frame_support/traits/tokens/fungibles/

regular.rs

// This file is part of Substrate.

// Copyright (C) Parity Technologies (UK) Ltd.
// SPDX-License-Identifier: Apache-2.0

// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// 	http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

//! `Inspect` and `Mutate` traits for working with regular balances.
//!
//! See the [`crate::traits::fungibles`] doc for more information about fungibles traits.

use core::marker::PhantomData;

use crate::{
	ensure,
	traits::{
		tokens::{
			misc::{
				Balance, DepositConsequence,
				Fortitude::{self, Force, Polite},
				Precision::{self, BestEffort, Exact},
				Preservation::{self, Expendable},
				Provenance::{self, Extant},
				WithdrawConsequence,
			},
			AssetId,
		},
		SameOrOther, TryDrop,
	},
};
use sp_arithmetic::traits::{CheckedAdd, CheckedSub, One, Zero};
use sp_runtime::{traits::Saturating, ArithmeticError, DispatchError, TokenError};

use super::{Credit, Debt, HandleImbalanceDrop, Imbalance};

/// Trait for providing balance-inspection access to a set of named fungible assets.
pub trait Inspect<AccountId>: Sized {
	/// Means of identifying one asset class from another.
	type AssetId: AssetId;

	/// Scalar type for representing balance of an account.
	type Balance: Balance;

	/// The total amount of issuance in the system.
	fn total_issuance(asset: Self::AssetId) -> Self::Balance;

	/// The total amount of issuance in the system excluding those which are controlled by the
	/// system.
	fn active_issuance(asset: Self::AssetId) -> Self::Balance {
		Self::total_issuance(asset)
	}

	/// The minimum balance any single account may have.
	fn minimum_balance(asset: Self::AssetId) -> Self::Balance;

	/// Get the total amount of funds whose ultimate beneficial ownership can be determined as
	/// `who`.
	///
	/// This may include funds which are wholly inaccessible to `who`, either temporarily or even
	/// indefinitely.
	///
	/// For the amount of the balance which is currently free to be removed from the account without
	/// error, use `reducible_balance`.
	///
	/// For the amount of the balance which may eventually be free to be removed from the account,
	/// use `balance()`.
	fn total_balance(asset: Self::AssetId, who: &AccountId) -> Self::Balance;

	/// Get the balance of `who` which does not include funds which are exclusively allocated to
	/// subsystems of the chain ("on hold" or "reserved").
	///
	/// In general this isn't especially useful outside of tests, and for practical purposes, you'll
	/// want to use `reducible_balance()`.
	fn balance(asset: Self::AssetId, who: &AccountId) -> Self::Balance;

	/// Get the maximum amount that `who` can withdraw/transfer successfully based on whether the
	/// account should be kept alive (`preservation`) or whether we are willing to force the
	/// transfer and potentially go below user-level restrictions on the minimum amount of the
	/// account.
	///
	/// Always less than `free_balance()`.
	fn reducible_balance(
		asset: Self::AssetId,
		who: &AccountId,
		preservation: Preservation,
		force: Fortitude,
	) -> Self::Balance;

	/// Returns `true` if the `asset` balance of `who` may be increased by `amount`.
	///
	/// - `asset`: The asset that should be deposited.
	/// - `who`: The account of which the balance should be increased by `amount`.
	/// - `amount`: How much should the balance be increased?
	/// - `mint`: Will `amount` be minted to deposit it into `account`?
	fn can_deposit(
		asset: Self::AssetId,
		who: &AccountId,
		amount: Self::Balance,
		provenance: Provenance,
	) -> DepositConsequence;

	/// Returns `Failed` if the `asset` balance of `who` may not be decreased by `amount`, otherwise
	/// the consequence.
	fn can_withdraw(
		asset: Self::AssetId,
		who: &AccountId,
		amount: Self::Balance,
	) -> WithdrawConsequence<Self::Balance>;

	/// Returns `true` if an `asset` exists.
	fn asset_exists(asset: Self::AssetId) -> bool;

