Treasury burning can be directed (#6671)

* Treasury burning can be directed

Also, Society is a imbalance handler

* Build

* Introduce from_permill in perthings.

* Rename to from_perthousand to avoid confusion with Permill

* Fixes

substrate/frame/treasury/src/lib.rs

@@ -184,6 +184,9 @@ pub trait Trait: frame_system::Trait {
 	/// Percentage of spare funds (if any) that are burnt per spend period.
 	type Burn: Get<Permill>;
 
+	/// Handler for the unbalanced decrease when treasury funds are burned.
+	type BurnDestination: OnUnbalanced<NegativeImbalanceOf<Self>>;
+
 	/// Weight information for extrinsics in this pallet.
 	type WeightInfo: WeightInfo;
 }
@@ -771,7 +774,10 @@ impl<T: Trait> Module<T> {
 			// burn some proportion of the remaining budget if we run a surplus.
 			let burn = (T::Burn::get() * budget_remaining).min(budget_remaining);
 			budget_remaining -= burn;
-			imbalance.subsume(T::Currency::burn(burn));
+
+			let (debit, credit) = T::Currency::pair(burn);
+			imbalance.subsume(debit);
+			T::BurnDestination::on_unbalanced(credit);
 			Self::deposit_event(RawEvent::Burnt(burn))
 		}
 

substrate/frame/treasury/src/tests.rs

@@ -149,6 +149,7 @@ impl Trait for Test {
 	type ProposalBondMinimum = ProposalBondMinimum;
 	type SpendPeriod = SpendPeriod;
 	type Burn = Burn;
+	type BurnDestination = ();  // Just gets burned.
 	type WeightInfo = ();
 }
 type System = frame_system::Module<Test>;