lib.rs

// This file is part of Substrate.

// Copyright (C) 2017-2021 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.

//! # Balances Pallet
//!
//! The Balances pallet provides functionality for handling accounts and balances.
//!
//! - [`Config`]
//! - [`Call`]
//! - [`Pallet`]
//!
//! ## Overview
//!
//! The Balances pallet provides functions for:
//!
//! - Getting and setting free balances.
//! - Retrieving total, reserved and unreserved balances.
//! - Repatriating a reserved balance to a beneficiary account that exists.
//! - Transferring a balance between accounts (when not reserved).
//! - Slashing an account balance.
//! - Account creation and removal.
//! - Managing total issuance.
//! - Setting and managing locks.
//!
//! ### Terminology
//!
//! - **Existential Deposit:** The minimum balance required to create or keep an account open. This prevents
//! "dust accounts" from filling storage. When the free plus the reserved balance (i.e. the total balance)
//!   fall below this, then the account is said to be dead; and it loses its functionality as well as any
//!   prior history and all information on it is removed from the chain's state.
//!   No account should ever have a total balance that is strictly between 0 and the existential
//!   deposit (exclusive). If this ever happens, it indicates either a bug in this pallet or an
//!   erroneous raw mutation of storage.
//!
//! - **Total Issuance:** The total number of units in existence in a system.
//!
//! - **Reaping an account:** The act of removing an account by resetting its nonce. Happens after its
//! total balance has become zero (or, strictly speaking, less than the Existential Deposit).
//!
//! - **Free Balance:** The portion of a balance that is not reserved. The free balance is the only
//!   balance that matters for most operations.
//!
//! - **Reserved Balance:** Reserved balance still belongs to the account holder, but is suspended.
//!   Reserved balance can still be slashed, but only after all the free balance has been slashed.
//!
//! - **Imbalance:** A condition when some funds were credited or debited without equal and opposite accounting
//! (i.e. a difference between total issuance and account balances). Functions that result in an imbalance will
//! return an object of the `Imbalance` trait that can be managed within your runtime logic. (If an imbalance is
//! simply dropped, it should automatically maintain any book-keeping such as total issuance.)
//!
//! - **Lock:** A freeze on a specified amount of an account's free balance until a specified block number. Multiple
//! locks always operate over the same funds, so they "overlay" rather than "stack".
//!
//! ### Implementations
//!
//! The Balances pallet provides implementations for the following traits. If these traits provide the functionality
//! that you need, then you can avoid coupling with the Balances pallet.
//!
//! - [`Currency`](frame_support::traits::Currency): Functions for dealing with a
//! fungible assets system.
//! - [`ReservableCurrency`](frame_support::traits::ReservableCurrency):
//! Functions for dealing with assets that can be reserved from an account.
//! - [`LockableCurrency`](frame_support::traits::LockableCurrency): Functions for
//! dealing with accounts that allow liquidity restrictions.
//! - [`Imbalance`](frame_support::traits::Imbalance): Functions for handling
//! imbalances between total issuance in the system and account balances. Must be used when a function
//! creates new funds (e.g. a reward) or destroys some funds (e.g. a system fee).
//!
//! ## Interface
//!
//! ### Dispatchable Functions
//!
//! - `transfer` - Transfer some liquid free balance to another account.
//! - `set_balance` - Set the balances of a given account. The origin of this call must be root.
//!
//! ## Usage
//!
//! The following examples show how to use the Balances pallet in your custom pallet.
//!
//! ### Examples from the FRAME
//!
//! The Contract pallet uses the `Currency` trait to handle gas payment, and its types inherit from `Currency`:
//!
//! ```
//! use frame_support::traits::Currency;
//! # pub trait Config: frame_system::Config {
//! # 	type Currency: Currency<Self::AccountId>;
//! # }
//!
//! pub type BalanceOf<T> = <<T as Config>::Currency as Currency<<T as frame_system::Config>::AccountId>>::Balance;
//! pub type NegativeImbalanceOf<T> = <<T as Config>::Currency as Currency<<T as frame_system::Config>::AccountId>>::NegativeImbalance;
//!
//! # fn main() {}
//! ```
//!
//! The Staking pallet uses the `LockableCurrency` trait to lock a stash account's funds:
//!
//! ```
//! use frame_support::traits::{WithdrawReasons, LockableCurrency};
//! use sp_runtime::traits::Bounded;
//! pub trait Config: frame_system::Config {
//! 	type Currency: LockableCurrency<Self::AccountId, Moment=Self::BlockNumber>;
//! }
//! # struct StakingLedger<T: Config> {
//! # 	stash: <T as frame_system::Config>::AccountId,
//! # 	total: <<T as Config>::Currency as frame_support::traits::Currency<<T as frame_system::Config>::AccountId>>::Balance,
//! # 	phantom: std::marker::PhantomData<T>,
//! # }
//! # const STAKING_ID: [u8; 8] = *b"staking ";
//!
//! fn update_ledger<T: Config>(
//! 	controller: &T::AccountId,
//! 	ledger: &StakingLedger<T>
//! ) {
//! 	T::Currency::set_lock(
//! 		STAKING_ID,
//! 		&ledger.stash,
//! 		ledger.total,
//! 		WithdrawReasons::all()
//! 	);
//! 	// <Ledger<T>>::insert(controller, ledger); // Commented out as we don't have access to Staking's storage here.
//! }
//! # fn main() {}
//! ```
//!
//! ## Genesis config
//!
//! The Balances pallet depends on the [`GenesisConfig`].
//!
//! ## Assumptions
//!
//! * Total issued balanced of all accounts should be less than `Config::Balance::max_value()`.

