extract amount method for fungible/s Imbalance (paritytech#1847)

Introduces an `extract` amount method for `fungible/s` `Imbalance`.

substrate/frame/balances/src/impl_currency.rs

@@ -100,6 +100,11 @@ mod imbalances {
 			mem::forget(self);
 			(Self(first), Self(second))
 		}
+		fn extract(&mut self, amount: T::Balance) -> Self {
+			let new = self.0.min(amount);
+			self.0 = self.0 - new;
+			Self(new)
+		}
 		fn merge(mut self, other: Self) -> Self {
 			self.0 = self.0.saturating_add(other.0);
 			mem::forget(other);
@@ -159,6 +164,11 @@ mod imbalances {
 			mem::forget(self);
 			(Self(first), Self(second))
 		}
+		fn extract(&mut self, amount: T::Balance) -> Self {
+			let new = self.0.min(amount);
+			self.0 = self.0 - new;
+			Self(new)
+		}
 		fn merge(mut self, other: Self) -> Self {
 			self.0 = self.0.saturating_add(other.0);
 			mem::forget(other);

substrate/frame/support/src/traits/tokens/fungible/imbalance.rs

@@ -113,6 +113,13 @@ impl<B: Balance, OnDrop: HandleImbalanceDrop<B>, OppositeOnDrop: HandleImbalance
 		sp_std::mem::forget(self);
 		(Imbalance::new(first), Imbalance::new(second))
 	}
+
+	fn extract(&mut self, amount: B) -> Self {
+		let new = self.amount.min(amount);
+		self.amount = self.amount - new;
+		Imbalance::new(new)
+	}
+
 	fn merge(mut self, other: Self) -> Self {
 		self.amount = self.amount.saturating_add(other.amount);
 		sp_std::mem::forget(other);

substrate/frame/support/src/traits/tokens/fungibles/imbalance.rs

@@ -109,6 +109,15 @@ impl<
 		sp_std::mem::forget(self);
 		(Imbalance::new(asset.clone(), first), Imbalance::new(asset, second))
 	}
+
+	/// Mutate `self` by extracting a new instance with at most `amount` value, reducing `self`
+	/// accordingly.
+	pub fn extract(&mut self, amount: B) -> Self {
+		let new = self.amount.min(amount);
+		self.amount = self.amount - new;
+		Imbalance::new(self.asset.clone(), new)
+	}
+
 	pub fn merge(mut self, other: Self) -> Result<Self, (Self, Self)> {
 		if self.asset == other.asset {
 			self.amount = self.amount.saturating_add(other.amount);

substrate/frame/support/src/traits/tokens/imbalance.rs

@@ -72,6 +72,10 @@ pub trait Imbalance<Balance>: Sized + TryDrop + Default {
 	/// is guaranteed to be at most `amount` and the second will be the remainder.
 	fn split(self, amount: Balance) -> (Self, Self);
 
+	/// Mutate `self` by extracting a new instance with at most `amount` value, reducing `self`
+	/// accordingly.
+	fn extract(&mut self, amount: Balance) -> Self;
+
 	/// Consume `self` and return two independent instances; the amounts returned will be in
 	/// approximately the same ratio as `first`:`second`.
 	///
@@ -190,6 +194,9 @@ impl<Balance: Default> Imbalance<Balance> for () {
 	fn split(self, _: Balance) -> (Self, Self) {
 		((), ())
 	}
+	fn extract(&mut self, _: Balance) -> Self {
+		()
+	}
 	fn ration(self, _: u32, _: u32) -> (Self, Self)
 	where
 		Balance: From<u32> + Saturating + Div<Output = Balance>,