	/// Returns `true` if the `asset` exists and is sufficient, `false` otherwise.
	///
	/// Defaults to `false`; implementations which track sufficiency should override this.
	fn is_sufficient(_asset: Self::AssetId) -> bool {
		false
	}
}

/// Special dust type which can be type-safely converted into a `Credit`.
#[must_use]
pub struct Dust<A, T: Unbalanced<A>>(pub T::AssetId, pub T::Balance);

impl<A, T: Balanced<A>> Dust<A, T> {
	/// Convert `Dust` into an instance of `Credit`.
	pub fn into_credit(self) -> Credit<A, T> {
		Credit::<A, T>::new(self.0, self.1)
	}
}

/// A fungible token class where the balance can be set arbitrarily.
///
/// **WARNING**
/// Do not use this directly unless you want trouble, since it allows you to alter account balances
/// without keeping the issuance up to date. It has no safeguards against accidentally creating
/// token imbalances in your system leading to accidental inflation or deflation. It's really just
/// for the underlying datatype to implement so the user gets the much safer `Balanced` trait to
/// use.
pub trait Unbalanced<AccountId>: Inspect<AccountId> {
	/// Create some dust and handle it with `Self::handle_dust`. This is an unbalanced operation
	/// and it must only be used when an account is modified in a raw fashion, outside of the entire
	/// fungibles API. The `amount` is capped at `Self::minimum_balance() - 1`.
	///
	/// This should not be reimplemented.
	fn handle_raw_dust(asset: Self::AssetId, amount: Self::Balance) {
		Self::handle_dust(Dust(
			asset.clone(),
			amount.min(Self::minimum_balance(asset).saturating_sub(One::one())),
		))
	}

	/// Do something with the dust which has been destroyed from the system. `Dust` can be converted
	/// into a `Credit` with the `Balanced` trait impl.
	fn handle_dust(dust: Dust<AccountId, Self>);

	/// Forcefully set the balance of `who` to `amount`.
	///
	/// If this call executes successfully, you can `assert_eq!(Self::balance(), amount);`.
	///
	/// For implementations which include one or more balances on hold, then these are *not*
	/// included in the `amount`.
	///
	/// This function does its best to force the balance change through, but will not break system
	/// invariants such as any Existential Deposits needed or overflows/underflows.
	/// If this cannot be done for some reason (e.g. because the account cannot be created, deleted
	/// or would overflow) then an `Err` is returned.
	fn write_balance(
		asset: Self::AssetId,
		who: &AccountId,
		amount: Self::Balance,
	) -> Result<Option<Self::Balance>, DispatchError>;

	/// Set the total issuance to `amount`.
	fn set_total_issuance(asset: Self::AssetId, amount: Self::Balance);

	/// Reduce the balance of `who` by `amount`.
	///
	/// If `precision` is `Exact` and it cannot be reduced by that amount for
	/// some reason, return `Err` and don't reduce it at all. If `precision` is `BestEffort`, then
	/// reduce the balance of `who` by the most that is possible, up to `amount`.
	///
	/// In either case, if `Ok` is returned then the inner is the amount by which is was reduced.
	/// Minimum balance will be respected and thus the returned amount may be up to
	/// `Self::minimum_balance() - 1` greater than `amount` in the case that the reduction caused
	/// the account to be deleted.
	///
	/// Any such excess must still be in the system when this returns — it must not have been
	/// disposed of through [`Unbalanced::handle_dust`]. Callers account for the returned amount,
	/// so an implementation that both reports the excess and traps it as dust makes them issue it
	/// a second time.
	fn decrease_balance(
		asset: Self::AssetId,
		who: &AccountId,
		mut amount: Self::Balance,
		precision: Precision,
		preservation: Preservation,
		force: Fortitude,
	) -> Result<Self::Balance, DispatchError> {
		let old_balance = Self::balance(asset.clone(), who);
		let reducible = Self::reducible_balance(asset.clone(), who, preservation, force);
		match precision {
			BestEffort => amount = amount.min(reducible),
			Exact => ensure!(reducible >= amount, TokenError::FundsUnavailable),
		}
		let new_balance = old_balance.checked_sub(&amount).ok_or(TokenError::FundsUnavailable)?;
		if let Some(dust) = Self::write_balance(asset.clone(), who, new_balance)? {
			Self::handle_dust(Dust(asset, dust));
		}
		Ok(old_balance.saturating_sub(new_balance))
	}