#![cfg_attr(not(feature = "std"), no_std)]

#[macro_use]
mod tests;
mod tests_local;
mod tests_composite;
mod tests_reentrancy;
mod benchmarking;
pub mod weights;

use sp_std::prelude::*;
use sp_std::{cmp, result, mem, fmt::Debug, ops::BitOr};
use codec::{Codec, Encode, Decode};
use frame_support::{
	ensure,
	traits::{
		Currency, OnUnbalanced, TryDrop, StoredMap,
		WithdrawReasons, LockIdentifier, LockableCurrency, ExistenceRequirement,
		Imbalance, SignedImbalance, ReservableCurrency, Get, ExistenceRequirement::KeepAlive,
		ExistenceRequirement::AllowDeath, BalanceStatus as Status,
	}
};
#[cfg(feature = "std")]
use frame_support::traits::GenesisBuild;
use sp_runtime::{
	RuntimeDebug, DispatchResult, DispatchError,
	traits::{
		Zero, AtLeast32BitUnsigned, StaticLookup, CheckedAdd, CheckedSub,
		MaybeSerializeDeserialize, Saturating, Bounded, StoredMapError,
	},
};
use frame_system as system;
pub use self::imbalances::{PositiveImbalance, NegativeImbalance};
pub use weights::WeightInfo;

pub use pallet::*;

#[frame_support::pallet]
pub mod pallet {
	use frame_support::pallet_prelude::*;
	use frame_system::pallet_prelude::*;
	use super::*;

	#[pallet::config]
	pub trait Config<I: 'static = ()>: frame_system::Config {
		/// The balance of an account.
		type Balance: Parameter + Member + AtLeast32BitUnsigned + Codec + Default + Copy +
			MaybeSerializeDeserialize + Debug;

		/// Handler for the unbalanced reduction when removing a dust account.
		type DustRemoval: OnUnbalanced<NegativeImbalance<Self, I>>;

		/// The overarching event type.
		type Event: From<Event<Self, I>> + IsType<<Self as frame_system::Config>::Event>;

		/// The minimum amount required to keep an account open.
		#[pallet::constant]
		type ExistentialDeposit: Get<Self::Balance>;

		/// The means of storing the balances of an account.
		type AccountStore: StoredMap<Self::AccountId, AccountData<Self::Balance>>;

		/// Weight information for extrinsics in this pallet.
		type WeightInfo: WeightInfo;

		/// The maximum number of locks that should exist on an account.
		/// Not strictly enforced, but used for weight estimation.
		type MaxLocks: Get<u32>;
	}

	#[pallet::pallet]
	#[pallet::generate_store(pub(super) trait Store)]
	pub struct Pallet<T, I=()>(PhantomData<(T, I)>);

	#[pallet::hooks]
	impl<T: Config<I>, I: 'static> Hooks<BlockNumberFor<T>> for Pallet<T, I> {
	}

	#[pallet::call]
	impl<T: Config<I>, I: 'static> Pallet<T, I> {
		/// Transfer some liquid free balance to another account.
		///
		/// `transfer` will set the `FreeBalance` of the sender and receiver.
		/// It will decrease the total issuance of the system by the `TransferFee`.
		/// If the sender's account is below the existential deposit as a result
		/// of the transfer, the account will be reaped.
		///
		/// The dispatch origin for this call must be `Signed` by the transactor.
		///
		/// # <weight>
		/// - Dependent on arguments but not critical, given proper implementations for
		///   input config types. See related functions below.
		/// - It contains a limited number of reads and writes internally and no complex computation.
		///
		/// Related functions:
		///
		///   - `ensure_can_withdraw` is always called internally but has a bounded complexity.
		///   - Transferring balances to accounts that did not exist before will cause
		///      `T::OnNewAccount::on_new_account` to be called.
		///   - Removing enough funds from an account will trigger `T::DustRemoval::on_unbalanced`.
		///   - `transfer_keep_alive` works the same way as `transfer`, but has an additional
		///     check that the transfer will not kill the origin account.
		/// ---------------------------------
		/// - Base Weight: 73.64 µs, worst case scenario (account created, account removed)
		/// - DB Weight: 1 Read and 1 Write to destination account
		/// - Origin account is already in memory, so no DB operations for them.
		/// # </weight>
		#[pallet::weight(T::WeightInfo::transfer())]
		pub fn transfer(
			origin: OriginFor<T>,
			dest: <T::Lookup as StaticLookup>::Source,
			#[pallet::compact] value: T::Balance,
		) -> DispatchResultWithPostInfo {
			let transactor = ensure_signed(origin)?;
			let dest = T::Lookup::lookup(dest)?;
			<Self as Currency<_>>::transfer(&transactor, &dest, value, ExistenceRequirement::AllowDeath)?;
			Ok(().into())
		}

