Do nothing for zero imbalances (#4089)

substrate/srml/support/src/traits.rs

@@ -132,13 +132,19 @@ pub trait KeyOwnerProofSystem<Key> {
 ///
 /// - Someone got slashed.
 /// - Someone paid for a transaction to be included.
-pub trait OnUnbalanced<Imbalance> {
+pub trait OnUnbalanced<Imbalance: TryDrop> {
 	/// Handler for some imbalance. Infallible.
-	fn on_unbalanced(amount: Imbalance);
+	fn on_unbalanced(amount: Imbalance) {
+		amount.try_drop().unwrap_or_else(Self::on_nonzero_unbalanced)
+	}
+
+	/// Actually handle a non-zero imbalance. You probably want to implement this rather than
+	/// `on_unbalanced`.
+	fn on_nonzero_unbalanced(amount: Imbalance);
 }
 
-impl<Imbalance> OnUnbalanced<Imbalance> for () {
-	fn on_unbalanced(amount: Imbalance) { drop(amount); }
+impl<Imbalance: TryDrop> OnUnbalanced<Imbalance> for () {
+	fn on_nonzero_unbalanced(amount: Imbalance) { drop(amount); }
 }
 
 /// Simple boolean for whether an account needs to be kept in existence.
@@ -153,6 +159,12 @@ pub enum ExistenceRequirement {
 	AllowDeath,
 }
 
+/// A type for which some values make sense to be able to drop without further consideration.
+pub trait TryDrop: Sized {
+	/// Drop an instance cleanly. Only works if its value represents "no-operation".
+	fn try_drop(self) -> Result<(), Self>;
+}
+
 /// A trait for a not-quite Linear Type that tracks an imbalance.
 ///
 /// Functions that alter account balances return an object of this trait to
@@ -182,14 +194,14 @@ pub enum ExistenceRequirement {
 ///
 /// You can always retrieve the raw balance value using `peek`.
 #[must_use]
-pub trait Imbalance<Balance>: Sized {
+pub trait Imbalance<Balance>: Sized + TryDrop {
 	/// The oppositely imbalanced type. They come in pairs.
 	type Opposite: Imbalance<Balance>;
 
 	/// The zero imbalance. Can be destroyed with `drop_zero`.
 	fn zero() -> Self;
 
-	/// Drop an instance cleanly. Only works if its `value()` is zero.
+	/// Drop an instance cleanly. Only works if its `self.value()` is zero.
 	fn drop_zero(self) -> Result<(), Self>;
 
 	/// Consume `self` and return two independent instances; the first
@@ -297,7 +309,7 @@ impl<
 	Target2: OnUnbalanced<I>,
 > OnUnbalanced<I> for SplitTwoWays<Balance, I, Part1, Target1, Part2, Target2>
 {
-	fn on_unbalanced(amount: I) {
+	fn on_nonzero_unbalanced(amount: I) {
 		let total: u32 = Part1::VALUE + Part2::VALUE;
 		let amount1 = amount.peek().saturating_mul(Part1::VALUE.into()) / total.into();
 		let (imb1, imb2) = amount.split(amount1);