Migrate fee payment from Currency to fungible (#2292)

Part of https://github.com/paritytech/polkadot-sdk/issues/226 
Related https://github.com/paritytech/polkadot-sdk/issues/1833

- Deprecate `CurrencyAdapter` and introduce `FungibleAdapter`
- Deprecate `ToStakingPot` and replace usage with `ResolveTo`
- Required creating a new `StakingPotAccountId` struct that implements
`TypedGet` for the staking pot account ID
- Update parachain common utils `DealWithFees`, `ToAuthor` and
`AssetsToBlockAuthor` implementations to use `fungible`
- Update runtime XCM Weight Traders to use `ResolveTo` instead of
`ToStakingPot`
- Update runtime Transaction Payment pallets to use `FungibleAdapter`
instead of `CurrencyAdapter`
- [x] Blocked by https://github.com/paritytech/polkadot-sdk/pull/1296,
needs the `Unbalanced::decrease_balance` fix

substrate/frame/transaction-payment/src/mock.rs

@@ -25,7 +25,7 @@ use frame_support::{
 	derive_impl,
 	dispatch::DispatchClass,
 	parameter_types,
-	traits::{ConstU32, ConstU64, Imbalance, OnUnbalanced},
+	traits::{fungible, ConstU32, ConstU64, Imbalance, OnUnbalanced},
 	weights::{Weight, WeightToFee as WeightToFeeT},
 };
 use frame_system as system;
@@ -137,9 +137,13 @@ parameter_types! {
 }
 
 pub struct DealWithFees;
-impl OnUnbalanced<pallet_balances::NegativeImbalance<Runtime>> for DealWithFees {
+impl OnUnbalanced<fungible::Credit<<Runtime as frame_system::Config>::AccountId, Balances>>
+	for DealWithFees
+{
 	fn on_unbalanceds<B>(
-		mut fees_then_tips: impl Iterator<Item = pallet_balances::NegativeImbalance<Runtime>>,
+		mut fees_then_tips: impl Iterator<
+			Item = fungible::Credit<<Runtime as frame_system::Config>::AccountId, Balances>,
+		>,
 	) {
 		if let Some(fees) = fees_then_tips.next() {
 			FeeUnbalancedAmount::mutate(|a| *a += fees.peek());
@@ -152,7 +156,7 @@ impl OnUnbalanced<pallet_balances::NegativeImbalance<Runtime>> for DealWithFees
 
 impl Config for Runtime {
 	type RuntimeEvent = RuntimeEvent;
-	type OnChargeTransaction = CurrencyAdapter<Balances, DealWithFees>;
+	type OnChargeTransaction = FungibleAdapter<Balances, DealWithFees>;
 	type OperationalFeeMultiplier = OperationalFeeMultiplier;
 	type WeightToFee = WeightToFee;
 	type LengthToFee = TransactionByteFee;

substrate/frame/transaction-payment/src/payment.rs

@@ -25,7 +25,11 @@ use sp_runtime::{
 };
 
 use frame_support::{
-	traits::{Currency, ExistenceRequirement, Imbalance, OnUnbalanced, WithdrawReasons},
+	traits::{
+		fungible::{Balanced, Credit, Debt, Inspect},
+		tokens::Precision,
+		Currency, ExistenceRequirement, Imbalance, OnUnbalanced, WithdrawReasons,
+	},
 	unsigned::TransactionValidityError,
 };
 
@@ -66,18 +70,95 @@ pub trait OnChargeTransaction<T: Config> {
 	) -> Result<(), TransactionValidityError>;
 }
 