		/// Set the balances of a given account.
		///
		/// This will alter `FreeBalance` and `ReservedBalance` in storage. it will
		/// also decrease the total issuance of the system (`TotalIssuance`).
		/// If the new free or reserved balance is below the existential deposit,
		/// it will reset the account nonce (`frame_system::AccountNonce`).
		///
		/// The dispatch origin for this call is `root`.
		///
		/// # <weight>
		/// - Independent of the arguments.
		/// - Contains a limited number of reads and writes.
		/// ---------------------
		/// - Base Weight:
		///     - Creating: 27.56 µs
		///     - Killing: 35.11 µs
		/// - DB Weight: 1 Read, 1 Write to `who`
		/// # </weight>
		#[pallet::weight(
			T::WeightInfo::set_balance_creating() // Creates a new account.
				.max(T::WeightInfo::set_balance_killing()) // Kills an existing account.
		)]
		pub(super) fn set_balance(
			origin: OriginFor<T>,
			who: <T::Lookup as StaticLookup>::Source,
			#[pallet::compact] new_free: T::Balance,
			#[pallet::compact] new_reserved: T::Balance,
		) -> DispatchResultWithPostInfo {
			ensure_root(origin)?;
			let who = T::Lookup::lookup(who)?;
			let existential_deposit = T::ExistentialDeposit::get();

			let wipeout = new_free + new_reserved < existential_deposit;
			let new_free = if wipeout { Zero::zero() } else { new_free };
			let new_reserved = if wipeout { Zero::zero() } else { new_reserved };

			let (free, reserved) = Self::mutate_account(&who, |account| {
				if new_free > account.free {
					mem::drop(PositiveImbalance::<T, I>::new(new_free - account.free));
				} else if new_free < account.free {
					mem::drop(NegativeImbalance::<T, I>::new(account.free - new_free));
				}

				if new_reserved > account.reserved {
					mem::drop(PositiveImbalance::<T, I>::new(new_reserved - account.reserved));
				} else if new_reserved < account.reserved {
					mem::drop(NegativeImbalance::<T, I>::new(account.reserved - new_reserved));
				}

				account.free = new_free;
				account.reserved = new_reserved;

				(account.free, account.reserved)
			})?;
			Self::deposit_event(Event::BalanceSet(who, free, reserved));
			Ok(().into())
		}

		/// Exactly as `transfer`, except the origin must be root and the source account may be
		/// specified.
		/// # <weight>
		/// - Same as transfer, but additional read and write because the source account is
		///   not assumed to be in the overlay.
		/// # </weight>
		#[pallet::weight(T::WeightInfo::force_transfer())]
		pub fn force_transfer(
			origin: OriginFor<T>,
			source: <T::Lookup as StaticLookup>::Source,
			dest: <T::Lookup as StaticLookup>::Source,
			#[pallet::compact] value: T::Balance,
		) -> DispatchResultWithPostInfo {
			ensure_root(origin)?;
			let source = T::Lookup::lookup(source)?;
			let dest = T::Lookup::lookup(dest)?;
			<Self as Currency<_>>::transfer(&source, &dest, value, ExistenceRequirement::AllowDeath)?;
			Ok(().into())
		}

		/// Same as the [`transfer`] call, but with a check that the transfer will not kill the
		/// origin account.
		///
		/// 99% of the time you want [`transfer`] instead.
		///
		/// [`transfer`]: struct.Pallet.html#method.transfer
		/// # <weight>
		/// - Cheaper than transfer because account cannot be killed.
		/// - Base Weight: 51.4 µs
		/// - DB Weight: 1 Read and 1 Write to dest (sender is in overlay already)
		/// #</weight>
		#[pallet::weight(T::WeightInfo::transfer_keep_alive())]
		pub fn transfer_keep_alive(
			origin: OriginFor<T>,
			dest: <T::Lookup as StaticLookup>::Source,
			#[pallet::compact] value: T::Balance,
		) -> DispatchResultWithPostInfo {
			let transactor = ensure_signed(origin)?;
			let dest = T::Lookup::lookup(dest)?;
			<Self as Currency<_>>::transfer(&transactor, &dest, value, KeepAlive)?;
			Ok(().into())
		}
	}

