedAssetsConvert::convert_back(&1).unwrap();
-
-			let amount_bought = WeightToFee::weight_to_fee(&bought);
-
-			assert!(
-				amount_bought < ExistentialDeposit::get(),
-				"we are testing what happens when the amount does not exceed ED"
-			);
-
-			let asset: MultiAsset = (asset_multilocation, amount_bought).into();
-
-			// Buy weight should return an error
-			assert_noop!(trader.buy_weight(bought, asset.into(), &ctx), XcmError::TooExpensive);
-
-			// not credited since the ED is higher than this value
-			assert_eq!(Assets::balance(1, AccountId::from(ALICE)), 0);
-
-			// We also need to ensure the total supply did not increase
-			assert_eq!(Assets::total_supply(1), 0);
-		});
-}
-
-#[test]
-fn test_that_buying_ed_refund_does_not_refund() {
-	ExtBuilder::<Runtime>::default()
-		.with_collators(vec![AccountId::from(ALICE)])
-		.with_session_keys(vec![(
-			AccountId::from(ALICE),
-			AccountId::from(ALICE),
-			SessionKeys { aura: AuraId::from(sp_core::sr25519::Public::from_raw(ALICE)) },
-		)])
-		.build()
-		.execute_with(|| {
-			// We need root origin to create a sufficient asset
-			// We set existential deposit to be identical to the one for Balances first
-			assert_ok!(Assets::force_create(
-				RuntimeHelper::root_origin(),
-				1.into(),
-				AccountId::from(ALICE).into(),
-				true,
-				ExistentialDeposit::get()
-			));
-
-			let mut trader = <XcmConfig as xcm_executor::Config>::Trader::new();
-			let ctx = XcmContext { origin: None, message_id: XcmHash::default(), topic: None };
-
-			// Set Alice as block author, who will receive fees
-			RuntimeHelper::run_to_block(2, AccountId::from(ALICE));
-
-			// We are gonna buy ED
-			let bought = Weight::from_parts(ExistentialDeposit::get().try_into().unwrap(), 0);
-
-			let asset_multilocation = AssetIdForTrustBackedAssetsConvert::convert_back(&1).unwrap();
-
-			let amount_bought = WeightToFee::weight_to_fee(&bought);
-
-			assert!(
-				amount_bought < ExistentialDeposit::get(),
-				"we are testing what happens when the amount does not exceed ED"
-			);
-
-			// We know we will have to buy at least ED, so lets make sure first it will
-			// fail with a payment of less than ED
-			let asset: MultiAsset = (asset_multilocation, amount_bought).into();
-			assert_noop!(trader.buy_weight(bought, asset.into(), &ctx), XcmError::TooExpensive);
-
-			// Now lets buy ED at least
-			let asset: MultiAsset = (asset_multilocation, ExistentialDeposit::get()).into();
-
-			// Buy weight should work
-			assert_ok!(trader.buy_weight(bought, asset.into(), &ctx));
-
-			// Should return None. We have a specific check making sure we dont go below ED for
-			// drop payment
-			assert_eq!(trader.refund_weight(bought, &ctx), None);
-
-			// Drop trader
+			// assert.
+			assert_eq!(Balances::balance(&staking_pot), initial_balance);
+			// only after `trader` is dropped we expect the fee to be resolved into the treasury
+			// account.
 			drop(trader);
-
-			// Make sure author(Alice) has received the amount
-			assert_eq!(Assets::balance(1, AccountId::from(ALICE)), ExistentialDeposit::get());
-
-			// We also need to ensure the total supply increased
-			assert_eq!(Assets::total_supply(1), ExistentialDeposit::get());
-		});
-}
-
-#[test]
-fn test_asset_xcm_trader_not_possible_for_non_sufficient_assets() {
-	ExtBuilder::<Runtime>::default()
-		.with_collators(vec![AccountId::from(ALICE)])
-		.with_session_keys(vec![(
-			AccountId::from(ALICE),
-			AccountId::from(ALICE),
-			SessionKeys { aura: AuraId::from(sp_core::sr25519::Public::from_raw(ALICE)) },
-		)])
-		.build()
-		.execute_with(|| {
-			// Create a non-sufficient asset with specific existential deposit
-			let minimum_asset_balance = 1_000_000_u128;
-			assert_ok!(Assets::force_create(
-				RuntimeHelper::root_origin(),
-				1.into(),
-				AccountId::from(ALICE).into(),
-				false,
-				minimum_asset_balance
-			));
-
-			// We first mint enough asset for the account to exist for assets
-			assert_ok!(Assets::mint(
-				RuntimeHelper::origin_of(AccountId::from(ALICE)),
-				1.into(),
-				AccountId::from(ALICE).into(),
-				minimum_asset_balance
-			));
-
-			let mut trader = <XcmConfig as xcm_executor::Config>::Trader::new();
-			let ctx = XcmContext { origin: None, message_id: XcmHash::default(), topic: None };
-
-			// Set Alice as block author, who will receive fees
-			RuntimeHelper::run_to_block(2, AccountId::from(ALICE));
-
-			// We are going to buy 4e9 weight
-			let bought = Weight::from_parts(4_000_000_000u64, 0);
-
-			// lets calculate amount needed
-			let asset_amount_needed = WeightToFee::weight_to_fee(&bought);
-
-			let asset_multilocation = AssetIdForTrustBackedAssetsConvert::convert_back(&1).unwrap();
-
-			let asset: MultiAsset = (asset_multilocation, asset_amount_needed).into();
-
-			// Make sure again buy_weight does return an error
-			assert_noop!(trader.buy_weight(bought, asset.into(), &ctx), XcmError::TooExpensive);
-
-			// Drop trader
-			drop(trader);
-
-			// Make sure author(Alice) has NOT received the amount
-			assert_eq!(Assets::balance(1, AccountId::from(ALICE)), minimum_asset_balance);
-
-			// We also need to ensure the total supply NOT increased
-			assert_eq!(Assets::total_supply(1), minimum_asset_balance);
-		});
+			assert_eq!(Balances::balance(&staking_pot), initial_balance + fee - refund);
+			assert_eq!(
+				ForeignAssets::total_issuance(foreign_location),
+				asset_total_issuance + asset_fee - asset_refund
+			);
+			assert_eq!(Balances::total_issuance(), native_total_issuance);
+		})
 }
 
 #[test]

cumulus/parachains/runtimes/assets/asset-hub-westend/src/lib.rs

@@ -295,7 +295,7 @@ impl pallet_assets::Config<PoolAssetsInstance> for Runtime {
 	type BenchmarkHelper = ();
 }
 
-/// Union fungibles implementation for `Assets`` and `ForeignAssets`.
+/// Union fungibles implementation for `Assets` and `ForeignAssets`.
 pub type LocalAndForeignAssets = fungibles::UnionOf<
 	Assets,
 	ForeignAssets,
@@ -307,18 +307,21 @@ pub type LocalAndForeignAssets = fungibles::UnionOf<
 	AccountId,
 >;
 
+/// Union fungibles implementation for [`LocalAndForeignAssets`] and `Balances`.
+pub type NativeAndAssets = fungible::UnionOf<
+	Balances,
+	LocalAndForeignAssets,
+	TargetFromLeft<WestendLocation>,
+	MultiLocation,
+	AccountId,
+>;
+
 impl pallet_asset_conversion::Config for Runtime {
 	type RuntimeEvent = RuntimeEvent;
 	type Balance = Balance;
 	type HigherPrecisionBalance = sp_core::U256;
 	type AssetKind = MultiLocation;
-	type Assets = fungible::UnionOf<
-		Balances,
-		LocalAndForeignAssets,
-		TargetFromLeft<WestendLocation>,
-		Self::AssetKind,
-		Self::AccountId,
-	>;
+	type Assets = NativeAndAssets;
 	type PoolId = (Self::AssetKind, Self::AssetKind);
 	type PoolLocator =
 		pallet_asset_conversion::WithFirstAsset<WestendLocation, AccountId, Self::AssetKind>;

cumulus/parachains/runtimes/assets/asset-hub-westend/src/xcm_config.rs

@@ -15,17 +15,21 @@
 
 use super::{
 	AccountId, AllPalletsWithSystem, Assets, Authorship, Balance, Balances, BaseDeliveryFee,
-	FeeAssetId, ForeignAssets, ForeignAssetsInstance, ParachainInfo, ParachainSystem, PolkadotXcm,
-	PoolAssets, Runtime, RuntimeCall, RuntimeEvent, RuntimeOrigin, ToRococoXcmRouter,
-	TransactionByteFee, TrustBackedAssetsInstance, WeightToFee, XcmpQueue,
+	CollatorSelection, FeeAssetId, ForeignAssets, ForeignAssetsInstance, ParachainInfo,
+	ParachainSystem, PolkadotXcm, PoolAssets, Runtime, RuntimeCall, RuntimeEvent, RuntimeOrigin,
+	ToRococoXcmRouter, TransactionByteFee, TrustBackedAssetsInstance, WeightToFee, XcmpQueue,
 };
 use assets_common::{
 	local_and_foreign_assets::MatchesLocalAndForeignAssetsMultiLocation,
 	matching::{FromSiblingParachain, IsForeignConcreteAsset},
+	TrustBackedAssetsAsMultiLocation,
 };
 use frame_support::{
 	match_types, parameter_types,
-	traits::{ConstU32, Contains, Equals, Everything, Nothing, PalletInfoAccess},
+	traits::{
+		tokens::imbalance::ResolveAssetTo, ConstU32, Contains, Equals, Everything, Nothing,
+		PalletInfoAccess,
+	},
 };
 use frame_system::EnsureRoot;
 use pallet_xcm::XcmPassthrough;
@@ -70,6 +74,7 @@ parameter_types! {
 	pub PoolAssetsPalletLocation: MultiLocation =
 		PalletInstance(<PoolAssets as PalletInfoAccess>::index() as u8).into();
 	pub CheckingAccount: AccountId = PolkadotXcm::check_account();
+	pub StakingPot: AccountId = CollatorSelection::account_id();
 	pub TreasuryAccount: AccountId = TREASURY_PALLET_ID.into_account_truncating();
 	pub RelayTreasuryLocation: MultiLocation = (Parent, PalletInstance(westend_runtime_constants::TREASURY_PALLET_ID)).into();
 }
@@ -567,31 +572,17 @@ impl xcm_executor::Config for XcmConfig {
 	>;
 	type Trader = (
 		UsingComponents<WeightToFee, WestendLocation, AccountId, Balances, ToStakingPot<Runtime>>,
-		// This trader allows to pay with `is_sufficient=true` "Trust Backed" assets from dedicated
-		// `pallet_assets` instance - `Assets`.
-		cumulus_primitives_utility::TakeFirstAssetTrader<
-			AccountId,
-			AssetFeeAsExistentialDepositMultiplierFeeCharger,
-			TrustBackedAssetsConvertedConcreteId,
-			Assets,
-			cumulus_primitives_utility::XcmFeesTo32ByteAccount<
-				FungiblesTransactor,
-				AccountId,
-				XcmAssetFeesReceiver,
-			>,
-		>,
-		// This trader allows to pay with `is_sufficient=true` "Foreign" assets from dedicated
-		// `pallet_assets` instance - `ForeignAssets`.
-		cumulus_primitives_utility::TakeFirstAssetTrader<
-			AccountId,
-			ForeignAssetFeeAsExistentialDepositMultiplierFeeCharger,
+		cumulus_primitives_utility::SwapFirstAssetTrader<
+			WestendLocation,
+			crate::AssetConversion,
+			WeightToFee,
+			crate::NativeAndAssets,
+			(
+				TrustBackedAssetsAsMultiLocation<TrustBackedAssetsPalletLocation, Balance>,
 				ForeignAssetsConvertedConcreteId,
-			ForeignAssets,
-			cumulus_primitives_utility::XcmFeesTo32ByteAccount<
-				ForeignFungiblesTransactor,
+			),
+			ResolveAssetTo<StakingPot, crate::NativeAndAssets>,
 			AccountId,
-				XcmAssetFeesReceiver,
-			>,
 		>,
 	);
 	type ResponseHandler = PolkadotXcm;