-/// Implements the transaction payment for a pallet implementing the `Currency`
-/// trait (eg. the pallet_balances) using an unbalance handler (implementing
-/// `OnUnbalanced`).
+/// Implements transaction payment for a pallet implementing the [`frame_support::traits::fungible`]
+/// trait (eg. pallet_balances) using an unbalance handler (implementing
+/// [`OnUnbalanced`]).
 ///
-/// The unbalance handler is given 2 unbalanceds in [`OnUnbalanced::on_unbalanceds`]: fee and
-/// then tip.
+/// The unbalance handler is given 2 unbalanceds in [`OnUnbalanced::on_unbalanceds`]: `fee` and
+/// then `tip`.
+pub struct FungibleAdapter<F, OU>(PhantomData<(F, OU)>);
+
+impl<T, F, OU> OnChargeTransaction<T> for FungibleAdapter<F, OU>
+where
+	T: Config,
+	F: Balanced<T::AccountId>,
+	OU: OnUnbalanced<Credit<T::AccountId, F>>,
+{
+	type LiquidityInfo = Option<Credit<T::AccountId, F>>;
+	type Balance = <F as Inspect<<T as frame_system::Config>::AccountId>>::Balance;
+
+	fn withdraw_fee(
+		who: &<T>::AccountId,
+		_call: &<T>::RuntimeCall,
+		_dispatch_info: &DispatchInfoOf<<T>::RuntimeCall>,
+		fee: Self::Balance,
+		_tip: Self::Balance,
+	) -> Result<Self::LiquidityInfo, TransactionValidityError> {
+		if fee.is_zero() {
+			return Ok(None)
+		}
+
+		match F::withdraw(
+			who,
+			fee,
+			Precision::Exact,
+			frame_support::traits::tokens::Preservation::Preserve,
+			frame_support::traits::tokens::Fortitude::Polite,
+		) {
+			Ok(imbalance) => Ok(Some(imbalance)),
+			Err(_) => Err(InvalidTransaction::Payment.into()),
+		}
+	}
+
+	fn correct_and_deposit_fee(
+		who: &<T>::AccountId,
+		_dispatch_info: &DispatchInfoOf<<T>::RuntimeCall>,
+		_post_info: &PostDispatchInfoOf<<T>::RuntimeCall>,
+		corrected_fee: Self::Balance,
+		tip: Self::Balance,
+		already_withdrawn: Self::LiquidityInfo,
+	) -> Result<(), TransactionValidityError> {
+		if let Some(paid) = already_withdrawn {
+			// Calculate how much refund we should return
+			let refund_amount = paid.peek().saturating_sub(corrected_fee);
+			// refund to the the account that paid the fees if it exists. otherwise, don't refind
+			// anything.
+			let refund_imbalance = if F::total_balance(who) > F::Balance::zero() {
+				F::deposit(who, refund_amount, Precision::BestEffort)
+					.unwrap_or_else(|_| Debt::<T::AccountId, F>::zero())
+			} else {
+				Debt::<T::AccountId, F>::zero()
+			};
+			// merge the imbalance caused by paying the fees and refunding parts of it again.
+			let adjusted_paid: Credit<T::AccountId, F> = paid
+				.offset(refund_imbalance)
+				.same()
+				.map_err(|_| TransactionValidityError::Invalid(InvalidTransaction::Payment))?;
+			// Call someone else to handle the imbalance (fee and tip separately)
+			let (tip, fee) = adjusted_paid.split(tip);
+			OU::on_unbalanceds(Some(fee).into_iter().chain(Some(tip)));
+		}
+
+		Ok(())
+	}
+}
+
+/// Implements the transaction payment for a pallet implementing the [`Currency`]
+/// trait (eg. the pallet_balances) using an unbalance handler (implementing
+/// [`OnUnbalanced`]).
+///
+/// The unbalance handler is given 2 unbalanceds in [`OnUnbalanced::on_unbalanceds`]: `fee` and
+/// then `tip`.
+#[deprecated(
+	note = "Please use the fungible trait and FungibleAdapter. This struct will be removed some time after March 2024."
+)]
 pub struct CurrencyAdapter<C, OU>(PhantomData<(C, OU)>);
 
 /// Default implementation for a Currency and an OnUnbalanced handler.
 ///
-/// The unbalance handler is given 2 unbalanceds in [`OnUnbalanced::on_unbalanceds`]: fee and
-/// then tip.
+/// The unbalance handler is given 2 unbalanceds in [`OnUnbalanced::on_unbalanceds`]: `fee` and
+/// then `tip`.
+#[allow(deprecated)]
 impl<T, C, OU> OnChargeTransaction<T> for CurrencyAdapter<C, OU>
 where
 	T: Config,