feature: add transferrable balance to currency trait

substrate/frame/balances/src/impl_currency.rs

@@ -25,7 +25,12 @@ use frame_support::{
 	ensure,
 	pallet_prelude::DispatchResult,
 	traits::{
-		tokens::{fungible, BalanceStatus as Status, Fortitude::Polite, Precision::BestEffort},
+		tokens::{
+			fungible, BalanceStatus as Status,
+			Fortitude::Polite,
+			Precision::BestEffort,
+			Preservation::{self, Preserve, Protect},
+		},
 		Currency, DefensiveSaturating, ExistenceRequirement,
 		ExistenceRequirement::AllowDeath,
 		Get, Imbalance, LockIdentifier, LockableCurrency, NamedReservableCurrency,
@@ -487,6 +492,24 @@ where
 		)
 		.unwrap_or_else(|_| SignedImbalance::Positive(Self::PositiveImbalance::zero()))
 	}
+
+	fn transferrable_balance(who: &T::AccountId, preservation: Preservation) -> Self::Balance {
+		let a = Self::account(who);
+		let mut untouchable = a.frozen;
+		// If we want to keep our provider ref..
+		if preservation == Preserve
+			// ..or we don't want the account to die and our provider ref is needed for it to live..
+			|| preservation == Protect && !a.free.is_zero() &&
+				frame_system::Pallet::<T>::providers(who) == 1
+			// ..or we don't care about the account dying but our provider ref is required..
+			|| preservation == Expendable && !a.free.is_zero() &&
+				!frame_system::Pallet::<T>::can_dec_provider(who)
+		{
+			// ..then the ED needed..
+			untouchable = untouchable.max(T::ExistentialDeposit::get());
+		}
+		a.free.saturating_sub(untouchable)
+	}
 }
 
 impl<T: Config<I>, I: 'static> ReservableCurrency<T::AccountId> for Pallet<T, I>

substrate/frame/support/src/traits/tokens/currency.rs

@@ -19,7 +19,7 @@
 
 use super::{
 	imbalance::{Imbalance, SignedImbalance},
-	misc::{Balance, ExistenceRequirement, WithdrawReasons},
+	misc::{Balance, ExistenceRequirement, Preservation, WithdrawReasons},
 };
 use crate::{dispatch::DispatchResult, traits::Get};
 use sp_runtime::{traits::MaybeSerializeDeserialize, DispatchError};
@@ -207,6 +207,12 @@ pub trait Currency<AccountId> {
 		who: &AccountId,
 		balance: Self::Balance,
 	) -> SignedImbalance<Self::Balance, Self::PositiveImbalance>;
+
+	/// 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
+	/// reduction and potentially go below user-level restrictions on the minimum amount of the
+	/// account.
+	fn transferrable_balance(who: &AccountId, preservation: Preservation) -> Self::Balance;
 }
 
 /// A non-const `Get` implementation parameterised by a `Currency` impl which provides the result
@@ -313,4 +319,7 @@ impl<AccountId> Currency<AccountId> for () {
 	) -> SignedImbalance<Self::Balance, Self::PositiveImbalance> {
 		SignedImbalance::Positive(())
 	}
+	fn transferrable_balance(_: &AccountId, _: Preservation) -> u32 {
+		0
+	}
 }

substrate/frame/support/src/traits/tokens/currency/reservable.rs

@@ -23,7 +23,7 @@ use sp_core::Get;
 use super::{super::misc::BalanceStatus, Currency};
 use crate::{
 	dispatch::DispatchResult,
-	traits::{ExistenceRequirement, SignedImbalance, WithdrawReasons},
+	traits::{tokens::Preservation, ExistenceRequirement, SignedImbalance, WithdrawReasons},
 };
 use sp_runtime::DispatchError;
 
@@ -338,6 +338,9 @@ impl<
 	) -> SignedImbalance<Self::Balance, Self::PositiveImbalance> {
 		NamedReservable::make_free_balance_be(who, balance)
 	}
+	fn transferrable_balance(who: &AccountId, preservation: Preservation) -> Self::Balance {
+		NamedReservable::transferrable_balance(who, preservation)
+	}
 }
 impl<
 		NamedReservable: NamedReservableCurrency<AccountId>,