refactor: maintain storage layout compability between ZRC20 v1 and v2

v2/src/zevm/GatewayZEVM.sol

@@ -118,7 +118,7 @@ contract GatewayZEVM is
     /// @param zrc20 The address of the ZRC20 token.
     function withdraw(bytes memory receiver, uint256 amount, address zrc20) external nonReentrant whenNotPaused {
         uint256 gasFee = _withdrawZRC20(amount, zrc20);
-        emit Withdrawal(msg.sender, 0, receiver, zrc20, amount, gasFee, IZRC20(zrc20).protocolFlatFee(), "");
+        emit Withdrawal(msg.sender, 0, receiver, zrc20, amount, gasFee, IZRC20(zrc20).PROTOCOL_FLAT_FEE(), "");
     }
 
     /// @notice Withdraw ZRC20 tokens and call a smart contract on an external chain.
@@ -137,7 +137,7 @@ contract GatewayZEVM is
         whenNotPaused
     {
         uint256 gasFee = _withdrawZRC20(amount, zrc20);
-        emit Withdrawal(msg.sender, 0, receiver, zrc20, amount, gasFee, IZRC20(zrc20).protocolFlatFee(), message);
+        emit Withdrawal(msg.sender, 0, receiver, zrc20, amount, gasFee, IZRC20(zrc20).PROTOCOL_FLAT_FEE(), message);
     }
 
     /// @notice Withdraw ZETA tokens to an external chain.

v2/src/zevm/ZRC20.sol

@@ -29,13 +29,14 @@ contract ZRC20 is IZRC20Metadata, ZRC20Errors, ZRC20Events {
     /// @notice Coin type, checkout Interfaces.sol.
     CoinType public immutable COIN_TYPE;
     /// @notice System contract address.
-    address public systemContractAddress;
-    /// @notice Gateway contract address.
-    address public gatewayAddress;
+    /// @dev Name is in upper case to maintain compatibility with ZRC20.sol v1
+    address public SYSTEM_CONTRACT_ADDRESS;
     /// @notice Gas limit.
-    uint256 public gasLimit;
+    /// @dev Name is in upper case to maintain compatibility with ZRC20.sol v1
+    uint256 public GAS_LIMIT;
     /// @notice Protocol flat fee.
-    uint256 public override protocolFlatFee;
+    /// @dev Name is in upper case to maintain compatibility with ZRC20.sol v1
+    uint256 public override PROTOCOL_FLAT_FEE;
 
     mapping(address => uint256) private _balances;
     mapping(address => mapping(address => uint256)) private _allowances;
@@ -44,6 +45,10 @@ contract ZRC20 is IZRC20Metadata, ZRC20Errors, ZRC20Events {
     string private _symbol;
     uint8 private _decimals;
 
+    /// @notice Gateway contract address.
+    /// @dev This variable is added at last position to maintain storage layout with ZRC20.sol v1
+    address public gatewayAddress;
+
     function _msgSender() internal view virtual returns (address) {
         return msg.sender;
     }
@@ -79,8 +84,8 @@ contract ZRC20 is IZRC20Metadata, ZRC20Errors, ZRC20Events {
         _decimals = decimals_;
         CHAIN_ID = chainid_;
         COIN_TYPE = coinType_;
-        gasLimit = gasLimit_;
-        systemContractAddress = systemContractAddress_;
+        GAS_LIMIT = gasLimit_;
+        SYSTEM_CONTRACT_ADDRESS = systemContractAddress_;
         gatewayAddress = gatewayAddress_;
     }
 