	#[pallet::event]
	#[pallet::generate_deposit(pub(super) fn deposit_event)]
	#[pallet::metadata(T::AccountId = "AccountId", T::Balance = "Balance")]
	pub enum Event<T: Config<I>, I: 'static = ()> {
		/// An account was created with some free balance. \[account, free_balance\]
		Endowed(T::AccountId, T::Balance),
		/// An account was removed whose balance was non-zero but below ExistentialDeposit,
		/// resulting in an outright loss. \[account, balance\]
		DustLost(T::AccountId, T::Balance),
		/// Transfer succeeded. \[from, to, value\]
		Transfer(T::AccountId, T::AccountId, T::Balance),
		/// A balance was set by root. \[who, free, reserved\]
		BalanceSet(T::AccountId, T::Balance, T::Balance),
		/// Some amount was deposited (e.g. for transaction fees). \[who, deposit\]
		Deposit(T::AccountId, T::Balance),
		/// Some balance was reserved (moved from free to reserved). \[who, value\]
		Reserved(T::AccountId, T::Balance),
		/// Some balance was unreserved (moved from reserved to free). \[who, value\]
		Unreserved(T::AccountId, T::Balance),
		/// Some balance was moved from the reserve of the first account to the second account.
		/// Final argument indicates the destination balance type.
		/// \[from, to, balance, destination_status\]
		ReserveRepatriated(T::AccountId, T::AccountId, T::Balance, Status),
	}

	/// Old name generated by `decl_event`.
	#[deprecated(note = "use `Event` instead")]
	pub type RawEvent<T, I = ()> = Event<T, I>;

	#[pallet::error]
	pub enum Error<T, I = ()> {
		/// Vesting balance too high to send value
		VestingBalance,
		/// Account liquidity restrictions prevent withdrawal
		LiquidityRestrictions,
		/// Got an overflow after adding
		Overflow,
		/// Balance too low to send value
		InsufficientBalance,
		/// Value too low to create account due to existential deposit
		ExistentialDeposit,
		/// Transfer/payment would kill account
		KeepAlive,
		/// A vesting schedule already exists for this account
		ExistingVestingSchedule,
		/// Beneficiary account must pre-exist
		DeadAccount,
	}

	/// The total units issued in the system.
	#[pallet::storage]
	#[pallet::getter(fn total_issuance)]
	pub type TotalIssuance<T: Config<I>, I: 'static = ()> = StorageValue<_, T::Balance, ValueQuery>;

	/// The balance of an account.
	///
	/// NOTE: This is only used in the case that this pallet is used to store balances.
	#[pallet::storage]
	pub type Account<T: Config<I>, I: 'static = ()> = StorageMap<
		_,
		Blake2_128Concat,
		T::AccountId,
		AccountData<T::Balance>,
		ValueQuery
	>;

	/// Any liquidity locks on some account balances.
	/// NOTE: Should only be accessed when setting, changing and freeing a lock.
	#[pallet::storage]
	#[pallet::getter(fn locks)]
	pub type Locks<T: Config<I>, I: 'static = ()> = StorageMap<
		_,
		Blake2_128Concat,
		T::AccountId,
		Vec<BalanceLock<T::Balance>>,
		ValueQuery
	>;

	/// Storage version of the pallet.
	///
	/// This is set to v2.0.0 for new networks.
	#[pallet::storage]
	pub(super) type StorageVersion<T: Config<I>, I: 'static = ()> = StorageValue<
		_,
		Releases,
		ValueQuery
	>;

	#[pallet::genesis_config]
	pub struct GenesisConfig<T: Config<I>, I: 'static = ()> {
		pub balances: Vec<(T::AccountId, T::Balance)>,
	}

	#[cfg(feature = "std")]
	impl<T: Config<I>, I: 'static> Default for GenesisConfig<T, I> {
		fn default() -> Self {
			Self {
				balances: Default::default(),
			}
		}
	}

	#[pallet::genesis_build]
	impl<T: Config<I>, I: 'static> GenesisBuild<T, I> for GenesisConfig<T, I> {
		fn build(&self) {
			let total = self.balances
				.iter()
				.fold(Zero::zero(), |acc: T::Balance, &(_, n)| acc + n);
			<TotalIssuance<T, I>>::put(total);

			<StorageVersion<T, I>>::put(Releases::V2_0_0);

			for (_, balance) in &self.balances {
				assert!(
					*balance >= <T as Config<I>>::ExistentialDeposit::get(),
					"the balance of any account should always be at least the existential deposit.",
				)
			}

			// ensure no duplicates exist.
			let endowed_accounts = self.balances.iter().map(|(x, _)| x).cloned().collect::<std::collections::BTreeSet<_>>();

			assert!(endowed_accounts.len() == self.balances.len(), "duplicate balances in genesis.");

			for &(ref who, free) in self.balances.iter() {
				assert!(T::AccountStore::insert(who, AccountData { free, ..Default::default() }).is_ok());
			}
		}
	}
}