	/// Increase the balance of `who` by `amount`.
	///
	/// If it cannot be increased by that amount for some reason, return `Err` and don't increase
	/// it at all. If Ok, return the imbalance.
	/// Minimum balance will be respected and an error will be returned if
	/// `amount < Self::minimum_balance()` when the account of `who` is zero.
	fn increase_balance(
		asset: Self::AssetId,
		who: &AccountId,
		amount: Self::Balance,
		precision: Precision,
	) -> Result<Self::Balance, DispatchError> {
		let old_balance = Self::balance(asset.clone(), who);
		let new_balance = if let BestEffort = precision {
			old_balance.saturating_add(amount)
		} else {
			old_balance.checked_add(&amount).ok_or(ArithmeticError::Overflow)?
		};
		if new_balance < Self::minimum_balance(asset.clone()) {
			// Attempt to increase from 0 to below minimum -> stays at zero.
			if let BestEffort = precision {
				Ok(Self::Balance::default())
			} else {
				Err(TokenError::BelowMinimum.into())
			}
		} else {
			if new_balance == old_balance {
				Ok(Self::Balance::default())
			} else {
				if let Some(dust) = Self::write_balance(asset.clone(), who, new_balance)? {
					Self::handle_dust(Dust(asset, dust));
				}
				Ok(new_balance.saturating_sub(old_balance))
			}
		}
	}

	/// Reduce the active issuance by some amount.
	fn deactivate(_asset: Self::AssetId, _: Self::Balance) {}

	/// Increase the active issuance by some amount, up to the outstanding amount reduced.
	fn reactivate(_asset: Self::AssetId, _: Self::Balance) {}
}

/// Trait for providing a basic fungible asset.
pub trait Mutate<AccountId>: Inspect<AccountId> + Unbalanced<AccountId>
where
	AccountId: Eq,
{
	/// Increase the balance of `who` by exactly `amount`, minting new tokens. If that isn't
	/// possible then an `Err` is returned and nothing is changed.
	fn mint_into(
		asset: Self::AssetId,
		who: &AccountId,
		amount: Self::Balance,
	) -> Result<Self::Balance, DispatchError> {
		Self::total_issuance(asset.clone())
			.checked_add(&amount)
			.ok_or(ArithmeticError::Overflow)?;
		let actual = Self::increase_balance(asset.clone(), who, amount, Exact)?;
		Self::set_total_issuance(
			asset.clone(),
			Self::total_issuance(asset.clone()).saturating_add(actual),
		);
		Self::done_mint_into(asset, who, amount);
		Ok(actual)
	}

	/// Decrease the balance of `who` by at least `amount`, possibly slightly more in the case of
	/// minimum-balance requirements, burning the tokens. If that isn't possible then an `Err` is
	/// returned and nothing is changed. If successful, the amount of tokens reduced is returned.
	fn burn_from(
		asset: Self::AssetId,
		who: &AccountId,
		amount: Self::Balance,
		preservation: Preservation,
		precision: Precision,
		force: Fortitude,
	) -> Result<Self::Balance, DispatchError> {
		let actual = Self::reducible_balance(asset.clone(), who, preservation, force).min(amount);
		ensure!(actual == amount || precision == BestEffort, TokenError::FundsUnavailable);
		Self::total_issuance(asset.clone())
			.checked_sub(&actual)
			.ok_or(ArithmeticError::Overflow)?;
		let actual =
			Self::decrease_balance(asset.clone(), who, actual, BestEffort, preservation, force)?;
		Self::set_total_issuance(
			asset.clone(),
			Self::total_issuance(asset.clone()).saturating_sub(actual),
		);
		Self::done_burn_from(asset, who, actual);
		Ok(actual)
	}

