✨ Add accruedRecipient and withdrawAccruedToAccruedRecipient function to allow permissionless withdrawals

.gas-snapshot

@@ -1,8 +1,9 @@
-GasbackTest:testConvertGasback() (gas: 50450)
-GasbackTest:testConvertGasback(uint256,uint256) (runs: 256, μ: 370473, ~: 135584)
-GasbackTest:testConvertGasbackBaseFeeVault() (gas: 24310)
-GasbackTest:testConvertGasbackMaxBaseFee() (gas: 21878)
-GasbackTest:testConvertGasbackMinVaultBalance() (gas: 24154)
-GasbackTest:test__codesize() (gas: 8178)
+GasbackTest:testConvertGasback() (gas: 73039)
+GasbackTest:testConvertGasback(uint256,uint256) (runs: 257, μ: 423506, ~: 308109)
+GasbackTest:testConvertGasbackBaseFeeVault() (gas: 27070)
+GasbackTest:testConvertGasbackMaxBaseFee() (gas: 44525)
+GasbackTest:testConvertGasbackMinVaultBalance() (gas: 26953)
+GasbackTest:testConvertGasbackWithAccruedToAccruedRecipient() (gas: 69305)
+GasbackTest:test__codesize() (gas: 9846)
 SoladyTest:test__codesize() (gas: 4099)
 TestPlus:test__codesize() (gas: 393)

src/Gasback.sol

@@ -34,6 +34,8 @@ contract Gasback {
         uint256 minVaultBalance;
         // The amount of ETH accrued by taking a cut from the gas burned.
         uint256 accrued;
+        // The recipient of the accrued ETH.
+        address accruedRecipient;
         // A mapping of addresses authorized to withdraw the accrued ETH.
         mapping(address => bool) accuralWithdrawers;
     }
@@ -58,6 +60,7 @@ contract Gasback {
         $.gasbackMaxBaseFee = type(uint256).max;
         $.baseFeeVault = 0x4200000000000000000000000000000000000019;
         $.minVaultBalance = 0.42 ether;
+        $.accruedRecipient = 0x4200000000000000000000000000000000000019;
     }
 
     /*«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-*/
@@ -120,6 +123,27 @@ contract Gasback {
         return true;
     }
 
+    /// @dev Withdraws from the accrued amount to the accrued recipient.
+    function withdrawAccruedToAccruedRecipient(uint256 amount) public virtual returns (bool) {
+        // Checked math prevents underflow.
+        _getGasbackStorage().accrued -= amount;
+
+        address accruedRecipient = _getGasbackStorage().accruedRecipient;
+        /// @solidity memory-safe-assembly
+        assembly {
+            if iszero(call(gas(), accruedRecipient, amount, 0x00, 0x00, 0x00, 0x00)) {
+                revert(0x00, 0x00)
+            }
+        }
+        return true;
+    }
+
+    /// @dev Sets the accrued recipient.
+    function setAccruedRecipient(address value) public onlySystemOrThis returns (bool) {
+        _getGasbackStorage().accruedRecipient = value;
+        return true;
+    }
+
     /*«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-«-*/
     /*                      ADMIN FUNCTIONS                       */
     /*-»-»-»-»-»-»-»-»-»-»-»-»-»-»-»-»-»-»-»-»-»-»-»-»-»-»-»-»-»-»*/

test/Gasback.t.sol

@@ -75,4 +75,30 @@ contract GasbackTest is SoladyTest {
         assertTrue(success);
         assertEq(pranker.balance, 0);
     }
+
+    function testConvertGasbackWithAccruedToAccruedRecipient() public {
+        address system = 0xffffFFFfFFffffffffffffffFfFFFfffFFFfFFfE;
+        vm.prank(system);
+        gasback.setAccruedRecipient(address(42));
+
+        uint256 baseFee = 1 ether;
+        uint256 gasToBurn = 333;
+
+        address pranker = address(111);
+        vm.fee(baseFee);
+        vm.deal(pranker, 1000 ether);
+
+        vm.prank(pranker);
+        (bool success,) = address(gasback).call(abi.encode(gasToBurn));
+        assertTrue(success);
+
+        uint256 accrued = gasback.accrued();
+
+        assertNotEq(accrued, 0);
+
+        vm.prank(pranker);
+        gasback.withdrawAccruedToAccruedRecipient(accrued);
+
+        assertEq(address(42).balance, accrued);
+    }
 }