cumulus/parachains/runtimes/assets/asset-hub-westend/tests/tests.rs

@@ -18,28 +18,36 @@
 //! Tests for the Westmint (Westend Assets Hub) chain.
 
 use asset_hub_westend_runtime::{
+	xcm_config,
 	xcm_config::{
-		self, bridging, AssetFeeAsExistentialDepositMultiplierFeeCharger, CheckingAccount,
-		ForeignCreatorsSovereignAccountOf, LocationToAccountId, TrustBackedAssetsPalletLocation,
-		WestendLocation, XcmConfig,
+		bridging, ForeignCreatorsSovereignAccountOf, LocationToAccountId, WestendLocation,
 	},
-	AllPalletsWithoutSystem, AssetDeposit, Assets, Balances, ExistentialDeposit, ForeignAssets,
-	ForeignAssetsInstance, MetadataDepositBase, MetadataDepositPerByte, ParachainSystem,
-	PolkadotXcm, Runtime, RuntimeCall, RuntimeEvent, RuntimeOrigin, SessionKeys,
-	ToRococoXcmRouterInstance, TrustBackedAssetsInstance, XcmpQueue,
+	AllPalletsWithoutSystem, MetadataDepositBase, MetadataDepositPerByte, PolkadotXcm, RuntimeCall,
+	RuntimeEvent, RuntimeOrigin, ToRococoXcmRouterInstance, XcmpQueue,
+};
+pub use asset_hub_westend_runtime::{
+	xcm_config::{CheckingAccount, TrustBackedAssetsPalletLocation, XcmConfig},
+	AssetConversion, AssetDeposit, Assets, Balances, CollatorSelection, ExistentialDeposit,
+	ForeignAssets, ForeignAssetsInstance, ParachainSystem, Runtime, SessionKeys, System,
+	TrustBackedAssetsInstance,
 };
 use asset_test_utils::{
 	test_cases_over_bridge::TestBridgingConfig, CollatorSessionKey, CollatorSessionKeys, ExtBuilder,
 };
 use codec::{Decode, Encode};
-use cumulus_primitives_utility::ChargeWeightInFungibles;
 use frame_support::{
-	assert_noop, assert_ok,
-	traits::fungibles::InspectEnumerable,
+	assert_ok,
+	traits::{
+		fungible::{Inspect, Mutate},
+		fungibles::{
+			Create, Inspect as FungiblesInspect, InspectEnumerable, Mutate as FungiblesMutate,
+		},
+	},
 	weights::{Weight, WeightToFee as WeightToFeeT},
 };
 use parachains_common::{
-	westend::fee::WeightToFee, AccountId, AssetIdForTrustBackedAssets, AuraId, Balance,
+	westend::{currency::UNITS, fee::WeightToFee},
+	AccountId, AssetIdForTrustBackedAssets, AuraId, Balance,
 };
 use sp_runtime::traits::MaybeEquivalence;
 use std::convert::Into;
@@ -67,7 +75,7 @@ fn collator_session_keys() -> CollatorSessionKeys<Runtime> {
 }
 
 #[test]
-fn test_asset_xcm_trader() {
+fn test_buy_and_refund_weight_in_native() {
 	ExtBuilder::<Runtime>::default()
 		.with_collators(vec![AccountId::from(ALICE)])
 		.with_session_keys(vec![(
@@ -77,77 +85,55 @@ fn test_asset_xcm_trader() {
 		)])
 		.build()
 		.execute_with(|| {
-			// We need root origin to create a sufficient asset
-			let minimum_asset_balance = 3333333_u128;
-			let local_asset_id = 1;
-			assert_ok!(Assets::force_create(
-				RuntimeHelper::root_origin(),
-				local_asset_id.into(),
-				AccountId::from(ALICE).into(),
-				true,
-				minimum_asset_balance
-			));
+			let bob: AccountId = SOME_ASSET_ADMIN.into();
+			let staking_pot = CollatorSelection::account_id();
+			let native_location = WestendLocation::get();
+			let initial_balance = 200 * UNITS;
 
-			// We first mint enough asset for the account to exist for assets
-			assert_ok!(Assets::mint(
-				RuntimeHelper::origin_of(AccountId::from(ALICE)),
-				local_asset_id.into(),
-				AccountId::from(ALICE).into(),
-				minimum_asset_balance
-			));
+			assert_ok!(Balances::mint_into(&bob, initial_balance));
+			assert_ok!(Balances::mint_into(&staking_pot, initial_balance));
 
-			// get asset id as multilocation
-			let asset_multilocation =
-				AssetIdForTrustBackedAssetsConvert::convert_back(&local_asset_id).unwrap();
+			// keep initial total issuance to assert later.
+			let total_issuance = Balances::total_issuance();
 
-			// Set Alice as block author, who will receive fees
-			RuntimeHelper::run_to_block(2, AccountId::from(ALICE));
-
-			// We are going to buy 4e9 weight
-			let bought = Weight::from_parts(4_000_000_000u64, 0);
-
-			// Lets calculate amount needed
-			let asset_amount_needed =
-				AssetFeeAsExistentialDepositMultiplierFeeCharger::charge_weight_in_fungibles(
-					local_asset_id,
-					bought,
-				)
-				.expect("failed to compute");
-
-			// Lets pay with: asset_amount_needed + asset_amount_extra
-			let asset_amount_extra = 100_u128;
-			let asset: MultiAsset =
-				(asset_multilocation, asset_amount_needed + asset_amount_extra).into();
-
-			let mut trader = <XcmConfig as xcm_executor::Config>::Trader::new();
+			// prepare input to buy weight.
+			let weight = Weight::from_parts(4_000_000_000, 0);
+			let fee = WeightToFee::weight_to_fee(&weight);
+			let extra_amount = 100;
 			let ctx = XcmContext { origin: None, message_id: XcmHash::default(), topic: None };
+			let payment: MultiAsset = (native_location, fee + extra_amount).into();
 
-			// Lets buy_weight and make sure buy_weight does not return an error
-			let unused_assets = trader.buy_weight(bought, asset.into(), &ctx).expect("Expected Ok");
-			// Check whether a correct amount of unused assets is returned
-			assert_ok!(
-				unused_assets.ensure_contains(&(asset_multilocation, asset_amount_extra).into())
-			);
+			// init trader and buy weight.
+			let mut trader = <XcmConfig as xcm_executor::Config>::Trader::new();
+			let unused_asset =
+				trader.buy_weight(weight, payment.into(), &ctx).expect("Expected Ok");
 
-			// Drop trader
+			// assert.
+			let unused_amount =
+				unused_asset.fungible.get(&native_location.into()).map_or(0, |a| *a);
+			assert_eq!(unused_amount, extra_amount);
+			assert_eq!(Balances::total_issuance(), total_issuance);
+
+			// prepare input to refund weight.
+			let refund_weight = Weight::from_parts(1_000_000_000, 0);
+			let refund = WeightToFee::weight_to_fee(&refund_weight);
+
+			// refund.
+			let actual_refund = trader.refund_weight(refund_weight, &ctx).unwrap();
+			assert_eq!(actual_refund, (native_location, refund).into());
+
+			// assert.
+			assert_eq!(Balances::balance(&staking_pot), initial_balance);
+			// only after `trader` is dropped we expect the fee to be resolved into the treasury
+			// account.
 			drop(trader);
-
-			// Make sure author(Alice) has received the amount
-			assert_eq!(
-				Assets::balance(local_asset_id, AccountId::from(ALICE)),
-				minimum_asset_balance + asset_amount_needed
-			);
-
-			// We also need to ensure the total supply increased
-			assert_eq!(
-				Assets::total_supply(local_asset_id),
-				minimum_asset_balance + asset_amount_needed
-			);
-		});
+			assert_eq!(Balances::balance(&staking_pot), initial_balance + fee - refund);
+			assert_eq!(Balances::total_issuance(), total_issuance + fee - refund);
+		})
 }
 
 #[test]
-fn test_asset_xcm_trader_with_refund() {
+fn test_buy_and_refund_weight_with_swap_local_asset_xcm_trader() {
 	ExtBuilder::<Runtime>::default()
 		.with_collators(vec![AccountId::from(ALICE)])
 		.with_session_keys(vec![(
@@ -157,247 +143,192 @@ fn test_asset_xcm_trader_with_refund() {
 		)])
 		.build()
 		.execute_with(|| {
-			// We need root origin to create a sufficient asset
-			// We set existential deposit to be identical to the one for Balances first
-			assert_ok!(Assets::force_create(
-				RuntimeHelper::root_origin(),
-				1.into(),
-				AccountId::from(ALICE).into(),
-				true,
-				ExistentialDeposit::get()
+			let bob: AccountId = SOME_ASSET_ADMIN.into();
+			let staking_pot = CollatorSelection::account_id();
+			let asset_1: u32 = 1;
+			let native_location = WestendLocation::get();
+			let asset_1_location =
+				AssetIdForTrustBackedAssetsConvert::convert_back(&asset_1).unwrap();
+			// bob's initial balance for native and `asset1` assets.
+			let initial_balance = 200 * UNITS;
+			// liquidity for both arms of (native, asset1) pool.
+			let pool_liquidity = 100 * UNITS;
+
+			// init asset, balances and pool.
+			assert_ok!(<Assets as Create<_>>::create(asset_1, bob.clone(), true, 10));
+
+			assert_ok!(Assets::mint_into(asset_1, &bob, initial_balance));
+			assert_ok!(Balances::mint_into(&bob, initial_balance));
+			assert_ok!(Balances::mint_into(&staking_pot, initial_balance));
+
+			assert_ok!(AssetConversion::create_pool(
+				RuntimeHelper::origin_of(bob.clone()),
+				Box::new(native_location),
+				Box::new(asset_1_location)
 			));
 