@@ -232,7 +237,8 @@ contract ZRC20 is IZRC20Metadata, ZRC20Errors, ZRC20Events {
      */
     function deposit(address to, uint256 amount) external override returns (bool) {
         if (
-            msg.sender != FUNGIBLE_MODULE_ADDRESS && msg.sender != systemContractAddress && msg.sender != gatewayAddress
+            msg.sender != FUNGIBLE_MODULE_ADDRESS && msg.sender != SYSTEM_CONTRACT_ADDRESS
+                && msg.sender != gatewayAddress
         ) revert InvalidSender();
         _mint(to, amount);
         emit Deposit(abi.encodePacked(FUNGIBLE_MODULE_ADDRESS), to, amount);
@@ -245,14 +251,14 @@ contract ZRC20 is IZRC20Metadata, ZRC20Errors, ZRC20Events {
      * withdraw()
      */
     function withdrawGasFee() public view override returns (address, uint256) {
-        address gasZRC20 = ISystem(systemContractAddress).gasCoinZRC20ByChainId(CHAIN_ID);
+        address gasZRC20 = ISystem(SYSTEM_CONTRACT_ADDRESS).gasCoinZRC20ByChainId(CHAIN_ID);
         if (gasZRC20 == address(0)) revert ZeroGasCoin();
 
-        uint256 gasPrice = ISystem(systemContractAddress).gasPriceByChainId(CHAIN_ID);
+        uint256 gasPrice = ISystem(SYSTEM_CONTRACT_ADDRESS).gasPriceByChainId(CHAIN_ID);
         if (gasPrice == 0) {
             revert ZeroGasPrice();
         }
-        uint256 gasFee = gasPrice * gasLimit + protocolFlatFee;
+        uint256 gasFee = gasPrice * GAS_LIMIT + PROTOCOL_FLAT_FEE;
         return (gasZRC20, gasFee);
     }
 
@@ -270,7 +276,7 @@ contract ZRC20 is IZRC20Metadata, ZRC20Errors, ZRC20Events {
             revert GasFeeTransferFailed();
         }
         _burn(msg.sender, amount);
-        emit Withdrawal(msg.sender, to, amount, gasFee, protocolFlatFee);
+        emit Withdrawal(msg.sender, to, amount, gasFee, PROTOCOL_FLAT_FEE);
         return true;
     }
 
@@ -279,7 +285,7 @@ contract ZRC20 is IZRC20Metadata, ZRC20Errors, ZRC20Events {
      * @param addr, new system contract address.
      */
     function updateSystemContractAddress(address addr) external onlyFungible {
-        systemContractAddress = addr;
+        SYSTEM_CONTRACT_ADDRESS = addr;
         emit UpdatedSystemContract(addr);
     }
 
@@ -297,16 +303,16 @@ contract ZRC20 is IZRC20Metadata, ZRC20Errors, ZRC20Events {
      * @param gasLimit_, new gas limit.
      */
     function updateGasLimit(uint256 gasLimit_) external onlyFungible {
-        gasLimit = gasLimit_;
-        emit UpdatedGasLimit(gasLimit);
+        GAS_LIMIT = gasLimit_;
+        emit UpdatedGasLimit(GAS_LIMIT);
     }
 
     /**
      * @dev Updates protocol flat fee. Can only be updated by the fungible module.
      * @param protocolFlatFee_, new protocol flat fee.
      */
     function updateProtocolFlatFee(uint256 protocolFlatFee_) external onlyFungible {
-        protocolFlatFee = protocolFlatFee_;
-        emit UpdatedProtocolFlatFee(protocolFlatFee);
+        PROTOCOL_FLAT_FEE = protocolFlatFee_;
+        emit UpdatedProtocolFlatFee(PROTOCOL_FLAT_FEE);
     }
 }

v2/src/zevm/interfaces/IZRC20.sol

@@ -24,7 +24,8 @@ interface IZRC20 {
 
     function withdrawGasFee() external view returns (address, uint256);
 
-    function protocolFlatFee() external view returns (uint256);
+    /// @dev Name is in upper case to maintain compatibility with ZRC20.sol v1
+    function PROTOCOL_FLAT_FEE() external view returns (uint256);
 }
 
