[FRAME] Short-circuit fungible self transfer (#2118)

Changes:
- Change the fungible(s) logic to treat a self-transfer as No-OP (as
long as all pre-checks pass).

Note that the self-transfer case will not emit an event since no state
was changed.

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

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

@@ -219,7 +219,7 @@ impl<
 impl<
 		F: fungibles::Mutate<AccountId>,
 		A: Get<<F as fungibles::Inspect<AccountId>>::AssetId>,
-		AccountId,
+		AccountId: Eq,
 	> Mutate<AccountId> for ItemOf<F, A, AccountId>
 {
 	fn mint_into(who: &AccountId, amount: Self::Balance) -> Result<Self::Balance, DispatchError> {

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

@@ -235,7 +235,10 @@ pub trait Unbalanced<AccountId>: Inspect<AccountId> {
 }
 
 /// Trait for providing a basic fungible asset.
-pub trait Mutate<AccountId>: Inspect<AccountId> + Unbalanced<AccountId> {
+pub trait Mutate<AccountId>: Inspect<AccountId> + Unbalanced<AccountId>
+where
+	AccountId: Eq,
+{
 	/// Increase the balance of `who` by exactly `amount`, minting new tokens. If that isn't
 	/// possible then an `Err` is returned and nothing is changed.
 	fn mint_into(who: &AccountId, amount: Self::Balance) -> Result<Self::Balance, DispatchError> {
@@ -303,6 +306,9 @@ pub trait Mutate<AccountId>: Inspect<AccountId> + Unbalanced<AccountId> {
 	}
 
 	/// Transfer funds from one account into another.
+	///
+	/// A transfer where the source and destination account are identical is treated as No-OP after
+	/// checking the preconditions.
 	fn transfer(
 		source: &AccountId,
 		dest: &AccountId,
@@ -311,6 +317,10 @@ pub trait Mutate<AccountId>: Inspect<AccountId> + Unbalanced<AccountId> {
 	) -> Result<Self::Balance, DispatchError> {
 		let _extra = Self::can_withdraw(source, amount).into_result(preservation != Expendable)?;
 		Self::can_deposit(dest, amount, Extant).into_result()?;
+		if source == dest {
+			return Ok(amount)
+		}
+
 		Self::decrease_balance(source, amount, BestEffort, preservation, Polite)?;
 		// This should never fail as we checked `can_deposit` earlier. But we do a best-effort
 		// anyway.

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

@@ -250,7 +250,10 @@ pub trait Unbalanced<AccountId>: Inspect<AccountId> {
 }
 
 /// Trait for providing a basic fungible asset.
-pub trait Mutate<AccountId>: Inspect<AccountId> + Unbalanced<AccountId> {
+pub trait Mutate<AccountId>: Inspect<AccountId> + Unbalanced<AccountId>
+where
+	AccountId: Eq,
+{
 	/// Increase the balance of `who` by exactly `amount`, minting new tokens. If that isn't
 	/// possible then an `Err` is returned and nothing is changed.
 	fn mint_into(
@@ -353,6 +356,9 @@ pub trait Mutate<AccountId>: Inspect<AccountId> + Unbalanced<AccountId> {
 	}
 
 	/// Transfer funds from one account into another.
+	///
+	/// A transfer where the source and destination account are identical is treated as No-OP after
+	/// checking the preconditions.
 	fn transfer(
 		asset: Self::AssetId,
 		source: &AccountId,
@@ -363,6 +369,10 @@ pub trait Mutate<AccountId>: Inspect<AccountId> + Unbalanced<AccountId> {
 		let _extra = Self::can_withdraw(asset.clone(), source, amount)
 			.into_result(preservation != Expendable)?;
 		Self::can_deposit(asset.clone(), dest, amount, Extant).into_result()?;
+		if source == dest {
+			return Ok(amount)
+		}
+
 		Self::decrease_balance(asset.clone(), source, amount, BestEffort, preservation, Polite)?;
 		// This should never fail as we checked `can_deposit` earlier. But we do a best-effort
 		// anyway.

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

@@ -82,8 +82,13 @@ pub enum PaymentStatus {
 }
 
 /// Simple implementation of `Pay` which makes a payment from a "pot" - i.e. a single account.
-pub struct PayFromAccount<F, A>(sp_std::marker::PhantomData<(F, A)>);
-impl<A: TypedGet, F: fungible::Mutate<A::Type>> Pay for PayFromAccount<F, A> {
+pub struct PayFromAccount<F, A>(core::marker::PhantomData<(F, A)>);
+impl<A, F> Pay for PayFromAccount<F, A>
+where
+	A: TypedGet,
+	F: fungible::Mutate<A::Type>,
+	A::Type: Eq,
+{
 	type Balance = F::Balance;
 	type Beneficiary = A::Type;
 	type AssetKind = ();
@@ -110,11 +115,12 @@ impl<A: TypedGet, F: fungible::Mutate<A::Type>> Pay for PayFromAccount<F, A> {
 
 /// Simple implementation of `Pay` for assets which makes a payment from a "pot" - i.e. a single
 /// account.
-pub struct PayAssetFromAccount<F, A>(sp_std::marker::PhantomData<(F, A)>);
+pub struct PayAssetFromAccount<F, A>(core::marker::PhantomData<(F, A)>);
 impl<A, F> frame_support::traits::tokens::Pay for PayAssetFromAccount<F, A>
 where
 	A: TypedGet,
 	F: fungibles::Mutate<A::Type> + fungibles::Create<A::Type>,
+	A::Type: Eq,
 {
 	type Balance = F::Balance;
 	type Beneficiary = A::Type;