Add an OnUnbalanced hook for contract rent payments (#3857)

* Add RentPayment trait to runtime

* clarify check

* improve proof

* Clarify further

* Simplify RentPayment::on_unbalance calling and get rid of NonZeroRentHook

substrate/srml/contracts/src/lib.rs

@@ -355,6 +355,9 @@ pub trait Trait: system::Trait {
 	/// Handler for the unbalanced reduction when making a gas payment.
 	type GasPayment: OnUnbalanced<NegativeImbalanceOf<Self>>;
 
+	/// Handler for rent payments.
+	type RentPayment: OnUnbalanced<NegativeImbalanceOf<Self>>;
+
 	/// Number of block delay an extrinsic claim surcharge has.
 	///
 	/// When claim surcharge is called by an extrinsic the rent is checked

substrate/srml/contracts/src/rent.rs

@@ -17,7 +17,7 @@
 use crate::{BalanceOf, ContractInfo, ContractInfoOf, TombstoneContractInfo, Trait, AliveContractInfo};
 use sr_primitives::traits::{Bounded, CheckedDiv, CheckedMul, Saturating, Zero,
 	SaturatedConversion};
-use support::traits::{Currency, ExistenceRequirement, Get, WithdrawReason};
+use support::traits::{Currency, ExistenceRequirement, Get, WithdrawReason, OnUnbalanced};
 use support::StorageMap;
 
 #[derive(PartialEq, Eq, Copy, Clone)]
@@ -126,7 +126,7 @@ fn try_evict_or_and_pay_rent<T: Trait>(
 
 	if can_withdraw_rent && (insufficient_rent || pay_rent) {
 		// Collect dues.
-		let _ = T::Currency::withdraw(
+		let imbalance = T::Currency::withdraw(
 			account,
 			dues_limited,
 			WithdrawReason::Fee,
@@ -137,6 +137,8 @@ fn try_evict_or_and_pay_rent<T: Trait>(
 			dues_limited < rent_budget < balance - subsistence < balance - existential_deposit;
 			qed",
 		);
+
+		T::RentPayment::on_unbalanced(imbalance);
 	}
 
 	if insufficient_rent || !can_withdraw_rent {

substrate/srml/contracts/src/tests.rs

@@ -159,6 +159,7 @@ impl Trait for Test {
 	type ComputeDispatchFee = DummyComputeDispatchFee;
 	type TrieIdGenerator = DummyTrieIdGenerator;
 	type GasPayment = ();
+	type RentPayment = ();
 	type SignedClaimHandicap = SignedClaimHandicap;
 	type TombstoneDeposit = TombstoneDeposit;
 	type StorageSizeOffset = StorageSizeOffset;