Add ExistenceRequirement to Currency trait

srml/balances/src/lib.rs

@@ -464,6 +464,23 @@ decl_module! {
 			let dest = T::Lookup::lookup(dest)?;
 			<Self as Currency<_>>::transfer(&source, &dest, value)?;
 		}
+
+		/// Transfer some liquid free balance to another account, while checking that the transfer
+		/// will not kill the account.
+		///
+		/// 99% of the time you want `transfer` instead.
+		#[weight = SimpleDispatchInfo::FixedNormal(1_000_000)]
+		pub fn transfer_some(
+			origin,
+			dest: <T::Lookup as StaticLookup>::Source,
+			#[compact] value: T::Balance
+		) {
+			let transactor = ensure_signed(origin)?;
+			let dest = T::Lookup::lookup(dest)?;
+
+			Self::transfer_inner(&transactor, &dest, value, ExistenceRequirement::KeepAlive)?;
+		}
+
 	}
 }
 
@@ -852,42 +869,7 @@ where
 	}
 
 	fn transfer(transactor: &T::AccountId, dest: &T::AccountId, value: Self::Balance) -> Result {
-		let from_balance = Self::free_balance(transactor);
-		let to_balance = Self::free_balance(dest);
-		let would_create = to_balance.is_zero();
-		let fee = if would_create { T::CreationFee::get() } else { T::TransferFee::get() };
-		let liability = match value.checked_add(&fee) {
-			Some(l) => l,
-			None => return Err("got overflow after adding a fee to value"),
-		};
-
-		let new_from_balance = match from_balance.checked_sub(&liability) {
-			None => return Err("balance too low to send value"),
-			Some(b) => b,
-		};
-		if would_create && value < T::ExistentialDeposit::get() {
-			return Err("value too low to create account");
-		}
-		Self::ensure_can_withdraw(transactor, value, WithdrawReason::Transfer.into(), new_from_balance)?;
-
-		// NOTE: total stake being stored in the same type means that this could never overflow
-		// but better to be safe than sorry.
-		let new_to_balance = match to_balance.checked_add(&value) {
-			Some(b) => b,
-			None => return Err("destination balance too high to receive value"),
-		};
-
-		if transactor != dest {
-			Self::set_free_balance(transactor, new_from_balance);
-			if !<FreeBalance<T, I>>::exists(dest) {
-				Self::new_account(dest, new_to_balance);
-			}
-			Self::set_free_balance(dest, new_to_balance);
-			T::TransferPayment::on_unbalanced(NegativeImbalance::new(fee));
-			Self::deposit_event(RawEvent::Transfer(transactor.clone(), dest.clone(), value, fee));
-		}
-
-		Ok(())
+		Self::transfer_inner(transactor, dest, value, ExistenceRequirement::AllowDeath)
 	}
 
 	fn withdraw(
@@ -1151,3 +1133,55 @@ where
 		Self::total_balance(who).is_zero()
 	}
 }
+
+impl<T: Trait<I>, I: Instance> Module<T, I> {
+	fn transfer_inner(
+		transactor: &T::AccountId,
+		dest: &T::AccountId,
+		value: T::Balance,
+		existential_requirement: ExistenceRequirement,
+	) -> Result {
+		let from_balance = Self::free_balance(transactor);
+		let to_balance = Self::free_balance(dest);
+		let would_create = to_balance.is_zero();
+		let fee = if would_create { T::CreationFee::get() } else { T::TransferFee::get() };
+		let liability = match value.checked_add(&fee) {
+			Some(l) => l,
+			None => return Err("got overflow after adding a fee to value"),
+		};
+
+		let new_from_balance = match from_balance.checked_sub(&liability) {
+			None => return Err("balance too low to send value"),
+			Some(b) => b,
+		};
+		if would_create && value < T::ExistentialDeposit::get() {
+			return Err("value too low to create account");
+		}
+		Self::ensure_can_withdraw(transactor, value, WithdrawReason::Transfer.into(), new_from_balance)?;
+
+		// NOTE: total stake being stored in the same type means that this could never overflow
+		// but better to be safe than sorry.
+		let new_to_balance = match to_balance.checked_add(&value) {
+			Some(b) => b,
+			None => return Err("destination balance too high to receive value"),
+		};
+
+		if transactor != dest {
+			if existential_requirement == ExistenceRequirement::KeepAlive {
+				if new_from_balance < Self::minimum_balance() {
+					return Err("payment would kill account");
+				}
+			}
+
+			Self::set_free_balance(transactor, new_from_balance);
+			if !<FreeBalance<T, I>>::exists(dest) {
+				Self::new_account(dest, new_to_balance);
+			}
+			Self::set_free_balance(dest, new_to_balance);
+			T::TransferPayment::on_unbalanced(NegativeImbalance::new(fee));
+			Self::deposit_event(RawEvent::Transfer(transactor.clone(), dest.clone(), value, fee));
+		}
+
+		Ok(())
+	}
+}