	/// Attempt to decrease the `asset` balance of `who` by `amount`.
	///
	/// Equivalent to `burn_from`, except with an expectation that within the bounds of some
	/// universal issuance, the total assets `suspend`ed and `resume`d will be equivalent. The
	/// implementation may be configured such that the total assets suspended may never be less than
	/// the total assets resumed (which is the invariant for an issuing system), or the reverse
	/// (which the invariant in a non-issuing system).
	///
	/// Because of this expectation, any metadata associated with the asset is expected to survive
	/// the suspect-resume cycle.
	fn shelve(
		asset: Self::AssetId,
		who: &AccountId,
		amount: Self::Balance,
	) -> Result<Self::Balance, DispatchError> {
		let actual = Self::reducible_balance(asset.clone(), who, Expendable, Polite).min(amount);
		ensure!(actual == amount, TokenError::FundsUnavailable);
		Self::total_issuance(asset.clone())
			.checked_sub(&actual)
			.ok_or(ArithmeticError::Overflow)?;
		let actual =
			Self::decrease_balance(asset.clone(), who, actual, BestEffort, Expendable, Polite)?;
		Self::set_total_issuance(
			asset.clone(),
			Self::total_issuance(asset.clone()).saturating_sub(actual),
		);
		Self::done_shelve(asset, who, actual);
		Ok(actual)
	}

	/// Attempt to increase the `asset` balance of `who` by `amount`.
	///
	/// Equivalent to `mint_into`, except with an expectation that within the bounds of some
	/// universal issuance, the total assets `suspend`ed and `resume`d will be equivalent. The
	/// implementation may be configured such that the total assets suspended may never be less than
	/// the total assets resumed (which is the invariant for an issuing system), or the reverse
	/// (which the invariant in a non-issuing system).
	///
	/// Because of this expectation, any metadata associated with the asset is expected to survive
	/// the suspect-resume cycle.
	fn restore(
		asset: Self::AssetId,
		who: &AccountId,
		amount: Self::Balance,
	) -> Result<Self::Balance, DispatchError> {
		Self::total_issuance(asset.clone())
			.checked_add(&amount)
			.ok_or(ArithmeticError::Overflow)?;
		let actual = Self::increase_balance(asset.clone(), who, amount, Exact)?;
		Self::set_total_issuance(
			asset.clone(),
			Self::total_issuance(asset.clone()).saturating_add(actual),
		);
		Self::done_restore(asset, who, amount);
		Ok(actual)
	}

	/// Transfer funds from one account into another.
	///
	/// Returns the amount credited to `dest`. With the default implementation this is `amount`
	/// unless [`Unbalanced::decrease_balance`] folded a sub-minimum remainder of `source` into
	/// its return, as `pallet-assets` does, in which case it exceeds `amount` by up to
	/// `minimum_balance() - 1` and that remainder is credited to `dest` as well. Adapters such as
	/// `ItemOf` and `UnionOf` forward that figure.
	///
	/// `source` may be debited by more than is returned, e.g. when a reaped remainder is passed to
	/// `handle_dust` as `pallet-balances` does.
	///
	/// A transfer where the source and destination account are identical, or where `amount` is
	/// zero, is treated as No-OP after checking the preconditions.
	fn transfer(
		asset: Self::AssetId,
		source: &AccountId,
		dest: &AccountId,
		amount: Self::Balance,
		preservation: Preservation,
	) -> Result<Self::Balance, DispatchError> {
		let _extra = Self::can_withdraw(asset.clone(), source, amount)
			.into_result(preservation != Expendable)?;
		Self::can_deposit(asset.clone(), dest, amount, Extant).into_result()?;
		if source == dest || amount.is_zero() {
			return Ok(amount);
		}

		let actual =
			Self::decrease_balance(asset.clone(), source, amount, Exact, preservation, Polite)?;
		Self::increase_balance(asset.clone(), dest, actual, Exact)?;
		Self::done_transfer(asset, source, dest, actual);
		Ok(actual)
	}