-			// We first mint enough asset for the account to exist for assets
-			assert_ok!(Assets::mint(
-				RuntimeHelper::origin_of(AccountId::from(ALICE)),
-				1.into(),
-				AccountId::from(ALICE).into(),
-				ExistentialDeposit::get()
+			assert_ok!(AssetConversion::add_liquidity(
+				RuntimeHelper::origin_of(bob.clone()),
+				Box::new(native_location),
+				Box::new(asset_1_location),
+				pool_liquidity,
+				pool_liquidity,
+				1,
+				1,
+				bob,
 			));
 
-			let mut trader = <XcmConfig as xcm_executor::Config>::Trader::new();
+			// keep initial total issuance to assert later.
+			let asset_total_issuance = Assets::total_issuance(asset_1);
+			let native_total_issuance = Balances::total_issuance();
+
+			// prepare input to buy weight.
+			let weight = Weight::from_parts(4_000_000_000, 0);
+			let fee = WeightToFee::weight_to_fee(&weight);
+			let asset_fee =
+				AssetConversion::get_amount_in(&fee, &pool_liquidity, &pool_liquidity).unwrap();
+			let extra_amount = 100;
 			let ctx = XcmContext { origin: None, message_id: XcmHash::default(), topic: None };
+			let payment: MultiAsset = (asset_1_location, asset_fee + extra_amount).into();
 
-			// Set Alice as block author, who will receive fees
-			RuntimeHelper::run_to_block(2, AccountId::from(ALICE));
+			// init trader and buy weight.
+			let mut trader = <XcmConfig as xcm_executor::Config>::Trader::new();
+			let unused_asset =
+				trader.buy_weight(weight, payment.into(), &ctx).expect("Expected Ok");
 
-			// We are going to buy 4e9 weight
-			let bought = Weight::from_parts(4_000_000_000u64, 0);
-			let asset_multilocation = AssetIdForTrustBackedAssetsConvert::convert_back(&1).unwrap();
+			// assert.
+			let unused_amount =
+				unused_asset.fungible.get(&asset_1_location.into()).map_or(0, |a| *a);
+			assert_eq!(unused_amount, extra_amount);
+			assert_eq!(Assets::total_issuance(asset_1), asset_total_issuance + asset_fee);
 
-			// lets calculate amount needed
-			let amount_bought = WeightToFee::weight_to_fee(&bought);
+			// prepare input to refund weight.
+			let refund_weight = Weight::from_parts(1_000_000_000, 0);
+			let refund = WeightToFee::weight_to_fee(&refund_weight);
+			let (reserve1, reserve2) =
+				AssetConversion::get_reserves(native_location, asset_1_location).unwrap();
+			let asset_refund =
+				AssetConversion::get_amount_out(&refund, &reserve1, &reserve2).unwrap();
 
-			let asset: MultiAsset = (asset_multilocation, amount_bought).into();
-
-			// Make sure buy_weight does not return an error
-			assert_ok!(trader.buy_weight(bought, asset.clone().into(), &ctx));
-
-			// Make sure again buy_weight does return an error
-			// This assert relies on the fact, that we use `TakeFirstAssetTrader` in `WeightTrader`
-			// tuple chain, which cannot be called twice
-			assert_noop!(trader.buy_weight(bought, asset.into(), &ctx), XcmError::TooExpensive);
-
-			// We actually use half of the weight
-			let weight_used = bought / 2;
-
-			// Make sure refurnd works.
-			let amount_refunded = WeightToFee::weight_to_fee(&(bought - weight_used));
+			// refund.
+			let actual_refund = trader.refund_weight(refund_weight, &ctx).unwrap();
+			assert_eq!(actual_refund, (asset_1_location, asset_refund).into());
 
+			// assert.
+			assert_eq!(Balances::balance(&staking_pot), initial_balance);
+			// only after `trader` is dropped we expect the fee to be resolved into the treasury
+			// account.
+			drop(trader);
+			assert_eq!(Balances::balance(&staking_pot), initial_balance + fee - refund);
 			assert_eq!(
-				trader.refund_weight(bought - weight_used, &ctx),
-				Some((asset_multilocation, amount_refunded).into())
+				Assets::total_issuance(asset_1),
+				asset_total_issuance + asset_fee - asset_refund
 			);
+			assert_eq!(Balances::total_issuance(), native_total_issuance);
+		})
+}
 
-			// Drop trader
-			drop(trader);
+#[test]
+fn test_buy_and_refund_weight_with_swap_foreign_asset_xcm_trader() {
+	ExtBuilder::<Runtime>::default()
+		.with_collators(vec![AccountId::from(ALICE)])
+		.with_session_keys(vec![(
+			AccountId::from(ALICE),
+			AccountId::from(ALICE),
+			SessionKeys { aura: AuraId::from(sp_core::sr25519::Public::from_raw(ALICE)) },
+		)])
+		.build()
+		.execute_with(|| {
+			let bob: AccountId = SOME_ASSET_ADMIN.into();
+			let staking_pot = CollatorSelection::account_id();
+			let native_location = WestendLocation::get();
+			let foreign_location =
+				MultiLocation { parents: 1, interior: X2(Parachain(1234), GeneralIndex(12345)) };
+			// bob's initial balance for native and `asset1` assets.
+			let initial_balance = 200 * UNITS;
+			// liquidity for both arms of (native, asset1) pool.
+			let pool_liquidity = 100 * UNITS;
 
-			// We only should have paid for half of the bought weight
-			let fees_paid = WeightToFee::weight_to_fee(&weight_used);
+			// init asset, balances and pool.
+			assert_ok!(<ForeignAssets as Create<_>>::create(
+				foreign_location,
+				bob.clone(),
+				true,
+				10
+			));
 
+			assert_ok!(ForeignAssets::mint_into(foreign_location, &bob, initial_balance));
+			assert_ok!(Balances::mint_into(&bob, initial_balance));
+			assert_ok!(Balances::mint_into(&staking_pot, initial_balance));
+
+			assert_ok!(AssetConversion::create_pool(
+				RuntimeHelper::origin_of(bob.clone()),
+				Box::new(native_location),
+				Box::new(foreign_location)
+			));
+
+			assert_ok!(AssetConversion::add_liquidity(
+				RuntimeHelper::origin_of(bob.clone()),
+				Box::new(native_location),
+				Box::new(foreign_location),
+				pool_liquidity,
+				pool_liquidity,
+				1,
+				1,
+				bob,
+			));
+
+			// keep initial total issuance to assert later.
+			let asset_total_issuance = ForeignAssets::total_issuance(foreign_location);
+			let native_total_issuance = Balances::total_issuance();
+
+			// prepare input to buy weight.
+			let weight = Weight::from_parts(4_000_000_000, 0);
+			let fee = WeightToFee::weight_to_fee(&weight);
+			let asset_fee =
+				AssetConversion::get_amount_in(&fee, &pool_liquidity, &pool_liquidity).unwrap();
+			let extra_amount = 100;
+			let ctx = XcmContext { origin: None, message_id: XcmHash::default(), topic: None };
+			let payment: MultiAsset = (foreign_location, asset_fee + extra_amount).into();
+
+			// init trader and buy weight.
+			let mut trader = <XcmConfig as xcm_executor::Config>::Trader::new();
+			let unused_asset =
+				trader.buy_weight(weight, payment.into(), &ctx).expect("Expected Ok");
+
+			// assert.
+			let unused_amount =
+				unused_asset.fungible.get(&foreign_location.into()).map_or(0, |a| *a);
+			assert_eq!(unused_amount, extra_amount);
 			assert_eq!(
-				Assets::balance(1, AccountId::from(ALICE)),
-				ExistentialDeposit::get() + fees_paid
+				ForeignAssets::total_issuance(foreign_location),
+				asset_total_issuance + asset_fee
 			);
 
-			// We also need to ensure the total supply increased
-			assert_eq!(Assets::total_supply(1), ExistentialDeposit::get() + fees_paid);
-		});
-}
+			// prepare input to refund weight.
+			let refund_weight = Weight::from_parts(1_000_000_000, 0);
+			let refund = WeightToFee::weight_to_fee(&refund_weight);
+			let (reserve1, reserve2) =
+				AssetConversion::get_reserves(native_location, foreign_location).unwrap();
+			let asset_refund =
+				AssetConversion::get_amount_out(&refund, &reserve1, &reserve2).unwrap();
 
-#[test]
-fn test_asset_xcm_trader_refund_not_possible_since_amount_less_than_ed() {
-	ExtBuilder::<Runtime>::default()
-		.with_collators(vec![AccountId::from(ALICE)])
-		.with_session_keys(vec![(
-			AccountId::from(ALICE),
-			AccountId::from(ALICE),
-			SessionKeys { aura: AuraId::from(sp_core::sr25519::Public::from_raw(ALICE)) },
-		)])
-		.build()
-		.execute_with(|| {
-			// We need root origin to create a sufficient asset
-			// We set existential deposit to be identical to the one for Balances first
-			assert_ok!(Assets::force_create(
-				RuntimeHelper::root_origin(),
-				1.into(),
-				AccountId::from(ALICE).into(),
-				true,
-				ExistentialDeposit::get()
-			));
+			// refund.
+			let actual_refund = trader.refund_weight(refund_weight, &ctx).unwrap();
+			assert_eq!(actual_refund, (foreign_location, asset_refund).into());
 
