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>

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),
+			}
+		}
+	}
+}