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/bin/node/runtime/src/lib.rs

@@ -601,6 +601,7 @@ impl pallet_treasury::Trait for Runtime {
 	type ProposalBondMinimum = ProposalBondMinimum;
 	type SpendPeriod = SpendPeriod;
 	type Burn = Burn;
+	type BurnDestination = ();
 	type WeightInfo = ();
 }
 

substrate/frame/society/src/lib.rs

@@ -264,11 +264,12 @@ use frame_support::{decl_error, decl_module, decl_storage, decl_event, ensure, d
 use frame_support::weights::Weight;
 use frame_support::traits::{
 	Currency, ReservableCurrency, Randomness, Get, ChangeMembers, BalanceStatus,
-	ExistenceRequirement::AllowDeath, EnsureOrigin
+	ExistenceRequirement::AllowDeath, EnsureOrigin, OnUnbalanced, Imbalance
 };
 use frame_system::{self as system, ensure_signed, ensure_root};
 
 type BalanceOf<T, I> = <<T as Trait<I>>::Currency as Currency<<T as system::Trait>::AccountId>>::Balance;
+type NegativeImbalanceOf<T> = <<T as Trait>::Currency as Currency<<T as frame_system::Trait>::AccountId>>::NegativeImbalance;
 
 /// The module's configuration trait.
 pub trait Trait<I=DefaultInstance>: system::Trait {
@@ -1143,6 +1144,8 @@ decl_event! {
 		NewMaxMembers(u32),
 		/// Society is unfounded.
 		Unfounded(AccountId),
+		/// Some funds were deposited into the society account.
+		Deposit(Balance),
 	}
 }
 
@@ -1665,3 +1668,14 @@ impl<T: Trait<I>, I: Instance> Module<T, I> {
 		}
 	}
 }
+
+impl<T: Trait> OnUnbalanced<NegativeImbalanceOf<T>> for Module<T> {
+	fn on_nonzero_unbalanced(amount: NegativeImbalanceOf<T>) {
+		let numeric_amount = amount.peek();
+
+		// Must resolve into existing but better to be safe.
+		let _ = T::Currency::resolve_creating(&Self::account_id(), amount);
+
+		Self::deposit_event(RawEvent::Deposit(numeric_amount));
+	}
+}

substrate/frame/support/src/traits.rs

@@ -897,6 +897,14 @@ pub trait Currency<AccountId> {
 	/// in the case of overflow.
 	fn issue(amount: Self::Balance) -> Self::NegativeImbalance;
 
+	/// Produce a pair of imbalances that cancel each other out exactly.
+	///
+	/// This is just the same as burning and issuing the same amount and has no effect on the
+	/// total issuance.
+	fn pair(amount: Self::Balance) -> (Self::PositiveImbalance, Self::NegativeImbalance) {
+		(Self::burn(amount.clone()), Self::issue(amount))
+	}
+
 	/// The 'free' balance of a given account.
 	///
 	/// This is the only balance that matters in terms of most operations on tokens. It alone

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>;

substrate/primitives/arithmetic/src/per_things.rs

@@ -1170,6 +1170,51 @@ macro_rules! implement_per_thing {
 	};
 }
 
+macro_rules! implement_per_thing_with_perthousand {
+	(
+		$name:ident,
+		$test_mod:ident,
+		$pt_test_mod:ident,
+		[$($test_units:tt),+],
+		$max:tt,
+		$type:ty,
+		$upper_type:ty,
+		$title:expr $(,)?
+	) => {
+		implement_per_thing! {
+			$name, $test_mod, [ $( $test_units ),+ ], $max, $type, $upper_type, $title,
+		}
+		impl $name {
+			/// Converts a percent into `Self`. Equal to `x / 1000`.
+			///
+			/// This can be created at compile time.
+			pub const fn from_perthousand(x: $type) -> Self {
+				Self(([x, 1000][(x > 1000) as usize] as $upper_type * $max as $upper_type / 1000) as $type)
+			}
+		}
+		#[cfg(test)]
+		mod $pt_test_mod {
+			use super::$name;
+			use crate::traits::Zero;
+
+			#[test]
+			fn from_perthousand_works() {
+				// some really basic stuff
+				assert_eq!($name::from_perthousand(00), $name::from_parts(Zero::zero()));
+				assert_eq!($name::from_perthousand(100), $name::from_parts($max / 10));
+				assert_eq!($name::from_perthousand(1000), $name::from_parts($max));
+				assert_eq!($name::from_perthousand(2000), $name::from_parts($max));
+			}
+
+			#[test]
+			#[allow(unused)]
+			fn const_fns_work() {
+				const C1: $name = $name::from_perthousand(500);
+			}
+		}
+	}
+}
+
 implement_per_thing!(
 	Percent,
 	test_per_cent,
@@ -1179,36 +1224,40 @@ implement_per_thing!(
 	u16,
 	"_Percent_",
 );
-implement_per_thing!(
+implement_per_thing_with_perthousand!(
 	PerU16,
 	test_peru16,
+	test_peru16_extra,
 	[u32, u64, u128],
 	65535_u16,
 	u16,
 	u32,
 	"_Parts per 65535_",
 );
-implement_per_thing!(
+implement_per_thing_with_perthousand!(
 	Permill,
 	test_permill,
+	test_permill_extra,
 	[u32, u64, u128],
 	1_000_000u32,
 	u32,
 	u64,
 	"_Parts per Million_",
 );
-implement_per_thing!(
+implement_per_thing_with_perthousand!(
 	Perbill,
 	test_perbill,
+	test_perbill_extra,
 	[u32, u64, u128],
 	1_000_000_000u32,
 	u32,
 	u64,
 	"_Parts per Billion_",
 );
-implement_per_thing!(
+implement_per_thing_with_perthousand!(
 	Perquintill,
 	test_perquintill,
+	test_perquintill_extra,
 	[u64, u128],
 	1_000_000_000_000_000_000u64,
 	u64,