	/// Simple infallible function to force an account to have a particular balance, good for use
	/// in tests and benchmarks but not recommended for production code owing to the lack of
	/// error reporting.
	///
	/// Returns the new balance.
	fn set_balance(asset: Self::AssetId, who: &AccountId, amount: Self::Balance) -> Self::Balance {
		let b = Self::balance(asset.clone(), who);
		if b > amount {
			Self::burn_from(asset, who, b - amount, Expendable, BestEffort, Force)
				.map(|d| b.saturating_sub(d))
		} else {
			Self::mint_into(asset, who, amount - b).map(|d| b.saturating_add(d))
		}
		.unwrap_or(b)
	}
	fn done_mint_into(_asset: Self::AssetId, _who: &AccountId, _amount: Self::Balance) {}
	fn done_burn_from(_asset: Self::AssetId, _who: &AccountId, _amount: Self::Balance) {}
	fn done_shelve(_asset: Self::AssetId, _who: &AccountId, _amount: Self::Balance) {}
	fn done_restore(_asset: Self::AssetId, _who: &AccountId, _amount: Self::Balance) {}
	fn done_transfer(
		_asset: Self::AssetId,
		_source: &AccountId,
		_dest: &AccountId,
		_amount: Self::Balance,
	) {
	}
}

/// Simple handler for an imbalance drop which increases the total issuance of the system by the
/// imbalance amount. Used for leftover debt.
pub struct IncreaseIssuance<AccountId, U>(PhantomData<(AccountId, U)>);
impl<AccountId, U: Unbalanced<AccountId>> HandleImbalanceDrop<U::AssetId, U::Balance>
	for IncreaseIssuance<AccountId, U>
{
	fn handle(asset: U::AssetId, amount: U::Balance) {
		U::set_total_issuance(asset.clone(), U::total_issuance(asset).saturating_add(amount))
	}
}

/// Simple handler for an imbalance drop which decreases the total issuance of the system by the
/// imbalance amount. Used for leftover credit.
pub struct DecreaseIssuance<AccountId, U>(PhantomData<(AccountId, U)>);
impl<AccountId, U: Unbalanced<AccountId>> HandleImbalanceDrop<U::AssetId, U::Balance>
	for DecreaseIssuance<AccountId, U>
{
	fn handle(asset: U::AssetId, amount: U::Balance) {
		U::set_total_issuance(asset.clone(), U::total_issuance(asset).saturating_sub(amount))
	}
}

/// A fungible token class where any creation and deletion of tokens is semi-explicit and where the
/// total supply is maintained automatically.
///
/// This is auto-implemented when a token class has `Unbalanced` implemented.
pub trait Balanced<AccountId>: Inspect<AccountId> + Unbalanced<AccountId> {
	/// The type for managing what happens when an instance of `Debt` is dropped without being used.
	type OnDropDebt: HandleImbalanceDrop<Self::AssetId, Self::Balance>;
	/// The type for managing what happens when an instance of `Credit` is dropped without being
	/// used.
	type OnDropCredit: HandleImbalanceDrop<Self::AssetId, Self::Balance>;

	/// Reduce the total issuance by `amount` and return the according imbalance. The imbalance will
	/// typically be used to reduce an account by the same amount with e.g. `settle`.
	///
	/// This is infallible, but doesn't guarantee that the entire `amount` is burnt, for example
	/// in the case of underflow.
	fn rescind(asset: Self::AssetId, amount: Self::Balance) -> Debt<AccountId, Self> {
		let old = Self::total_issuance(asset.clone());
		let new = old.saturating_sub(amount);
		Self::set_total_issuance(asset.clone(), new);
		let delta = old - new;
		Self::done_rescind(asset.clone(), delta);
		Imbalance::<Self::AssetId, Self::Balance, Self::OnDropDebt, Self::OnDropCredit>::new(
			asset, delta,
		)
	}