-			let mut trader = <XcmConfig as xcm_executor::Config>::Trader::new();
-			let ctx = XcmContext { origin: None, message_id: XcmHash::default(), topic: None };
-
-			// Set Alice as block author, who will receive fees
-			RuntimeHelper::run_to_block(2, AccountId::from(ALICE));
-
-			// We are going to buy small amount
-			let bought = Weight::from_parts(500_000_000u64, 0);
-
-			let asset_multilocation = AssetIdForTrustBackedAssetsConvert::convert_back(&1).unwrap();
-
-			let amount_bought = WeightToFee::weight_to_fee(&bought);
-
-			assert!(
-				amount_bought < ExistentialDeposit::get(),
-				"we are testing what happens when the amount does not exceed ED"
-			);
-
-			let asset: MultiAsset = (asset_multilocation, amount_bought).into();
-
-			// Buy weight should return an error
-			assert_noop!(trader.buy_weight(bought, asset.into(), &ctx), XcmError::TooExpensive);
-
-			// not credited since the ED is higher than this value
-			assert_eq!(Assets::balance(1, AccountId::from(ALICE)), 0);
-
-			// We also need to ensure the total supply did not increase
-			assert_eq!(Assets::total_supply(1), 0);
-		});
-}
-
-#[test]
-fn test_that_buying_ed_refund_does_not_refund() {
-	ExtBuilder::<Runtime>::default()
-		.with_collators(vec![AccountId::from(ALICE)])
-		.with_session_keys(vec![(
-			AccountId::from(ALICE),
-			AccountId::from(ALICE),
-			SessionKeys { aura: AuraId::from(sp_core::sr25519::Public::from_raw(ALICE)) },
-		)])
-		.build()
-		.execute_with(|| {
-			// We need root origin to create a sufficient asset
-			// We set existential deposit to be identical to the one for Balances first
-			assert_ok!(Assets::force_create(
-				RuntimeHelper::root_origin(),
-				1.into(),
-				AccountId::from(ALICE).into(),
-				true,
-				ExistentialDeposit::get()
-			));
-
-			let mut trader = <XcmConfig as xcm_executor::Config>::Trader::new();
-			let ctx = XcmContext { origin: None, message_id: XcmHash::default(), topic: None };
-
-			// Set Alice as block author, who will receive fees
-			RuntimeHelper::run_to_block(2, AccountId::from(ALICE));
-
-			let bought = Weight::from_parts(500_000_000u64, 0);
-
-			let asset_multilocation = AssetIdForTrustBackedAssetsConvert::convert_back(&1).unwrap();
-
-			let amount_bought = WeightToFee::weight_to_fee(&bought);
-
-			assert!(
-				amount_bought < ExistentialDeposit::get(),
-				"we are testing what happens when the amount does not exceed ED"
-			);
-
-			// We know we will have to buy at least ED, so lets make sure first it will
-			// fail with a payment of less than ED
-			let asset: MultiAsset = (asset_multilocation, amount_bought).into();
-			assert_noop!(trader.buy_weight(bought, asset.into(), &ctx), XcmError::TooExpensive);
-
-			// Now lets buy ED at least
-			let asset: MultiAsset = (asset_multilocation, ExistentialDeposit::get()).into();
-
-			// Buy weight should work
-			assert_ok!(trader.buy_weight(bought, asset.into(), &ctx));
-
-			// Should return None. We have a specific check making sure we dont go below ED for
-			// drop payment
-			assert_eq!(trader.refund_weight(bought, &ctx), None);
-
-			// Drop trader
+			// assert.
+			assert_eq!(Balances::balance(&staking_pot), initial_balance);
+			// only after `trader` is dropped we expect the fee to be resolved into the treasury
+			// account.
 			drop(trader);
-
-			// Make sure author(Alice) has received the amount
-			assert_eq!(Assets::balance(1, AccountId::from(ALICE)), ExistentialDeposit::get());
-
-			// We also need to ensure the total supply increased
-			assert_eq!(Assets::total_supply(1), ExistentialDeposit::get());
-		});
-}
-
-#[test]
-fn test_asset_xcm_trader_not_possible_for_non_sufficient_assets() {
-	ExtBuilder::<Runtime>::default()
-		.with_collators(vec![AccountId::from(ALICE)])
-		.with_session_keys(vec![(
-			AccountId::from(ALICE),
-			AccountId::from(ALICE),
-			SessionKeys { aura: AuraId::from(sp_core::sr25519::Public::from_raw(ALICE)) },
-		)])
-		.build()
-		.execute_with(|| {
-			// Create a non-sufficient asset with specific existential deposit
-			let minimum_asset_balance = 1_000_000_u128;
-			assert_ok!(Assets::force_create(
-				RuntimeHelper::root_origin(),
-				1.into(),
-				AccountId::from(ALICE).into(),
-				false,
-				minimum_asset_balance
-			));
-
-			// We first mint enough asset for the account to exist for assets
-			assert_ok!(Assets::mint(
-				RuntimeHelper::origin_of(AccountId::from(ALICE)),
-				1.into(),
-				AccountId::from(ALICE).into(),
-				minimum_asset_balance
-			));
-
-			let mut trader = <XcmConfig as xcm_executor::Config>::Trader::new();
-			let ctx = XcmContext { origin: None, message_id: XcmHash::default(), topic: None };
-
-			// Set Alice as block author, who will receive fees
-			RuntimeHelper::run_to_block(2, AccountId::from(ALICE));
-
-			// We are going to buy 4e9 weight
-			let bought = Weight::from_parts(4_000_000_000u64, 0);
-
-			// lets calculate amount needed
-			let asset_amount_needed = WeightToFee::weight_to_fee(&bought);
-
-			let asset_multilocation = AssetIdForTrustBackedAssetsConvert::convert_back(&1).unwrap();
-
-			let asset: MultiAsset = (asset_multilocation, asset_amount_needed).into();
-
-			// Make sure again buy_weight does return an error
-			assert_noop!(trader.buy_weight(bought, asset.into(), &ctx), XcmError::TooExpensive);
-
-			// Drop trader
-			drop(trader);
-
-			// Make sure author(Alice) has NOT received the amount
-			assert_eq!(Assets::balance(1, AccountId::from(ALICE)), minimum_asset_balance);
-
-			// We also need to ensure the total supply NOT increased
-			assert_eq!(Assets::total_supply(1), minimum_asset_balance);
-		});
+			assert_eq!(Balances::balance(&staking_pot), initial_balance + fee - refund);
+			assert_eq!(
+				ForeignAssets::total_issuance(foreign_location),
+				asset_total_issuance + asset_fee - asset_refund
+			);
+			assert_eq!(Balances::total_issuance(), native_total_issuance);
+		})
 }
 
 #[test]

cumulus/parachains/runtimes/assets/common/src/lib.rs

@@ -47,6 +47,16 @@ pub type TrustBackedAssetsConvertedConcreteId<TrustBackedAssetsPalletLocation, B
 /// AssetId used for identifying assets by MultiLocation.
 pub type MultiLocationForAssetId = MultiLocation;
 
+/// [`MatchedConvertedConcreteId`] converter dedicated for `TrustBackedAssets`
+pub type TrustBackedAssetsAsMultiLocation<TrustBackedAssetsPalletLocation, Balance> =
+	MatchedConvertedConcreteId<
+		MultiLocationForAssetId,
+		Balance,
+		StartsWith<TrustBackedAssetsPalletLocation>,
+		Identity,
+		JustTry,
+	>;
+
 /// [`MatchedConvertedConcreteId`] converter dedicated for storing `AssetId` as `MultiLocation`.
 pub type MultiLocationConvertedConcreteId<MultiLocationFilter, Balance> =
 	MatchedConvertedConcreteId<

cumulus/parachains/runtimes/assets/test-utils/Cargo.toml

@@ -16,6 +16,7 @@ codec = { package = "parity-scale-codec", version = "3.0.0", default-features =
 frame-support = { path = "../../../../../substrate/frame/support", default-features = false }
 frame-system = { path = "../../../../../substrate/frame/system", default-features = false }
 pallet-assets = { path = "../../../../../substrate/frame/assets", default-features = false }
+pallet-asset-conversion = { path = "../../../../../substrate/frame/asset-conversion", default-features = false }
 pallet-balances = { path = "../../../../../substrate/frame/balances", default-features = false }
 pallet-session = { path = "../../../../../substrate/frame/session", default-features = false }
 sp-consensus-aura = { path = "../../../../../substrate/primitives/consensus/aura", default-features = false }
@@ -64,6 +65,7 @@ std = [
 	"cumulus-test-relay-sproof-builder/std",
 	"frame-support/std",
 	"frame-system/std",
+	"pallet-asset-conversion/std",
 	"pallet-assets/std",
 	"pallet-balances/std",
 	"pallet-collator-selection/std",

cumulus/parachains/runtimes/assets/test-utils/src/test_cases_over_bridge.rs

@@ -30,6 +30,7 @@ use parachains_runtimes_test_utils::{
 	ValidatorIdOf, XcmReceivedFrom,
 };
 use sp_runtime::{traits::StaticLookup, Saturating};
+use sp_std::ops::Mul;
 use xcm::{latest::prelude::*, VersionedMultiAssets};
 use xcm_builder::{CreateMatcher, MatchXcm};
 use xcm_executor::{traits::ConvertLocation, XcmExecutor};
@@ -336,12 +337,13 @@ pub fn receive_reserve_asset_deposited_from_different_consensus_works<
 		+ pallet_collator_selection::Config
 		+ cumulus_pallet_parachain_system::Config
 		+ cumulus_pallet_xcmp_queue::Config
-		+ pallet_assets::Config<ForeignAssetsPalletInstance>,
+		+ pallet_assets::Config<ForeignAssetsPalletInstance>
+		+ pallet_asset_conversion::Config,
 	AllPalletsWithoutSystem:
 		OnInitialize<BlockNumberFor<Runtime>> + OnFinalize<BlockNumberFor<Runtime>>,
-	AccountIdOf<Runtime>: Into<[u8; 32]>,
+	AccountIdOf<Runtime>: Into<[u8; 32]> + From<[u8; 32]>,
 	ValidatorIdOf<Runtime>: From<AccountIdOf<Runtime>>,
-	BalanceOf<Runtime>: From<Balance>,
+	BalanceOf<Runtime>: From<Balance> + Into<Balance>,
 	XcmConfig: xcm_executor::Config,
 	LocationToAccountId: ConvertLocation<AccountIdOf<Runtime>>,
 	<Runtime as pallet_assets::Config<ForeignAssetsPalletInstance>>::AssetId:
@@ -354,6 +356,9 @@ pub fn receive_reserve_asset_deposited_from_different_consensus_works<
 		+ Into<AccountId>,
 	<<Runtime as frame_system::Config>::Lookup as StaticLookup>::Source:
 		From<<Runtime as frame_system::Config>::AccountId>,
+	<Runtime as pallet_asset_conversion::Config>::AssetKind:
+		From<MultiLocation> + Into<MultiLocation>,
+	<Runtime as pallet_asset_conversion::Config>::Balance: From<Balance>,
 	ForeignAssetsPalletInstance: 'static,
 {
 	ExtBuilder::<Runtime>::default()
@@ -400,6 +405,43 @@ pub fn receive_reserve_asset_deposited_from_different_consensus_works<
 				)
 			);
 