#[cfg(feature = "std")]
impl<T: Config<I>, I: 'static> GenesisConfig<T, I> {
	/// Direct implementation of `GenesisBuild::build_storage`.
	///
	/// Kept in order not to break dependency.
	pub fn build_storage(&self) -> Result<sp_runtime::Storage, String> {
		<Self as GenesisBuild<T, I>>::build_storage(self)
	}

	/// Direct implementation of `GenesisBuild::assimilate_storage`.
	///
	/// Kept in order not to break dependency.
	pub fn assimilate_storage(
		&self,
		storage: &mut sp_runtime::Storage
	) -> Result<(), String> {
		<Self as GenesisBuild<T, I>>::assimilate_storage(self, storage)
	}
}

/// Simplified reasons for withdrawing balance.
#[derive(Encode, Decode, Clone, Copy, PartialEq, Eq, RuntimeDebug)]
pub enum Reasons {
	/// Paying system transaction fees.
	Fee = 0,
	/// Any reason other than paying system transaction fees.
	Misc = 1,
	/// Any reason at all.
	All = 2,
}

impl From<WithdrawReasons> for Reasons {
	fn from(r: WithdrawReasons) -> Reasons {
		if r == WithdrawReasons::from(WithdrawReasons::TRANSACTION_PAYMENT) {
			Reasons::Fee
		} else if r.contains(WithdrawReasons::TRANSACTION_PAYMENT) {
			Reasons::All
		} else {
			Reasons::Misc
		}
	}
}

impl BitOr for Reasons {
	type Output = Reasons;
	fn bitor(self, other: Reasons) -> Reasons {
		if self == other { return self }
		Reasons::All
	}
}

/// A single lock on a balance. There can be many of these on an account and they "overlap", so the
/// same balance is frozen by multiple locks.
#[derive(Encode, Decode, Clone, PartialEq, Eq, RuntimeDebug)]
pub struct BalanceLock<Balance> {
	/// An identifier for this lock. Only one lock may be in existence for each identifier.
	pub id: LockIdentifier,
	/// The amount which the free balance may not drop below when this lock is in effect.
	pub amount: Balance,
	/// If true, then the lock remains in effect even for payment of transaction fees.
	pub reasons: Reasons,
}

/// All balance information for an account.
#[derive(Encode, Decode, Clone, PartialEq, Eq, Default, RuntimeDebug)]
pub struct AccountData<Balance> {
	/// Non-reserved part of the balance. There may still be restrictions on this, but it is the
	/// total pool what may in principle be transferred, reserved and used for tipping.
	///
	/// This is the only balance that matters in terms of most operations on tokens. It
	/// alone is used to determine the balance when in the contract execution environment.
	pub free: Balance,
	/// Balance which is reserved and may not be used at all.
	///
	/// This can still get slashed, but gets slashed last of all.
	///
	/// This balance is a 'reserve' balance that other subsystems use in order to set aside tokens
	/// that are still 'owned' by the account holder, but which are suspendable.
	pub reserved: Balance,
	/// The amount that `free` may not drop below when withdrawing for *anything except transaction
	/// fee payment*.
	pub misc_frozen: Balance,
	/// The amount that `free` may not drop below when withdrawing specifically for transaction
	/// fee payment.
	pub fee_frozen: Balance,
}

impl<Balance: Saturating + Copy + Ord> AccountData<Balance> {
	/// How much this account's balance can be reduced for the given `reasons`.
	fn usable(&self, reasons: Reasons) -> Balance {
		self.free.saturating_sub(self.frozen(reasons))
	}
	/// The amount that this account's free balance may not be reduced beyond for the given
	/// `reasons`.
	fn frozen(&self, reasons: Reasons) -> Balance {
		match reasons {
			Reasons::All => self.misc_frozen.max(self.fee_frozen),
			Reasons::Misc => self.misc_frozen,
			Reasons::Fee => self.fee_frozen,
		}
	}
	/// The total balance in this account including any that is reserved and ignoring any frozen.
	fn total(&self) -> Balance {
		self.free.saturating_add(self.reserved)
	}
}

// A value placed in storage that represents the current version of the Balances storage.
// This value is used by the `on_runtime_upgrade` logic to determine whether we run
// storage migration logic. This should match directly with the semantic versions of the Rust crate.
#[derive(Encode, Decode, Clone, Copy, PartialEq, Eq, RuntimeDebug)]
enum Releases {
	V1_0_0,
	V2_0_0,
}

impl Default for Releases {
	fn default() -> Self {
		Releases::V1_0_0
	}
}

pub struct DustCleaner<T: Config<I>, I: 'static = ()>(Option<(T::AccountId, NegativeImbalance<T, I>)>);

impl<T: Config<I>, I: 'static> Drop for DustCleaner<T, I> {
	fn drop(&mut self) {
		if let Some((who, dust)) = self.0.take() {
			Pallet::<T, I>::deposit_event(Event::DustLost(who, dust.peek()));
			T::DustRemoval::on_unbalanced(dust);
		}
	}
}

impl<T: Config<I>, I: 'static> Pallet<T, I> {
	/// Get the free balance of an account.
	pub fn free_balance(who: impl sp_std::borrow::Borrow<T::AccountId>) -> T::Balance {
		Self::account(who.borrow()).free
	}