	/// Increase the total issuance by `amount` and return the according imbalance. The imbalance
	/// will typically be used to increase an account by the same amount with e.g.
	/// `resolve_into_existing` or `resolve_creating`.
	///
	/// This is infallible, but doesn't guarantee that the entire `amount` is issued, for example
	/// in the case of overflow.
	fn issue(asset: Self::AssetId, amount: Self::Balance) -> Credit<AccountId, Self> {
		let old = Self::total_issuance(asset.clone());
		let new = old.saturating_add(amount);
		Self::set_total_issuance(asset.clone(), new);
		let delta = new - old;
		Self::done_issue(asset.clone(), delta);
		Imbalance::<Self::AssetId, Self::Balance, Self::OnDropCredit, Self::OnDropDebt>::new(
			asset, delta,
		)
	}

	/// Produce a pair of imbalances that cancel each other out exactly.
	///
	/// This is just the same as burning and issuing the same amount and has no effect on the
	/// total issuance.
	///
	/// This is infallible, but doesn't guarantee that the entire `amount` is used to create the
	/// pair, for example in the case where the amounts would cause overflow or underflow in
	/// [`Balanced::issue`] or [`Balanced::rescind`].
	fn pair(
		asset: Self::AssetId,
		amount: Self::Balance,
	) -> Result<(Debt<AccountId, Self>, Credit<AccountId, Self>), DispatchError> {
		let issued = Self::issue(asset.clone(), amount);
		let rescinded = Self::rescind(asset, amount);
		// Need to check amount in case by some edge case both issued and rescinded are below
		// `amount` by the exact same value
		if issued.peek() != rescinded.peek() || issued.peek() != amount {
			// Issued and rescinded will be dropped automatically
			Err("Failed to issue and rescind equal amounts".into())
		} else {
			Ok((rescinded, issued))
		}
	}

	/// Mints `value` into the account of `who`, creating it as needed.
	///
	/// If `precision` is `BestEffort` and `value` in full could not be minted (e.g. due to
	/// overflow), then the maximum is minted, up to `value`. If `precision` is `Exact`, then
	/// exactly `value` must be minted into the account of `who` or the operation will fail with an
	/// `Err` and nothing will change.
	///
	/// If the operation is successful, this will return `Ok` with a `Debt` of the total value
	/// added to the account.
	fn deposit(
		asset: Self::AssetId,
		who: &AccountId,
		value: Self::Balance,
		precision: Precision,
	) -> Result<Debt<AccountId, Self>, DispatchError> {
		let increase = Self::increase_balance(asset.clone(), who, value, precision)?;
		Self::done_deposit(asset.clone(), who, increase);
		Ok(Imbalance::<Self::AssetId, Self::Balance, Self::OnDropDebt, Self::OnDropCredit>::new(
			asset, increase,
		))
	}

	/// Removes `value` balance from `who` account if possible.
	///
	/// If `precision` is `BestEffort` and `value` in full could not be removed (e.g. due to
	/// underflow), then the maximum is removed, up to `value`. If `precision` is `Exact`, then
	/// exactly `value` must be removed from the account of `who` or the operation will fail with an
	/// `Err` and nothing will change.
	///
	/// If the removal is needed but not possible, then it returns `Err` and nothing is changed.
	/// If the account needed to be deleted, then slightly more than `value` may be removed from the
	/// account owning since up to (but not including) minimum balance may also need to be removed.
	///
	/// If the operation is successful, this will return `Ok` with a `Credit` of the total value
	/// removed from the account.
	fn withdraw(
		asset: Self::AssetId,
		who: &AccountId,
		value: Self::Balance,
		precision: Precision,
		preservation: Preservation,
		force: Fortitude,
	) -> Result<Credit<AccountId, Self>, DispatchError> {
		let decrease =
			Self::decrease_balance(asset.clone(), who, value, precision, preservation, force)?;
		Self::done_withdraw(asset.clone(), who, decrease);
		Ok(Imbalance::<Self::AssetId, Self::Balance, Self::OnDropCredit, Self::OnDropDebt>::new(
			asset, decrease,
		))
	}

