Do nothing for zero imbalances (#4089)

substrate/node/runtime/src/impls.rs

@@ -25,7 +25,7 @@ use crate::{Balances, System, Authorship, MaximumBlockWeight, NegativeImbalance}
 
 pub struct Author;
 impl OnUnbalanced<NegativeImbalance> for Author {
-	fn on_unbalanced(amount: NegativeImbalance) {
+	fn on_nonzero_unbalanced(amount: NegativeImbalance) {
 		Balances::resolve_creating(&Authorship::author(), amount);
 	}
 }

substrate/srml/balances/src/lib.rs

@@ -153,7 +153,7 @@ use codec::{Codec, Encode, Decode};
 use support::{
 	StorageValue, Parameter, decl_event, decl_storage, decl_module,
 	traits::{
-		UpdateBalanceOutcome, Currency, OnFreeBalanceZero, OnUnbalanced,
+		UpdateBalanceOutcome, Currency, OnFreeBalanceZero, OnUnbalanced, TryDrop,
 		WithdrawReason, WithdrawReasons, LockIdentifier, LockableCurrency, ExistenceRequirement,
 		Imbalance, SignedImbalance, ReservableCurrency, Get,
 	},
@@ -603,7 +603,7 @@ impl<T: Trait<I>, I: Instance> Module<T, I> {
 mod imbalances {
 	use super::{
 		result, Subtrait, DefaultInstance, Imbalance, Trait, Zero, Instance, Saturating,
-		StorageValue,
+		StorageValue, TryDrop,
 	};
 	use rstd::mem;
 
@@ -631,6 +631,12 @@ mod imbalances {
 		}
 	}
 
+	impl<T: Trait<I>, I: Instance> TryDrop for PositiveImbalance<T, I> {
+		fn try_drop(self) -> result::Result<(), Self> {
+			self.drop_zero()
+		}
+	}
+
 	impl<T: Trait<I>, I: Instance> Imbalance<T::Balance> for PositiveImbalance<T, I> {
 		type Opposite = NegativeImbalance<T, I>;
 
@@ -676,6 +682,12 @@ mod imbalances {
 		}
 	}
 
+	impl<T: Trait<I>, I: Instance> TryDrop for NegativeImbalance<T, I> {
+		fn try_drop(self) -> result::Result<(), Self> {
+			self.drop_zero()
+		}
+	}
+
 	impl<T: Trait<I>, I: Instance> Imbalance<T::Balance> for NegativeImbalance<T, I> {
 		type Opposite = PositiveImbalance<T, I>;
 

substrate/srml/generic-asset/src/lib.rs

@@ -155,7 +155,8 @@ use codec::{Decode, Encode, HasCompact, Input, Output, Error};
 
 use sr_primitives::RuntimeDebug;
 use sr_primitives::traits::{
-	CheckedAdd, CheckedSub, MaybeSerializeDeserialize, Member, One, Saturating, SimpleArithmetic, Zero, Bounded
+	CheckedAdd, CheckedSub, MaybeSerializeDeserialize, Member, One, Saturating, SimpleArithmetic,
+	Zero, Bounded,
 };
 
 use rstd::prelude::*;
@@ -165,7 +166,7 @@ use support::{
 	decl_event, decl_module, decl_storage, ensure,
 	traits::{
 		Currency, ExistenceRequirement, Imbalance, LockIdentifier, LockableCurrency, ReservableCurrency,
-		SignedImbalance, UpdateBalanceOutcome, WithdrawReason, WithdrawReasons,
+		SignedImbalance, UpdateBalanceOutcome, WithdrawReason, WithdrawReasons, TryDrop,
 	},
 	Parameter, StorageMap,
 };
@@ -862,7 +863,9 @@ pub trait AssetIdProvider {
 // wrapping these imbalanes in a private module is necessary to ensure absolute privacy
 // of the inner member.
 mod imbalances {
-	use super::{result, AssetIdProvider, Imbalance, Saturating, StorageMap, Subtrait, Zero};
+	use super::{
+		result, AssetIdProvider, Imbalance, Saturating, StorageMap, Subtrait, Zero, TryDrop
+	};
 	use rstd::mem;
 
 	/// Opaque, move-only struct with private fields that serves as a token denoting that
@@ -899,6 +902,16 @@ mod imbalances {
 		}
 	}
 
+	impl<T, U> TryDrop for PositiveImbalance<T, U>
+	where
+		T: Subtrait,
+		U: AssetIdProvider<AssetId = T::AssetId>,
+	{
+		fn try_drop(self) -> result::Result<(), Self> {
+			self.drop_zero()
+		}
+	}
+
 	impl<T, U> Imbalance<T::Balance> for PositiveImbalance<T, U>
 	where
 		T: Subtrait,
@@ -948,6 +961,16 @@ mod imbalances {
 		}
 	}
 
+	impl<T, U> TryDrop for NegativeImbalance<T, U>
+	where
+		T: Subtrait,
+		U: AssetIdProvider<AssetId = T::AssetId>,
+	{
+		fn try_drop(self) -> result::Result<(), Self> {
+			self.drop_zero()
+		}
+	}
+
 	impl<T, U> Imbalance<T::Balance> for NegativeImbalance<T, U>
 	where
 		T: Subtrait,

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);

substrate/srml/treasury/src/lib.rs

@@ -337,7 +337,7 @@ impl<T: Trait> Module<T> {
 }
 
 impl<T: Trait> OnUnbalanced<NegativeImbalanceOf<T>> for Module<T> {
-	fn on_unbalanced(amount: NegativeImbalanceOf<T>) {
+	fn on_nonzero_unbalanced(amount: NegativeImbalanceOf<T>) {
 		let numeric_amount = amount.peek();
 
 		// Must resolve into existing but better to be safe.