	/// Get the balance of an account that can be used for transfers, reservations, or any other
	/// non-locking, non-transaction-fee activity. Will be at most `free_balance`.
	pub fn usable_balance(who: impl sp_std::borrow::Borrow<T::AccountId>) -> T::Balance {
		Self::account(who.borrow()).usable(Reasons::Misc)
	}

	/// Get the balance of an account that can be used for paying transaction fees (not tipping,
	/// or any other kind of fees, though). Will be at most `free_balance`.
	pub fn usable_balance_for_fees(who: impl sp_std::borrow::Borrow<T::AccountId>) -> T::Balance {
		Self::account(who.borrow()).usable(Reasons::Fee)
	}

	/// Get the reserved balance of an account.
	pub fn reserved_balance(who: impl sp_std::borrow::Borrow<T::AccountId>) -> T::Balance {
		Self::account(who.borrow()).reserved
	}

	/// Get both the free and reserved balances of an account.
	fn account(who: &T::AccountId) -> AccountData<T::Balance> {
		T::AccountStore::get(&who)
	}

	/// Handles any steps needed after mutating an account.
	///
	/// This includes DustRemoval unbalancing, in the case than the `new` account's total balance
	/// is non-zero but below ED.
	///
	/// Returns two values:
	/// - `Some` containing the the `new` account, iff the account has sufficient balance.
	/// - `Some` containing the dust to be dropped, iff some dust should be dropped.
	fn post_mutation(
		_who: &T::AccountId,
		new: AccountData<T::Balance>,
	) -> (Option<AccountData<T::Balance>>, Option<NegativeImbalance<T, I>>) {
		let total = new.total();
		if total < T::ExistentialDeposit::get() {
			if total.is_zero() {
				(None, None)
			} else {
				(None, Some(NegativeImbalance::new(total)))
			}
		} else {
			(Some(new), None)
		}
	}

	fn deposit_consequence(
		_who: &T::AccountId,
		amount: T::Balance,
		account: &AccountData<T::Balance>,
	) -> DepositConsequence {
		if amount.is_zero() { return DepositConsequence::Success }

		if TotalIssuance::<T, I>::get().checked_add(&amount).is_none() {
			return DepositConsequence::Overflow
		}

		let new_total_balance = match account.total().checked_add(&amount) {
			Some(x) => x,
			None => return DepositConsequence::Overflow,
		};

		if new_total_balance < T::ExistentialDeposit::get() {
			return DepositConsequence::BelowMinimum
		}

		// NOTE: We assume that we are a provider, so don't need to do any checks in the
		// case of account creation.

		DepositConsequence::Success
	}

	fn withdraw_consequence(
		who: &T::AccountId,
		amount: T::Balance,
		account: &AccountData<T::Balance>,
	) -> WithdrawConsequence<T::Balance> {
		if amount.is_zero() { return WithdrawConsequence::Success }

		if TotalIssuance::<T, I>::get().checked_sub(&amount).is_none() {
			return WithdrawConsequence::Underflow
		}

		let new_total_balance = match account.total().checked_sub(&amount) {
			Some(x) => x,
			None => return WithdrawConsequence::NoFunds,
		};

		// Provider restriction - total account balance cannot be reduced to zero if it cannot
		// sustain the loss of a provider reference.
		// NOTE: This assumes that the pallet is a provider (which is true). Is this ever changes,
		// then this will need to adapt accordingly.
		let ed = T::ExistentialDeposit::get();
		let success = if new_total_balance < ed {
			if frame_system::Pallet::<T>::can_dec_provider(who) {
				WithdrawConsequence::ReducedToZero(new_total_balance)
			} else {
				return WithdrawConsequence::WouldDie
			}
		} else {
			WithdrawConsequence::Success
		};

		// Enough free funds to have them be reduced.
		let new_free_balance = match account.free.checked_sub(&amount) {
			Some(b) => b,
			None => return WithdrawConsequence::NoFunds,
		};

		// Eventual free funds must be no less than the frozen balance.
		let min_balance = account.frozen(Reasons::All);
		if new_free_balance < min_balance {
			return WithdrawConsequence::Frozen
		}

		success
	}

	/// Mutate an account to some new value, or delete it entirely with `None`. Will enforce
	/// `ExistentialDeposit` law, annulling the account as needed.
	///
	/// NOTE: Doesn't do any preparatory work for creating a new account, so should only be used
	/// when it is known that the account already exists.
	///
	/// NOTE: LOW-LEVEL: This will not attempt to maintain total issuance. It is expected that
	/// the caller will do this.
	pub fn mutate_account<R>(
		who: &T::AccountId,
		f: impl FnOnce(&mut AccountData<T::Balance>) -> R
	) -> Result<R, StoredMapError> {
		Self::try_mutate_account(who, |a, _| -> Result<R, StoredMapError> { Ok(f(a)) })
	}