	/// The balance of `who` is increased in order to counter `credit`. If the whole of `credit`
	/// cannot be countered, then nothing is changed and the original `credit` is returned in an
	/// `Err`.
	///
	/// Please note: If `credit.peek()` is less than `Self::minimum_balance()`, then `who` must
	/// already exist for this to succeed.
	fn resolve(
		who: &AccountId,
		credit: Credit<AccountId, Self>,
	) -> Result<(), Credit<AccountId, Self>> {
		let v = credit.peek();
		let debt = match Self::deposit(credit.asset(), who, v, Exact) {
			Err(_) => return Err(credit),
			Ok(d) => d,
		};
		if let Ok(result) = credit.offset(debt) {
			let result = result.try_drop();
			debug_assert!(result.is_ok(), "ok deposit return must be equal to credit value; qed");
		} else {
			debug_assert!(false, "debt.asset is credit.asset; qed");
		}
		Ok(())
	}

	/// The balance of `who` is decreased in order to counter `debt`. If the whole of `debt`
	/// cannot be countered, then nothing is changed and the original `debt` is returned in an
	/// `Err`.
	fn settle(
		who: &AccountId,
		debt: Debt<AccountId, Self>,
		preservation: Preservation,
	) -> Result<Credit<AccountId, Self>, Debt<AccountId, Self>> {
		let amount = debt.peek();
		let asset = debt.asset();
		let credit = match Self::withdraw(asset.clone(), who, amount, Exact, preservation, Polite) {
			Err(_) => return Err(debt),
			Ok(d) => d,
		};
		match credit.offset(debt) {
			Ok(SameOrOther::None) => Ok(Credit::<AccountId, Self>::zero(asset)),
			Ok(SameOrOther::Same(dust)) => Ok(dust),
			Ok(SameOrOther::Other(rest)) => {
				debug_assert!(false, "ok withdraw return must be at least debt value; qed");
				Err(rest)
			},
			Err(_) => {
				debug_assert!(false, "debt.asset is credit.asset; qed");
				Ok(Credit::<AccountId, Self>::zero(asset))
			},
		}
	}

	fn done_rescind(_asset: Self::AssetId, _amount: Self::Balance) {}
	fn done_issue(_asset: Self::AssetId, _amount: Self::Balance) {}
	fn done_deposit(_asset: Self::AssetId, _who: &AccountId, _amount: Self::Balance) {}
	fn done_withdraw(_asset: Self::AssetId, _who: &AccountId, _amount: Self::Balance) {}
}

/// Dummy implementation of [`Inspect`]
#[cfg(feature = "std")]
impl<AccountId> Inspect<AccountId> for () {
	type AssetId = u32;
	type Balance = u32;
	fn total_issuance(_: Self::AssetId) -> Self::Balance {
		0
	}
	fn minimum_balance(_: Self::AssetId) -> Self::Balance {
		0
	}
	fn total_balance(_: Self::AssetId, _: &AccountId) -> Self::Balance {
		0
	}
	fn balance(_: Self::AssetId, _: &AccountId) -> Self::Balance {
		0
	}
	fn reducible_balance(
		_: Self::AssetId,
		_: &AccountId,
		_: Preservation,
		_: Fortitude,
	) -> Self::Balance {
		0
	}
	fn can_deposit(
		_: Self::AssetId,
		_: &AccountId,
		_: Self::Balance,
		_: Provenance,
	) -> DepositConsequence {
		DepositConsequence::Success
	}
	fn can_withdraw(
		_: Self::AssetId,
		_: &AccountId,
		_: Self::Balance,
	) -> WithdrawConsequence<Self::Balance> {
		WithdrawConsequence::Success
	}
	fn asset_exists(_: Self::AssetId) -> bool {
		false
	}
}

/// Dummy implementation of [`Unbalanced`]
#[cfg(feature = "std")]
impl<AccountId> Unbalanced<AccountId> for () {
	fn handle_dust(_: Dust<AccountId, Self>) {}
	fn write_balance(
		_: Self::AssetId,
		_: &AccountId,
		_: Self::Balance,
	) -> Result<Option<Self::Balance>, DispatchError> {
		Ok(None)
	}
	fn set_total_issuance(_: Self::AssetId, _: Self::Balance) {}
}

/// Dummy implementation of [`Mutate`]
#[cfg(feature = "std")]
impl<AccountId: Eq> Mutate<AccountId> for () {}