test: add ERC-20 storage utils for state setup

test/token/ERC20/ERC20/ERC20BurnFacet.t.sol

@@ -7,7 +7,7 @@ pragma solidity >=0.8.30;
 
 import {Test} from "forge-std/Test.sol";
 import {ERC20BurnFacet} from "src/token/ERC20/ERC20/ERC20BurnFacet.sol";
-import {ERC20BurnFacetHarness} from "test/harnesses/token/ERC20/ERC20/ERC20BurnFacetHarness.sol";
+import {ERC20BurnFacetHarness} from "./harnesses/ERC20BurnFacetHarness.sol";
 
 contract ERC20BurnFacetTest is Test {
     ERC20BurnFacetHarness public token;

test/token/ERC20/ERC20/ERC20PermitFacet.t.sol

@@ -7,7 +7,7 @@ pragma solidity >=0.8.30;
 
 import {Test} from "forge-std/Test.sol";
 import {ERC20PermitFacet} from "src/token/ERC20/ERC20Permit/ERC20PermitFacet.sol";
-import {ERC20PermitFacetHarness} from "test/harnesses/token/ERC20/ERC20/ERC20PermitFacetHarness.sol";
+import {ERC20PermitFacetHarness} from "./harnesses/ERC20PermitFacetHarness.sol";
 
 contract ERC20BurnFacetTest is Test {
     ERC20PermitFacetHarness public token;

test/utils/storage/ERC20StorageUtils.sol

@@ -0,0 +1,79 @@
+// SPDX-License-Identifier: MIT
+pragma solidity >=0.8.30;
+
+/* Compose
+ * https://compose.diamonds
+ */
+
+import {Vm} from "forge-std/Vm.sol";
+
+/**
+ * @title ERC20StorageUtils
+ * @notice Storage manipulation utilities for ERC20 token testing
+ * @dev Uses vm.load and vm.store to directly manipulate storage slots
+ */
+library ERC20StorageUtils {
+    Vm private constant vm = Vm(address(uint160(uint256(keccak256("hevm cheat code")))));
+
+    bytes32 internal constant ERC20_TRANSFER_STORAGE_POSITION = keccak256("compose.erc20.transfer");
+
+    /*//////////////////////////////////////////////////////////////
+                                GETTERS
+    //////////////////////////////////////////////////////////////*/
+
+    /*
+     * @notice ERC-20 Transfer storage layout (ERC-8042 standard)
+     * @custom:storage-location erc8042:compose.erc20.transfer
+     *
+     * Slot 0: mapping(address owner => uint256 balance) balanceOf
+     * Slot 1: uint256 totalSupply
+     * Slot 2: mapping(address owner => mapping(address spender => uint256)) allowance
+     */
+
+    function balanceOf(address target, address owner) internal view returns (uint256) {
+        bytes32 slot = keccak256(abi.encode(owner, uint256(ERC20_TRANSFER_STORAGE_POSITION)));
+        return uint256(vm.load(target, slot));
+    }
+
+    function totalSupply(address target) internal view returns (uint256) {
+        bytes32 slot = bytes32(uint256(ERC20_TRANSFER_STORAGE_POSITION) + 1);
+        return uint256(vm.load(target, slot));
+    }
+
+    function allowance(address target, address owner, address spender) internal view returns (uint256) {
+        bytes32 ownerSlot = keccak256(abi.encode(owner, uint256(ERC20_TRANSFER_STORAGE_POSITION) + 2));
+        bytes32 slot = keccak256(abi.encode(spender, ownerSlot));
+        return uint256(vm.load(target, slot));
+    }
+
+    /*//////////////////////////////////////////////////////////////
+                                SETTERS
+    //////////////////////////////////////////////////////////////*/
+
+    function setBalance(address target, address owner, uint256 balance) internal {
+        bytes32 slot = keccak256(abi.encode(owner, uint256(ERC20_TRANSFER_STORAGE_POSITION)));
+        vm.store(target, slot, bytes32(balance));
+    }
+
+    function setTotalSupply(address target, uint256 supply) internal {
+        bytes32 slot = bytes32(uint256(ERC20_TRANSFER_STORAGE_POSITION) + 1);
+        vm.store(target, slot, bytes32(supply));
+    }
+
+    function setAllowance(address target, address owner, address spender, uint256 amount) internal {
+        bytes32 ownerSlot = keccak256(abi.encode(owner, uint256(ERC20_TRANSFER_STORAGE_POSITION) + 2));
+        bytes32 slot = keccak256(abi.encode(spender, ownerSlot));
+        vm.store(target, slot, bytes32(amount));
+    }
+
+    /**
+     * @notice Mint tokens by updating balance and totalSupply
+     */
+    function mint(address target, address to, uint256 amount) internal {
+        uint256 currentBalance = balanceOf(target, to);
+        uint256 currentSupply = totalSupply(target);
+
+        setBalance(target, to, currentBalance + amount);
+        setTotalSupply(target, currentSupply + amount);
+    }
+}