+			// setup a pool to pay fees with `foreign_asset_id_multilocation` tokens
+			let pool_owner: AccountIdOf<Runtime> = [1u8; 32].into();
+			let native_asset = MultiLocation::parent();
+			let pool_liquidity: u128 =
+				existential_deposit.into().max(foreign_asset_id_minimum_balance).mul(100_000);
+
+			let _ = <pallet_balances::Pallet<Runtime>>::deposit_creating(
+				&pool_owner,
+				(existential_deposit.into() + pool_liquidity).mul(2).into(),
+			);
+
+			assert_ok!(<pallet_assets::Pallet<Runtime, ForeignAssetsPalletInstance>>::mint(
+				RuntimeHelper::<Runtime, AllPalletsWithoutSystem>::origin_of(
+					sovereign_account_as_owner_of_foreign_asset
+				),
+				foreign_asset_id_multilocation.into(),
+				pool_owner.clone().into(),
+				(foreign_asset_id_minimum_balance + pool_liquidity).mul(2).into(),
+			));
+
+			assert_ok!(<pallet_asset_conversion::Pallet<Runtime>>::create_pool(
+				RuntimeHelper::<Runtime, AllPalletsWithoutSystem>::origin_of(pool_owner.clone()),
+				Box::new(native_asset.into()),
+				Box::new(foreign_asset_id_multilocation.into())
+			));
+
+			assert_ok!(<pallet_asset_conversion::Pallet<Runtime>>::add_liquidity(
+				RuntimeHelper::<Runtime, AllPalletsWithoutSystem>::origin_of(pool_owner.clone()),
+				Box::new(native_asset.into()),
+				Box::new(foreign_asset_id_multilocation.into()),
+				pool_liquidity.into(),
+				pool_liquidity.into(),
+				1.into(),
+				1.into(),
+				pool_owner,
+			));
+
 			// Balances before
 			assert_eq!(
 				<pallet_balances::Pallet<Runtime>>::free_balance(&target_account),
@@ -485,14 +527,12 @@ pub fn receive_reserve_asset_deposited_from_different_consensus_works<
 			);
 			assert_ok!(outcome.ensure_complete());
 
-			// author actual balance after (received fees from Trader for ForeignAssets)
-			let author_received_fees =
-				<pallet_assets::Pallet<Runtime, ForeignAssetsPalletInstance>>::balance(
-					foreign_asset_id_multilocation.into(),
-					&block_author_account,
+			// Balances after
+			// staking pot receives xcm fees in dot
+			assert!(
+				<pallet_balances::Pallet<Runtime>>::free_balance(&staking_pot) !=
+					existential_deposit
 			);
-
-			// Balances after (untouched)
 			assert_eq!(
 				<pallet_balances::Pallet<Runtime>>::free_balance(&target_account),
 				existential_deposit.clone()
@@ -501,25 +541,13 @@ pub fn receive_reserve_asset_deposited_from_different_consensus_works<
 				<pallet_balances::Pallet<Runtime>>::free_balance(&block_author_account),
 				0.into()
 			);
-			assert_eq!(
-				<pallet_balances::Pallet<Runtime>>::free_balance(&staking_pot),
-				existential_deposit.clone()
-			);
 
 			// ForeignAssets balances after
-			assert_eq!(
+			assert!(
 				<pallet_assets::Pallet<Runtime, ForeignAssetsPalletInstance>>::balance(
 					foreign_asset_id_multilocation.into(),
 					&target_account
-				),
-				(transfered_foreign_asset_id_amount - author_received_fees.into()).into()
-			);
-			assert_eq!(
-				<pallet_assets::Pallet<Runtime, ForeignAssetsPalletInstance>>::balance(
-					foreign_asset_id_multilocation.into(),
-					&block_author_account
-				),
-				author_received_fees
+				) > 0.into()
 			);
 			assert_eq!(
 				<pallet_assets::Pallet<Runtime, ForeignAssetsPalletInstance>>::balance(
@@ -528,6 +556,13 @@ pub fn receive_reserve_asset_deposited_from_different_consensus_works<
 				),
 				0.into()
 			);
+			assert_eq!(
+				<pallet_assets::Pallet<Runtime, ForeignAssetsPalletInstance>>::balance(
+					foreign_asset_id_multilocation.into(),
+					&block_author_account
+				),
+				0.into()
+			);
 		})
 }
 

cumulus/primitives/utility/Cargo.toml

@@ -18,6 +18,7 @@ frame-support = { path = "../../../substrate/frame/support", default-features =
 sp-io = { path = "../../../substrate/primitives/io", default-features = false }
 sp-runtime = { path = "../../../substrate/primitives/runtime", default-features = false }
 sp-std = { path = "../../../substrate/primitives/std", default-features = false }
+pallet-asset-conversion = { path = "../../../substrate/frame/asset-conversion", default-features = false }
 
 # Polkadot
 polkadot-runtime-common = { path = "../../../polkadot/runtime/common", default-features = false }
@@ -37,6 +38,7 @@ std = [
 	"cumulus-primitives-core/std",
 	"frame-support/std",
 	"log/std",
+	"pallet-asset-conversion/std",
 	"pallet-xcm-benchmarks/std",
 	"polkadot-runtime-common/std",
 	"polkadot-runtime-parachains/std",
@@ -51,6 +53,7 @@ std = [
 runtime-benchmarks = [
 	"cumulus-primitives-core/runtime-benchmarks",
 	"frame-support/runtime-benchmarks",
+	"pallet-asset-conversion/runtime-benchmarks",
 	"pallet-xcm-benchmarks/runtime-benchmarks",
 	"polkadot-runtime-common/runtime-benchmarks",
 	"polkadot-runtime-parachains/runtime-benchmarks",

cumulus/primitives/utility/src/lib.rs

@@ -22,19 +22,24 @@
 use codec::Encode;
 use cumulus_primitives_core::{MessageSendError, UpwardMessageSender};
 use frame_support::{
-	traits::{
-		tokens::{fungibles, fungibles::Inspect},
-		Get,
-	},
-	weights::Weight,
+	defensive,
+	traits::{tokens::fungibles, Get, OnUnbalanced as OnUnbalancedT},
+	weights::{Weight, WeightToFee as WeightToFeeT},
 };
+use pallet_asset_conversion::SwapCredit as SwapCreditT;
 use polkadot_runtime_common::xcm_sender::PriceForMessageDelivery;
-use sp_runtime::{traits::Saturating, SaturatedConversion};
+use sp_runtime::{
+	traits::{Saturating, Zero},
+	SaturatedConversion,
+};
 use sp_std::{marker::PhantomData, prelude::*};
 use xcm::{latest::prelude::*, WrapVersion};
 use xcm_builder::TakeRevenue;
 use xcm_executor::traits::{MatchesFungibles, TransactAsset, WeightTrader};
 