	/// Mutate an account to some new value, or delete it entirely with `None`. Will enforce
	/// `ExistentialDeposit` law, annulling the account as needed. This will do nothing if the
	/// result of `f` is an `Err`.
	///
	/// NOTE: Doesn't do any preparatory work for creating a new account, so should only be used
	/// when it is known that the account already exists.
	///
	/// NOTE: LOW-LEVEL: This will not attempt to maintain total issuance. It is expected that
	/// the caller will do this.
	fn try_mutate_account<R, E: From<StoredMapError>>(
		who: &T::AccountId,
		f: impl FnOnce(&mut AccountData<T::Balance>, bool) -> Result<R, E>
	) -> Result<R, E> {
		Self::try_mutate_account_with_dust(who, f)
			.map(|(result, dust_cleaner)| {
				drop(dust_cleaner);
				result
			})
	}

	/// Mutate an account to some new value, or delete it entirely with `None`. Will enforce
	/// `ExistentialDeposit` law, annulling the account as needed. This will do nothing if the
	/// result of `f` is an `Err`.
	///
	/// It returns both the result from the closure, and an optional `DustCleaner` instance which
	/// should be dropped once it is known that all nested mutates that could affect storage items
	/// what the dust handler touches have completed.
	///
	/// NOTE: Doesn't do any preparatory work for creating a new account, so should only be used
	/// when it is known that the account already exists.
	///
	/// NOTE: LOW-LEVEL: This will not attempt to maintain total issuance. It is expected that
	/// the caller will do this.
	fn try_mutate_account_with_dust<R, E: From<StoredMapError>>(
		who: &T::AccountId,
		f: impl FnOnce(&mut AccountData<T::Balance>, bool) -> Result<R, E>
	) -> Result<(R, DustCleaner<T, I>), E> {
		let result = T::AccountStore::try_mutate_exists(who, |maybe_account| {
			let is_new = maybe_account.is_none();
			let mut account = maybe_account.take().unwrap_or_default();
			f(&mut account, is_new).map(move |result| {
				let maybe_endowed = if is_new { Some(account.free) } else { None };
				let maybe_account_maybe_dust = Self::post_mutation(who, account);
				*maybe_account = maybe_account_maybe_dust.0;
				(maybe_endowed, maybe_account_maybe_dust.1, result)
			})
		});
		result.map(|(maybe_endowed, maybe_dust, result)| {
			if let Some(endowed) = maybe_endowed {
				Self::deposit_event(Event::Endowed(who.clone(), endowed));
			}
			let dust_cleaner = DustCleaner(maybe_dust.map(|dust| (who.clone(), dust)));
			(result, dust_cleaner)
		})
	}

	/// Update the account entry for `who`, given the locks.
	fn update_locks(who: &T::AccountId, locks: &[BalanceLock<T::Balance>]) {
		if locks.len() as u32 > T::MaxLocks::get() {
			log::warn!(
				target: "runtime::balances",
				"Warning: A user has more currency locks than expected. \
				A runtime configuration adjustment may be needed."
			);
		}
		// No way this can fail since we do not alter the existential balances.
		let res = Self::mutate_account(who, |b| {
			b.misc_frozen = Zero::zero();
			b.fee_frozen = Zero::zero();
			for l in locks.iter() {
				if l.reasons == Reasons::All || l.reasons == Reasons::Misc {
					b.misc_frozen = b.misc_frozen.max(l.amount);
				}
				if l.reasons == Reasons::All || l.reasons == Reasons::Fee {
					b.fee_frozen = b.fee_frozen.max(l.amount);
				}
			}
		});
		debug_assert!(res.is_ok());

		let existed = Locks::<T, I>::contains_key(who);
		if locks.is_empty() {
			Locks::<T, I>::remove(who);
			if existed {
				// TODO: use Locks::<T, I>::hashed_key
				// https://github.com/paritytech/substrate/issues/4969
				system::Pallet::<T>::dec_consumers(who);
			}
		} else {
			Locks::<T, I>::insert(who, locks);
			if !existed {
				if system::Pallet::<T>::inc_consumers(who).is_err() {
					// No providers for the locks. This is impossible under normal circumstances
					// since the funds that are under the lock will themselves be stored in the
					// account and therefore will need a reference.
					log::warn!(
						target: "runtime::balances",
						"Warning: Attempt to introduce lock consumer reference, yet no providers. \
						This is unexpected but should be safe."
					);
				}
			}
		}
	}
}