 /// @title IZRC20Metadata

v2/test/GatewayEVMZEVM.t.sol

@@ -167,7 +167,14 @@ contract GatewayEVMZEVMTest is
         bytes memory message = abi.encodeWithSelector(receiverEVM.receivePayable.selector, str, num, flag);
         vm.expectEmit(true, true, true, true, address(gatewayZEVM));
         emit Withdrawal(
-            ownerZEVM, 0, abi.encodePacked(receiverEVM), address(zrc20), 1_000_000, 0, zrc20.protocolFlatFee(), message
+            ownerZEVM,
+            0,
+            abi.encodePacked(receiverEVM),
+            address(zrc20),
+            1_000_000,
+            0,
+            zrc20.PROTOCOL_FLAT_FEE(),
+            message
         );
         vm.prank(ownerZEVM);
         gatewayZEVM.withdrawAndCall(abi.encodePacked(receiverEVM), 1_000_000, address(zrc20), message);

v2/test/GatewayZEVM.t.sol

@@ -66,7 +66,7 @@ contract GatewayZEVMInboundTest is Test, IGatewayZEVMEvents, IGatewayZEVMErrors
         uint256 ownerBalanceBefore = zrc20.balanceOf(owner);
 
         vm.expectEmit(true, true, true, true, address(gateway));
-        emit Withdrawal(owner, 0, abi.encodePacked(addr1), address(zrc20), amount, 0, zrc20.protocolFlatFee(), "");
+        emit Withdrawal(owner, 0, abi.encodePacked(addr1), address(zrc20), amount, 0, zrc20.PROTOCOL_FLAT_FEE(), "");
         gateway.withdraw(abi.encodePacked(addr1), amount, address(zrc20));
 
         uint256 ownerBalanceAfter = zrc20.balanceOf(owner);
@@ -112,7 +112,9 @@ contract GatewayZEVMInboundTest is Test, IGatewayZEVMEvents, IGatewayZEVMErrors
 
         bytes memory message = abi.encodeWithSignature("hello(address)", addr1);
         vm.expectEmit(true, true, true, true, address(gateway));
-        emit Withdrawal(owner, 0, abi.encodePacked(addr1), address(zrc20), amount, 0, zrc20.protocolFlatFee(), message);
+        emit Withdrawal(
+            owner, 0, abi.encodePacked(addr1), address(zrc20), amount, 0, zrc20.PROTOCOL_FLAT_FEE(), message
+        );
         gateway.withdrawAndCall(abi.encodePacked(addr1), amount, address(zrc20), message);
 
         uint256 ownerBalanceAfter = zrc20.balanceOf(owner);

v2/test/ZRC20.t.sol

@@ -208,7 +208,7 @@ contract ZRC20Test is Test, ZRC20Errors {
     function testUpdateSystemContractAddress() public {
         vm.prank(fungibleModule);
         zrc20.updateSystemContractAddress(address(0x3211));
-        assertEq(zrc20.systemContractAddress(), address(0x3211));
+        assertEq(zrc20.SYSTEM_CONTRACT_ADDRESS(), address(0x3211));
     }
 
     function testUpdateSystemContractAddressFailsIfSenderIsNotFungible() public {
@@ -230,7 +230,7 @@ contract ZRC20Test is Test, ZRC20Errors {
     function testUpdateGasLimit() public {
         vm.prank(fungibleModule);
         zrc20.updateGasLimit(10);
-        assertEq(10, zrc20.gasLimit());
+        assertEq(10, zrc20.GAS_LIMIT());
     }
 
     function testUpdateGasLimitFailsIfSenderIsNotFungible() public {
@@ -241,7 +241,7 @@ contract ZRC20Test is Test, ZRC20Errors {
     function testUpdateProtocolFlatFee() public {
         vm.prank(fungibleModule);
         zrc20.updateProtocolFlatFee(10);
-        assertEq(10, zrc20.protocolFlatFee());
+        assertEq(10, zrc20.PROTOCOL_FLAT_FEE());
     }
 
     function testUpdateProtocolFlatFeeFailsIfSenderIsNotFungible() public {