+#[cfg(test)]
+mod tests;
+
 /// Xcm router which recognises the `Parent` destination and handles it by sending the message into
 /// the given UMP `UpwardMessageSender` implementation. Thus this essentially adapts an
 /// `UpwardMessageSender` trait impl into a `SendXcm` trait impl.
@@ -286,23 +291,238 @@ impl<
 /// in such assetId for that amount of weight
 pub trait ChargeWeightInFungibles<AccountId, Assets: fungibles::Inspect<AccountId>> {
 	fn charge_weight_in_fungibles(
-		asset_id: <Assets as Inspect<AccountId>>::AssetId,
+		asset_id: <Assets as fungibles::Inspect<AccountId>>::AssetId,
 		weight: Weight,
-	) -> Result<<Assets as Inspect<AccountId>>::Balance, XcmError>;
+	) -> Result<<Assets as fungibles::Inspect<AccountId>>::Balance, XcmError>;
+}
+
+/// Provides an implementation of [`WeightTrader`] to charge for weight using the first asset
+/// specified in the `payment` argument.
+///
+/// The asset used to pay for the weight must differ from the `Target` asset and be exchangeable for
+/// the same `Target` asset through `SwapCredit`.
+///
+/// ### Parameters:
+/// - `Target`: the asset into which the user's payment will be exchanged using `SwapCredit`.
+/// - `SwapCredit`: mechanism used for the exchange of the user's payment asset into the `Target`.
+/// - `WeightToFee`: weight to the `Target` asset fee calculator.
+/// - `Fungibles`: registry of fungible assets.
+/// - `FungiblesAssetMatcher`: utility for mapping [`MultiAsset`] to `Fungibles::AssetId` and
+///   `Fungibles::Balance`.
+/// - `OnUnbalanced`: handler for the fee payment.
+/// - `AccountId`: the account identifier type.
+pub struct SwapFirstAssetTrader<
+	Target: Get<Fungibles::AssetId>,
+	SwapCredit: SwapCreditT<
+		AccountId,
+		Balance = Fungibles::Balance,
+		AssetKind = Fungibles::AssetId,
+		Credit = fungibles::Credit<AccountId, Fungibles>,
+	>,
+	WeightToFee: WeightToFeeT<Balance = Fungibles::Balance>,
+	Fungibles: fungibles::Balanced<AccountId>,
+	FungiblesAssetMatcher: MatchesFungibles<Fungibles::AssetId, Fungibles::Balance>,
+	OnUnbalanced: OnUnbalancedT<fungibles::Credit<AccountId, Fungibles>>,
+	AccountId,
+> where
+	Fungibles::Balance: Into<u128>,
+{
+	/// Accumulated fee paid for XCM execution.
+	total_fee: fungibles::Credit<AccountId, Fungibles>,
+	/// Last asset utilized by a client to settle a fee.
+	last_fee_asset: Option<AssetId>,
+	_phantom_data: PhantomData<(
+		Target,
+		SwapCredit,
+		WeightToFee,
+		Fungibles,
+		FungiblesAssetMatcher,
+		OnUnbalanced,
+		AccountId,
+	)>,
+}
+
+impl<
+		Target: Get<Fungibles::AssetId>,
+		SwapCredit: SwapCreditT<
+			AccountId,
+			Balance = Fungibles::Balance,
+			AssetKind = Fungibles::AssetId,
+			Credit = fungibles::Credit<AccountId, Fungibles>,
+		>,
+		WeightToFee: WeightToFeeT<Balance = Fungibles::Balance>,
+		Fungibles: fungibles::Balanced<AccountId>,
+		FungiblesAssetMatcher: MatchesFungibles<Fungibles::AssetId, Fungibles::Balance>,
+		OnUnbalanced: OnUnbalancedT<fungibles::Credit<AccountId, Fungibles>>,
+		AccountId,
+	> WeightTrader
+	for SwapFirstAssetTrader<
+		Target,
+		SwapCredit,
+		WeightToFee,
+		Fungibles,
+		FungiblesAssetMatcher,
+		OnUnbalanced,
+		AccountId,
+	> where
+	Fungibles::Balance: Into<u128>,
+{
+	fn new() -> Self {
+		Self {
+			total_fee: fungibles::Credit::<AccountId, Fungibles>::zero(Target::get()),
+			last_fee_asset: None,
+			_phantom_data: PhantomData,
+		}
+	}
+
+	fn buy_weight(
+		&mut self,
+		weight: Weight,
+		mut payment: xcm_executor::Assets,
+		_context: &XcmContext,
+	) -> Result<xcm_executor::Assets, XcmError> {
+		log::trace!(
+			target: "xcm::weight",
+			"SwapFirstAssetTrader::buy_weight weight: {:?}, payment: {:?}",
+			weight,
+			payment,
+		);
+		let first_asset: MultiAsset =
+			payment.fungible.pop_first().ok_or(XcmError::AssetNotFound)?.into();
+		let (fungibles_asset, balance) = FungiblesAssetMatcher::matches_fungibles(&first_asset)
+			.map_err(|_| XcmError::AssetNotFound)?;
+
+		let swap_asset = fungibles_asset.clone().into();
+		if Target::get().eq(&swap_asset) {
+			// current trader is not applicable.
+			return Err(XcmError::FeesNotMet)
+		}
+
+		let credit_in = Fungibles::issue(fungibles_asset, balance);
+		let fee = WeightToFee::weight_to_fee(&weight);
+
+		// swap the user's asset for the `Target` asset.
+		let (credit_out, credit_change) = SwapCredit::swap_tokens_for_exact_tokens(
+			vec![swap_asset, Target::get()],
+			credit_in,
+			fee,
+		)
+		.map_err(|(credit_in, _)| {
+			drop(credit_in);
+			XcmError::FeesNotMet
+		})?;
+
+		match self.total_fee.subsume(credit_out) {
+			Err(credit_out) => {
+				// error may occur if `total_fee.asset` differs from `credit_out.asset`, which does
+				// not apply in this context.
+				defensive!(
+					"`total_fee.asset` must be equal to `credit_out.asset`",
+					(self.total_fee.asset(), credit_out.asset())
+				);
+				return Err(XcmError::FeesNotMet)
+			},
+			_ => (),
+		};
+		self.last_fee_asset = Some(first_asset.id);
+
+		payment.fungible.insert(first_asset.id, credit_change.peek().into());
+		drop(credit_change);
+		Ok(payment)
+	}
+
+	fn refund_weight(&mut self, weight: Weight, _context: &XcmContext) -> Option<MultiAsset> {
+		log::trace!(
+			target: "xcm::weight",
+			"SwapFirstAssetTrader::refund_weight weight: {:?}, self.total_fee: {:?}",
+			weight,
+			self.total_fee,
+		);
+		if self.total_fee.peek().is_zero() {
+			// noting yet paid to refund.
+			return None
+		}
+		let mut refund_asset = if let Some(asset) = &self.last_fee_asset {
+			// create an initial zero refund in the asset used in the last `buy_weight`.
+			(*asset, Fungible(0)).into()
+		} else {
+			return None
+		};
+		let refund_amount = WeightToFee::weight_to_fee(&weight);
+		if refund_amount >= self.total_fee.peek() {
+			// not enough was paid to refund the `weight`.
+			return None
+		}
+
+		let refund_swap_asset = FungiblesAssetMatcher::matches_fungibles(&refund_asset)
+			.map(|(a, _)| a.into())
+			.ok()?;
+
+		let refund = self.total_fee.extract(refund_amount);
+		let refund = match SwapCredit::swap_exact_tokens_for_tokens(
+			vec![Target::get(), refund_swap_asset],
+			refund,
+			None,
+		) {
+			Ok(refund_in_target) => refund_in_target,
+			Err((refund, _)) => {
+				// return an attempted refund back to the `total_fee`.
+				let _ = self.total_fee.subsume(refund).map_err(|refund| {
+					// error may occur if `total_fee.asset` differs from `refund.asset`, which does
+					// not apply in this context.
+					defensive!(
+						"`total_fee.asset` must be equal to `refund.asset`",
+						(self.total_fee.asset(), refund.asset())
+					);
+				});
+				return None
+			},
+		};
+
+		refund_asset.fun = refund.peek().into().into();
+		drop(refund);
+		Some(refund_asset)
+	}
+}
+
+impl<
+		Target: Get<Fungibles::AssetId>,
+		SwapCredit: SwapCreditT<
+			AccountId,
+			Balance = Fungibles::Balance,
+			AssetKind = Fungibles::AssetId,
+			Credit = fungibles::Credit<AccountId, Fungibles>,
+		>,
+		WeightToFee: WeightToFeeT<Balance = Fungibles::Balance>,
+		Fungibles: fungibles::Balanced<AccountId>,
+		FungiblesAssetMatcher: MatchesFungibles<Fungibles::AssetId, Fungibles::Balance>,
+		OnUnbalanced: OnUnbalancedT<fungibles::Credit<AccountId, Fungibles>>,
+		AccountId,
+	> Drop
+	for SwapFirstAssetTrader<
+		Target,
+		SwapCredit,
+		WeightToFee,
+		Fungibles,
+		FungiblesAssetMatcher,
+		OnUnbalanced,
+		AccountId,
+	> where
+	Fungibles::Balance: Into<u128>,
+{
+	fn drop(&mut self) {
+		if self.total_fee.peek().is_zero() {
+			return
+		}
+		let total_fee = self.total_fee.extract(self.total_fee.peek());
+		OnUnbalanced::on_unbalanced(total_fee);
+	}
 }
 
 #[cfg(test)]
-mod tests {
+mod test_xcm_router {
 	use super::*;
 	use cumulus_primitives_core::UpwardMessage;
-	use frame_support::{
-		assert_ok,
-		traits::tokens::{
-			DepositConsequence, Fortitude, Preservation, Provenance, WithdrawConsequence,
-		},
-	};
-	use sp_runtime::DispatchError;
-	use xcm_executor::{traits::Error, Assets};
 
 	/// Validates [`validate`] for required Some(destination) and Some(message)
 	struct OkFixedXcmHashWithAssertingRequiredInputsSender;
@@ -398,6 +618,18 @@ mod tests {
 			)>(dest.into(), message)
 		);
 	}
+}
+#[cfg(test)]
+mod test_trader {
+	use super::*;
+	use frame_support::{
+		assert_ok,
+		traits::tokens::{
+			DepositConsequence, Fortitude, Preservation, Provenance, WithdrawConsequence,
+		},
+	};
+	use sp_runtime::DispatchError;
+	use xcm_executor::{traits::Error, Assets};
 
 	#[test]
 	fn take_first_asset_trader_buy_weight_called_twice_throws_error() {
@@ -491,9 +723,9 @@ mod tests {
 		struct FeeChargerAssetsHandleRefund;
 		impl ChargeWeightInFungibles<TestAccountId, TestAssets> for FeeChargerAssetsHandleRefund {
 			fn charge_weight_in_fungibles(
-				_: <TestAssets as Inspect<TestAccountId>>::AssetId,
+				_: <TestAssets as fungibles::Inspect<TestAccountId>>::AssetId,
 				_: Weight,
-			) -> Result<<TestAssets as Inspect<TestAccountId>>::Balance, XcmError> {
+			) -> Result<<TestAssets as fungibles::Inspect<TestAccountId>>::Balance, XcmError> {
 				Ok(AMOUNT)
 			}
 		}

cumulus/primitives/utility/src/tests/mod.rs

@@ -0,0 +1,17 @@
+// Copyright (C) Parity Technologies (UK) Ltd.
+// This file is part of Cumulus.
+
+// Substrate 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.
+
+// Substrate 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 Cumulus.  If not, see <http://www.gnu.org/licenses/>.
+
+mod swap_first;

