[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/balances/src/tests/currency_tests.rs

@@ -18,14 +18,18 @@
 //! Tests regarding the functionality of the `Currency` trait set implementations.
 
 use super::*;
-use crate::NegativeImbalance;
-use frame_support::traits::{
-	BalanceStatus::{Free, Reserved},
-	Currency,
-	ExistenceRequirement::{self, AllowDeath, KeepAlive},
-	Hooks, LockIdentifier, LockableCurrency, NamedReservableCurrency, ReservableCurrency,
-	WithdrawReasons,
+use crate::{Event, NegativeImbalance};
+use frame_support::{
+	traits::{
+		BalanceStatus::{Free, Reserved},
+		Currency,
+		ExistenceRequirement::{self, AllowDeath, KeepAlive},
+		Hooks, LockIdentifier, LockableCurrency, NamedReservableCurrency, ReservableCurrency,
+		WithdrawReasons,
+	},
+	StorageNoopGuard,
 };
+use frame_system::Event as SysEvent;
 
 const ID_1: LockIdentifier = *b"1       ";
 const ID_2: LockIdentifier = *b"2       ";
@@ -1363,3 +1367,39 @@ fn freezing_and_locking_should_work() {
 			assert_eq!(System::consumers(&1), 0);
 		});
 }
+
+#[test]
+fn self_transfer_noop() {
+	ExtBuilder::default().existential_deposit(100).build_and_execute_with(|| {
+		assert_eq!(Balances::total_issuance(), 0);
+		let _ = Balances::deposit_creating(&1, 100);
+
+		// The account is set up properly:
+		assert_eq!(
+			events(),
+			[
+				Event::Deposit { who: 1, amount: 100 }.into(),
+				SysEvent::NewAccount { account: 1 }.into(),
+				Event::Endowed { account: 1, free_balance: 100 }.into(),
+			]
+		);
+		assert_eq!(Balances::free_balance(1), 100);
+		assert_eq!(Balances::total_issuance(), 100);
+
+		// Transfers to self are No-OPs:
+		let _g = StorageNoopGuard::new();
+		for i in 0..200 {
+			let r = Balances::transfer_allow_death(Some(1).into(), 1, i);
+
+			if i <= 100 {
+				assert_ok!(r);
+			} else {
+				assert!(r.is_err());
+			}
+
+			assert!(events().is_empty());
+			assert_eq!(Balances::free_balance(1), 100, "Balance unchanged by self transfer");
+			assert_eq!(Balances::total_issuance(), 100, "TI unchanged by self transfers");
+		}
+	});
+}