use frame_support::traits::tokens::{fungible, DepositConsequence, WithdrawConsequence};

impl<T: Config<I>, I: 'static> fungible::Inspect<T::AccountId> for Pallet<T, I> {
	type Balance = T::Balance;

	fn total_issuance() -> Self::Balance {
		TotalIssuance::<T, I>::get()
	}
	fn minimum_balance() -> Self::Balance {
		T::ExistentialDeposit::get()
	}
	fn balance(who: &T::AccountId) -> Self::Balance {
		Self::account(who).total()
	}
	fn can_deposit(who: &T::AccountId, amount: Self::Balance) -> DepositConsequence {
		Self::deposit_consequence(who, amount, &Self::account(who))
	}
	fn can_withdraw(who: &T::AccountId, amount: Self::Balance) -> WithdrawConsequence<Self::Balance> {
		Self::withdraw_consequence(who, amount, &Self::account(who))
	}
}

impl<T: Config<I>, I: 'static> fungible::Mutate<T::AccountId> for Pallet<T, I> {
	fn deposit(who: &T::AccountId, amount: Self::Balance) -> DispatchResult {
		if amount.is_zero() { return Ok(()) }
		Self::try_mutate_account(who, |account, _is_new| -> DispatchResult {
			Self::deposit_consequence(who, amount, &account).into_result()?;
			account.free += amount;
			Ok(())
		})?;
		TotalIssuance::<T, I>::mutate(|t| *t += amount);
		Ok(())
	}

	fn withdraw(who: &T::AccountId, amount: Self::Balance) -> Result<Self::Balance, DispatchError> {
		if amount.is_zero() { return Ok(Self::Balance::zero()); }

		let actual = Self::try_mutate_account(who, |account, _is_new| -> Result<T::Balance, DispatchError> {
			let extra = Self::withdraw_consequence(who, amount, &account).into_result()?;
			let actual = amount + extra;
			account.free -= actual;
			Ok(actual)
		})?;
		TotalIssuance::<T, I>::mutate(|t| *t -= actual);
		Ok(actual)
	}
}

impl<T: Config<I>, I: 'static> fungible::Transfer<T::AccountId> for Pallet<T, I> {
	fn transfer(
		source: &T::AccountId,
		dest: &T::AccountId,
		amount: T::Balance,
	) -> Result<T::Balance, DispatchError> {
		<Self as fungible::Mutate::<T::AccountId>>::transfer(source, dest, amount)
	}
}

impl<T: Config<I>, I: 'static> fungible::Unbalanced<T::AccountId> for Pallet<T, I> {
	fn set_balance(who: &T::AccountId, amount: Self::Balance) -> DispatchResult {
		Self::mutate_account(who, |account| account.free = amount)?;
		Ok(())
	}

	fn set_total_issuance(amount: Self::Balance) {
		TotalIssuance::<T, I>::mutate(|t| *t = amount);
	}
}

// wrapping these imbalances in a private module is necessary to ensure absolute privacy
// of the inner member.
mod imbalances {
	use super::{
		result, Imbalance, Config, Zero, Saturating,
		TryDrop, RuntimeDebug,
	};
	use sp_std::mem;
	use frame_support::traits::SameOrOther;

	/// Opaque, move-only struct with private fields that serves as a token denoting that
	/// funds have been created without any equal and opposite accounting.
	#[must_use]
	#[derive(RuntimeDebug, PartialEq, Eq)]
	pub struct PositiveImbalance<T: Config<I>, I: 'static = ()>(T::Balance);

	impl<T: Config<I>, I: 'static> PositiveImbalance<T, I> {
		/// Create a new positive imbalance from a balance.
		pub fn new(amount: T::Balance) -> Self {
			PositiveImbalance(amount)
		}
	}

	/// Opaque, move-only struct with private fields that serves as a token denoting that
	/// funds have been destroyed without any equal and opposite accounting.
	#[must_use]
	#[derive(RuntimeDebug, PartialEq, Eq)]
	pub struct NegativeImbalance<T: Config<I>, I: 'static = ()>(T::Balance);

	impl<T: Config<I>, I: 'static> NegativeImbalance<T, I> {
		/// Create a new negative imbalance from a balance.
		pub fn new(amount: T::Balance) -> Self {
			NegativeImbalance(amount)
		}
	}

	impl<T: Config<I>, I: 'static> TryDrop for PositiveImbalance<T, I> {
		fn try_drop(self) -> result::Result<(), Self> {
			self.drop_zero()
		}
	}

	impl<T: Config<I>, I: 'static> Default for PositiveImbalance<T, I> {
		fn default() -> Self {
			Self::zero()
		}
	}

	impl<T: Config<I>, I: 'static> Imbalance<T::Balance> for PositiveImbalance<T, I> {
		type Opposite = NegativeImbalance<T, I>;

		fn zero() -> Self {
			Self(Zero::zero())
		}