cumulus/primitives/utility/src/tests/swap_first.rs

@@ -0,0 +1,551 @@
+// Copyright (C) Parity Technologies (UK) Ltd.
+// This file is part of Cumulus.
+
+// Substrate 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.
+
+// Substrate 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 Cumulus.  If not, see <http://www.gnu.org/licenses/>.
+
+use crate::*;
+use frame_support::{parameter_types, traits::fungibles::Inspect};
+use mock::{setup_pool, AccountId, AssetId, Balance, Fungibles};
+use xcm::latest::AssetId as XcmAssetId;
+use xcm_executor::Assets as HoldingAsset;
+
+fn create_holding_asset(asset_id: AssetId, amount: Balance) -> HoldingAsset {
+	create_asset(asset_id, amount).into()
+}
+
+fn create_asset(asset_id: AssetId, amount: Balance) -> MultiAsset {
+	MultiAsset { id: create_asset_id(asset_id), fun: Fungible(amount) }
+}
+
+fn create_asset_id(asset_id: AssetId) -> XcmAssetId {
+	Concrete(MultiLocation::new(0, X1(GeneralIndex(asset_id.into()))))
+}
+
+fn xcm_context() -> XcmContext {
+	XcmContext { origin: None, message_id: [0u8; 32], topic: None }
+}
+
+fn weight_worth_of(fee: Balance) -> Weight {
+	Weight::from_parts(fee.try_into().unwrap(), 0)
+}
+
+const TARGET_ASSET: AssetId = 1;
+const CLIENT_ASSET: AssetId = 2;
+const CLIENT_ASSET_2: AssetId = 3;
+
+parameter_types! {
+	pub const TargetAsset: AssetId = TARGET_ASSET;
+}
+
+pub type Trader = SwapFirstAssetTrader<
+	TargetAsset,
+	mock::Swap,
+	mock::WeightToFee,
+	mock::Fungibles,
+	mock::FungiblesMatcher,
+	(),
+	AccountId,
+>;
+
+#[test]
+fn holding_asset_swap_for_target() {
+	let client_asset_total = 15;
+	let fee = 5;
+
+	setup_pool(CLIENT_ASSET, 1000, TARGET_ASSET, 1000);
+
+	let holding_asset = create_holding_asset(CLIENT_ASSET, client_asset_total);
+	let holding_change = create_holding_asset(CLIENT_ASSET, client_asset_total - fee);
+
+	let target_total = Fungibles::total_issuance(TARGET_ASSET);
+	let client_total = Fungibles::total_issuance(CLIENT_ASSET);
+
+	let mut trader = Trader::new();
+	assert_eq!(
+		trader.buy_weight(weight_worth_of(fee), holding_asset, &xcm_context()).unwrap(),
+		holding_change
+	);
+
+	assert_eq!(trader.total_fee.peek(), fee);
+	assert_eq!(trader.last_fee_asset, Some(create_asset_id(CLIENT_ASSET)));
+
+	assert_eq!(Fungibles::total_issuance(TARGET_ASSET), target_total);
+	assert_eq!(Fungibles::total_issuance(CLIENT_ASSET), client_total + fee);
+}
+
+#[test]
+fn holding_asset_swap_for_target_twice() {
+	let client_asset_total = 20;
+	let fee1 = 5;
+	let fee2 = 6;
+
+	setup_pool(CLIENT_ASSET, 1000, TARGET_ASSET, 1000);
+
+	let holding_asset = create_holding_asset(CLIENT_ASSET, client_asset_total);
+	let holding_change1 = create_holding_asset(CLIENT_ASSET, client_asset_total - fee1);
+	let holding_change2 = create_holding_asset(CLIENT_ASSET, client_asset_total - fee1 - fee2);
+
+	let target_total = Fungibles::total_issuance(TARGET_ASSET);
+	let client_total = Fungibles::total_issuance(CLIENT_ASSET);
+
+	let mut trader = Trader::new();
+	assert_eq!(
+		trader.buy_weight(weight_worth_of(fee1), holding_asset, &xcm_context()).unwrap(),
+		holding_change1
+	);
+	assert_eq!(
+		trader
+			.buy_weight(weight_worth_of(fee2), holding_change1, &xcm_context())
+			.unwrap(),
+		holding_change2
+	);
+
+	assert_eq!(trader.total_fee.peek(), fee1 + fee2);
+	assert_eq!(trader.last_fee_asset, Some(create_asset_id(CLIENT_ASSET)));
+
+	assert_eq!(Fungibles::total_issuance(TARGET_ASSET), target_total);
+	assert_eq!(Fungibles::total_issuance(CLIENT_ASSET), client_total + fee1 + fee2);
+}
+
+#[test]
+fn buy_and_refund_twice_for_target() {
+	let client_asset_total = 15;
+	let fee = 5;
+	let refund1 = 4;
+	let refund2 = 2;
+
+	setup_pool(CLIENT_ASSET, 1000, TARGET_ASSET, 1000);
+	// create pool for refund swap.
+	setup_pool(TARGET_ASSET, 1000, CLIENT_ASSET, 1000);
+
+	let holding_asset = create_holding_asset(CLIENT_ASSET, client_asset_total);
+	let holding_change = create_holding_asset(CLIENT_ASSET, client_asset_total - fee);
+	let refund_asset = create_asset(CLIENT_ASSET, refund1);
+
+	let target_total = Fungibles::total_issuance(TARGET_ASSET);
+	let client_total = Fungibles::total_issuance(CLIENT_ASSET);
+
+	let mut trader = Trader::new();
+	assert_eq!(
+		trader.buy_weight(weight_worth_of(fee), holding_asset, &xcm_context()).unwrap(),
+		holding_change
+	);
+
+	assert_eq!(trader.total_fee.peek(), fee);
+	assert_eq!(trader.last_fee_asset, Some(create_asset_id(CLIENT_ASSET)));
+
+	assert_eq!(trader.refund_weight(weight_worth_of(refund1), &xcm_context()), Some(refund_asset));
+
+	assert_eq!(trader.total_fee.peek(), fee - refund1);
+	assert_eq!(trader.last_fee_asset, Some(create_asset_id(CLIENT_ASSET)));
+
+	assert_eq!(trader.refund_weight(weight_worth_of(refund2), &xcm_context()), None);
+
+	assert_eq!(trader.total_fee.peek(), fee - refund1);
+	assert_eq!(trader.last_fee_asset, Some(create_asset_id(CLIENT_ASSET)));
+
+	assert_eq!(Fungibles::total_issuance(TARGET_ASSET), target_total);
+	assert_eq!(Fungibles::total_issuance(CLIENT_ASSET), client_total + fee - refund1);
+}
+
+#[test]
+fn buy_with_various_assets_and_refund_for_target() {
+	let client_asset_total = 10;
+	let client_asset_2_total = 15;
+	let fee1 = 5;
+	let fee2 = 6;
+	let refund1 = 6;
+	let refund2 = 4;
+
+	setup_pool(CLIENT_ASSET, 1000, TARGET_ASSET, 1000);
+	setup_pool(CLIENT_ASSET_2, 1000, TARGET_ASSET, 1000);
+	// create pool for refund swap.
+	setup_pool(TARGET_ASSET, 1000, CLIENT_ASSET_2, 1000);
+
+	let holding_asset = create_holding_asset(CLIENT_ASSET, client_asset_total);
+	let holding_asset_2 = create_holding_asset(CLIENT_ASSET_2, client_asset_2_total);
+	let holding_change = create_holding_asset(CLIENT_ASSET, client_asset_total - fee1);
+	let holding_change_2 = create_holding_asset(CLIENT_ASSET_2, client_asset_2_total - fee2);
+	// both refunds in the latest buy asset (`CLIENT_ASSET_2`).
+	let refund_asset = create_asset(CLIENT_ASSET_2, refund1);
+	let refund_asset_2 = create_asset(CLIENT_ASSET_2, refund2);
+
+	let target_total = Fungibles::total_issuance(TARGET_ASSET);
+	let client_total = Fungibles::total_issuance(CLIENT_ASSET);
+	let client_total_2 = Fungibles::total_issuance(CLIENT_ASSET_2);
+
+	let mut trader = Trader::new();
+	// first purchase with `CLIENT_ASSET`.
+	assert_eq!(
+		trader.buy_weight(weight_worth_of(fee1), holding_asset, &xcm_context()).unwrap(),
+		holding_change
+	);
+
+	assert_eq!(trader.total_fee.peek(), fee1);
+	assert_eq!(trader.last_fee_asset, Some(create_asset_id(CLIENT_ASSET)));
+
+	// second purchase with `CLIENT_ASSET_2`.
+	assert_eq!(
+		trader
+			.buy_weight(weight_worth_of(fee2), holding_asset_2, &xcm_context())
+			.unwrap(),
+		holding_change_2
+	);
+
+	assert_eq!(trader.total_fee.peek(), fee1 + fee2);
+	assert_eq!(trader.last_fee_asset, Some(create_asset_id(CLIENT_ASSET_2)));
+
+	// first refund in the last asset used with `buy_weight`.
+	assert_eq!(trader.refund_weight(weight_worth_of(refund1), &xcm_context()), Some(refund_asset));
+
+	assert_eq!(trader.total_fee.peek(), fee1 + fee2 - refund1);
+	assert_eq!(trader.last_fee_asset, Some(create_asset_id(CLIENT_ASSET_2)));
+
+	// second refund in the last asset used with `buy_weight`.
+	assert_eq!(
+		trader.refund_weight(weight_worth_of(refund2), &xcm_context()),
+		Some(refund_asset_2)
+	);
+
+	assert_eq!(trader.total_fee.peek(), fee1 + fee2 - refund1 - refund2);
+	assert_eq!(trader.last_fee_asset, Some(create_asset_id(CLIENT_ASSET_2)));
+
+	assert_eq!(Fungibles::total_issuance(TARGET_ASSET), target_total);
+	assert_eq!(Fungibles::total_issuance(CLIENT_ASSET), client_total + fee1);
+	assert_eq!(
+		Fungibles::total_issuance(CLIENT_ASSET_2),
+		client_total_2 + fee2 - refund1 - refund2
+	);
+}
+
+#[test]
+fn not_enough_to_refund() {
+	let client_asset_total = 15;
+	let fee = 5;
+	let refund = 6;
+
+	setup_pool(CLIENT_ASSET, 1000, TARGET_ASSET, 1000);
+
+	let holding_asset = create_holding_asset(CLIENT_ASSET, client_asset_total);
+	let holding_change = create_holding_asset(CLIENT_ASSET, client_asset_total - fee);
+
+	let target_total = Fungibles::total_issuance(TARGET_ASSET);
+	let client_total = Fungibles::total_issuance(CLIENT_ASSET);
+
+	let mut trader = Trader::new();
+	assert_eq!(
+		trader.buy_weight(weight_worth_of(fee), holding_asset, &xcm_context()).unwrap(),
+		holding_change
+	);
+
+	assert_eq!(trader.total_fee.peek(), fee);
+	assert_eq!(trader.last_fee_asset, Some(create_asset_id(CLIENT_ASSET)));
+
+	assert_eq!(trader.refund_weight(weight_worth_of(refund), &xcm_context()), None);
+
+	assert_eq!(Fungibles::total_issuance(TARGET_ASSET), target_total);
+	assert_eq!(Fungibles::total_issuance(CLIENT_ASSET), client_total + fee);
+}
+
+#[test]
+fn not_exchangeable_to_refund() {
+	let client_asset_total = 15;
+	let fee = 5;
+	let refund = 1;
+
+	setup_pool(CLIENT_ASSET, 1000, TARGET_ASSET, 1000);
+
+	let holding_asset = create_holding_asset(CLIENT_ASSET, client_asset_total);
+	let holding_change = create_holding_asset(CLIENT_ASSET, client_asset_total - fee);
+
+	let target_total = Fungibles::total_issuance(TARGET_ASSET);
+	let client_total = Fungibles::total_issuance(CLIENT_ASSET);
+
+	let mut trader = Trader::new();
+	assert_eq!(
+		trader.buy_weight(weight_worth_of(fee), holding_asset, &xcm_context()).unwrap(),
+		holding_change
+	);
+
+	assert_eq!(trader.total_fee.peek(), fee);
+	assert_eq!(trader.last_fee_asset, Some(create_asset_id(CLIENT_ASSET)));
+
+	assert_eq!(trader.refund_weight(weight_worth_of(refund), &xcm_context()), None);
+
+	assert_eq!(Fungibles::total_issuance(TARGET_ASSET), target_total);
+	assert_eq!(Fungibles::total_issuance(CLIENT_ASSET), client_total + fee);
+}
+
+#[test]
+fn nothing_to_refund() {
+	let fee = 5;
+
+	let mut trader = Trader::new();
+	assert_eq!(trader.refund_weight(weight_worth_of(fee), &xcm_context()), None);
+}
+
+#[test]
+fn holding_asset_not_exchangeable_for_target() {
+	let holding_asset = create_holding_asset(CLIENT_ASSET, 10);
+
+	let target_total = Fungibles::total_issuance(TARGET_ASSET);
+	let client_total = Fungibles::total_issuance(CLIENT_ASSET);
+
+	let mut trader = Trader::new();
+	assert_eq!(
+		trader
+			.buy_weight(Weight::from_all(10), holding_asset, &xcm_context())
+			.unwrap_err(),
+		XcmError::FeesNotMet
+	);
+
+	assert_eq!(Fungibles::total_issuance(TARGET_ASSET), target_total);
+	assert_eq!(Fungibles::total_issuance(CLIENT_ASSET), client_total);
+}
+
+#[test]
+fn empty_holding_asset() {
+	let mut trader = Trader::new();
+	assert_eq!(
+		trader
+			.buy_weight(Weight::from_all(10), HoldingAsset::new(), &xcm_context())
+			.unwrap_err(),
+		XcmError::AssetNotFound
+	);
+}
+
+#[test]
+fn fails_to_match_holding_asset() {
+	let mut trader = Trader::new();
+	let holding_asset =
+		MultiAsset { id: Concrete(MultiLocation::new(1, X1(Parachain(1)))), fun: Fungible(10) };
+	assert_eq!(
+		trader
+			.buy_weight(Weight::from_all(10), holding_asset.into(), &xcm_context())
+			.unwrap_err(),
+		XcmError::AssetNotFound
+	);
+}
+
+#[test]
+fn holding_asset_equal_to_target_asset() {
+	let mut trader = Trader::new();
+	let holding_asset = create_holding_asset(TargetAsset::get(), 10);
+	assert_eq!(
+		trader
+			.buy_weight(Weight::from_all(10), holding_asset, &xcm_context())
+			.unwrap_err(),
+		XcmError::FeesNotMet
+	);
+}
+
+pub mod mock {
+	use crate::*;
+	use core::cell::RefCell;
+	use frame_support::{
+		ensure,
+		traits::{
+			fungibles::{Balanced, DecreaseIssuance, Dust, IncreaseIssuance, Inspect, Unbalanced},
+			tokens::{
+				DepositConsequence, Fortitude, Fortitude::Polite, Precision::Exact, Preservation,
+				Preservation::Preserve, Provenance, WithdrawConsequence,
+			},
+		},
+	};
+	use sp_runtime::{traits::One, DispatchError};
+	use std::collections::HashMap;
+	use xcm::latest::Junction;
+
+	pub type AccountId = u64;
+	pub type AssetId = u32;
+	pub type Balance = u128;
+	pub type Credit = fungibles::Credit<AccountId, Fungibles>;
+
+	thread_local! {
+	   pub static TOTAL_ISSUANCE: RefCell<HashMap<AssetId, Balance>> = RefCell::new(HashMap::new());
+	   pub static ACCOUNT: RefCell<HashMap<(AssetId, AccountId), Balance>> = RefCell::new(HashMap::new());
+	   pub static SWAP: RefCell<HashMap<(AssetId, AssetId), AccountId>> = RefCell::new(HashMap::new());
+	}
+
+	pub struct Swap {}
+	impl SwapCreditT<AccountId> for Swap {
+		type Balance = Balance;
+		type AssetKind = AssetId;
+		type Credit = Credit;
+		fn max_path_len() -> u32 {
+			2
+		}
+		fn swap_exact_tokens_for_tokens(
+			path: Vec<Self::AssetKind>,
+			credit_in: Self::Credit,
+			amount_out_min: Option<Self::Balance>,
+		) -> Result<Self::Credit, (Self::Credit, DispatchError)> {
+			ensure!(2 == path.len(), (credit_in, DispatchError::Unavailable));
+			ensure!(
+				credit_in.peek() >= amount_out_min.unwrap_or(Self::Balance::zero()),
+				(credit_in, DispatchError::Unavailable)
+			);
+			let swap_res = SWAP.with(|b| b.borrow().get(&(path[0], path[1])).map(|v| *v));
+			let pool_account = match swap_res {
+				Some(a) => a,
+				None => return Err((credit_in, DispatchError::Unavailable)),
+			};
+			let credit_out = match Fungibles::withdraw(
+				path[1],
+				&pool_account,
+				credit_in.peek(),
+				Exact,
+				Preserve,
+				Polite,
+			) {
+				Ok(c) => c,
+				Err(_) => return Err((credit_in, DispatchError::Unavailable)),
+			};
+			let _ = Fungibles::resolve(&pool_account, credit_in)
+				.map_err(|c| (c, DispatchError::Unavailable))?;
+			Ok(credit_out)
+		}
+		fn swap_tokens_for_exact_tokens(
+			path: Vec<Self::AssetKind>,
+			credit_in: Self::Credit,
+			amount_out: Self::Balance,
+		) -> Result<(Self::Credit, Self::Credit), (Self::Credit, DispatchError)> {
+			ensure!(2 == path.len(), (credit_in, DispatchError::Unavailable));
+			ensure!(credit_in.peek() >= amount_out, (credit_in, DispatchError::Unavailable));
+			let swap_res = SWAP.with(|b| b.borrow().get(&(path[0], path[1])).map(|v| *v));
+			let pool_account = match swap_res {
+				Some(a) => a,
+				None => return Err((credit_in, DispatchError::Unavailable)),
+			};
+			let credit_out = match Fungibles::withdraw(
+				path[1],
+				&pool_account,
+				amount_out,
+				Exact,
+				Preserve,
+				Polite,
+			) {
+				Ok(c) => c,
+				Err(_) => return Err((credit_in, DispatchError::Unavailable)),
+			};
+			let (credit_in, change) = credit_in.split(amount_out);
+			let _ = Fungibles::resolve(&pool_account, credit_in)
+				.map_err(|c| (c, DispatchError::Unavailable))?;
+			Ok((credit_out, change))
+		}
+	}
+
+	pub fn pool_account(asset1: AssetId, asset2: AssetId) -> AccountId {
+		(1000 + asset1 * 10 + asset2 * 100).into()
+	}
+
+	pub fn setup_pool(asset1: AssetId, liquidity1: Balance, asset2: AssetId, liquidity2: Balance) {
+		let account = pool_account(asset1, asset2);
+		SWAP.with(|b| b.borrow_mut().insert((asset1, asset2), account));
+		let debt1 = Fungibles::deposit(asset1, &account, liquidity1, Exact);
+		let debt2 = Fungibles::deposit(asset2, &account, liquidity2, Exact);
+		drop(debt1);
+		drop(debt2);
+	}
+
+	pub struct WeightToFee;
+	impl WeightToFeeT for WeightToFee {
+		type Balance = Balance;
+		fn weight_to_fee(weight: &Weight) -> Self::Balance {
+			(weight.ref_time() + weight.proof_size()).into()
+		}
+	}
+
+	pub struct Fungibles {}
+	impl Inspect<AccountId> for Fungibles {
+		type AssetId = AssetId;
+		type Balance = Balance;
+		fn total_issuance(asset: Self::AssetId) -> Self::Balance {
+			TOTAL_ISSUANCE.with(|b| b.borrow().get(&asset).map_or(Self::Balance::zero(), |b| *b))
+		}
+		fn minimum_balance(_: Self::AssetId) -> Self::Balance {
+			Self::Balance::one()
+		}
+		fn total_balance(asset: Self::AssetId, who: &AccountId) -> Self::Balance {
+			ACCOUNT.with(|b| b.borrow().get(&(asset, *who)).map_or(Self::Balance::zero(), |b| *b))
+		}
+		fn balance(asset: Self::AssetId, who: &AccountId) -> Self::Balance {
+			ACCOUNT.with(|b| b.borrow().get(&(asset, *who)).map_or(Self::Balance::zero(), |b| *b))
+		}
+		fn reducible_balance(
+			asset: Self::AssetId,
+			who: &AccountId,
+			_: Preservation,
+			_: Fortitude,
+		) -> Self::Balance {
+			ACCOUNT.with(|b| b.borrow().get(&(asset, *who)).map_or(Self::Balance::zero(), |b| *b))
+		}
+		fn can_deposit(
+			_: Self::AssetId,
+			_: &AccountId,
+			_: Self::Balance,
+			_: Provenance,
+		) -> DepositConsequence {
+			unimplemented!()
+		}
+		fn can_withdraw(
+			_: Self::AssetId,
+			_: &AccountId,
+			_: Self::Balance,
+		) -> WithdrawConsequence<Self::Balance> {
+			unimplemented!()
+		}
+		fn asset_exists(_: Self::AssetId) -> bool {
+			unimplemented!()
+		}
+	}
+
+	impl Unbalanced<AccountId> for Fungibles {
+		fn set_total_issuance(asset: Self::AssetId, amount: Self::Balance) {
+			TOTAL_ISSUANCE.with(|b| b.borrow_mut().insert(asset, amount));
+		}
+		fn handle_dust(_: Dust<AccountId, Self>) {
+			unimplemented!()
+		}
+		fn write_balance(
+			asset: Self::AssetId,
+			who: &AccountId,
+			amount: Self::Balance,
+		) -> Result<Option<Self::Balance>, DispatchError> {
+			let _ = ACCOUNT.with(|b| b.borrow_mut().insert((asset, *who), amount));
+			Ok(None)
+		}
+	}
+
+	impl Balanced<AccountId> for Fungibles {
+		type OnDropCredit = DecreaseIssuance<AccountId, Self>;
+		type OnDropDebt = IncreaseIssuance<AccountId, Self>;
+	}
+
+	pub struct FungiblesMatcher;
+	impl MatchesFungibles<AssetId, Balance> for FungiblesMatcher {
+		fn matches_fungibles(
+			a: &MultiAsset,
+		) -> core::result::Result<(AssetId, Balance), xcm_executor::traits::Error> {
+			match a {
+				MultiAsset {
+					fun: Fungible(amount),
+					id:
+						Concrete(MultiLocation { parents: 0, interior: X1(Junction::GeneralIndex(id)) }),
+				} => Ok(((*id).try_into().unwrap(), *amount)),
+				_ => Err(xcm_executor::traits::Error::AssetNotHandled),
+			}
+		}
+	}
+}

prdoc/pr_1845.prdoc

@@ -0,0 +1,16 @@
+# 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: "XCM WeightTrader: Swap Fee Asset for Native Asset"
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      Implements an XCM executor `WeightTrader`, facilitating fee payments in any asset that can be exchanged for a native asset.
+
+      A few constraints need to be observed:
+      - `buy_weight` and `refund` operations must be atomic, as another weight trader implementation might be attempted in case of failure.
+      - swap credit must be utilized since there isn’t an account to which an asset of some class can be deposited with a guarantee to meet the existential deposit requirement.
+
+crates:
+  - name: cumulus-primitives-utility