respond to PR comments

contracts/libs/AddressUtils.sol

@@ -0,0 +1,30 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.20;
+
+
+/**
+ * Utility library of inline functions on addresses
+ */
+library AddressUtils {
+
+  /**
+   * Returns whether the target address is a contract
+   * @dev This function will return false if invoked during the constructor of a contract,
+   * as the code is not actually created until after the constructor finishes.
+   * @param _addr address to check
+   * @return whether the target address is a contract
+   */
+  function isContract(address _addr) internal view returns (bool) {
+    uint256 size;
+    // XXX Currently there is no better way to check if there is a contract in an address
+    // than to check the size of the code at that address.
+    // See https://ethereum.stackexchange.com/a/14016/36603
+    // for more details about how this works.
+    // TODO Check this again before the Serenity release, because all addresses will be
+    // contracts then.
+    // solium-disable-next-line security/no-inline-assembly
+    assembly { size := extcodesize(_addr) }
+    return size > 0;
+  }
+
+}

contracts/sponsorship/SponsorshipPaymaster.sol

@@ -9,6 +9,7 @@ import {UserOperation, UserOperationLib} from "@account-abstraction/contracts/in
 import "../BasePaymaster.sol";
 import {SponsorshipPaymasterErrors} from "../common/Errors.sol";
 import {MathLib} from "../libs/MathLib.sol";
+import {AddressUtils} from "../libs/AddressUtils.sol";
 import {ISponsorshipPaymaster} from "../interfaces/paymasters/ISponsorshipPaymaster.sol";
 
 /**
@@ -29,6 +30,7 @@ contract SponsorshipPaymaster is
     ISponsorshipPaymaster
 {
     using ECDSA for bytes32;
+    using AddressUtils for address;
     using UserOperationLib for UserOperation;
 
     uint32 private constant PRICE_DENOMINATOR = 1e6;
@@ -72,7 +74,7 @@ contract SponsorshipPaymaster is
      * @param paymasterId dapp identifier for which deposit is being made
      */
     function depositFor(address paymasterId) external payable nonReentrant {
-        if(isContract(paymasterId)) revert PaymasterIdCannotBeContract();
+        if(paymasterId.isContract()) revert PaymasterIdCannotBeContract();
         if (paymasterId == address(0)) revert PaymasterIdCannotBeZero();
         if (msg.value == 0) revert DepositCanNotBeZero();
         paymasterIdBalances[paymasterId] += msg.value;
@@ -109,7 +111,7 @@ contract SponsorshipPaymaster is
     function setFeeCollector(
         address _newFeeCollector
     ) external payable onlyOwner {
-        if(isContract(_newFeeCollector)) revert FeeCollectorCannotBeContract();
+        if(_newFeeCollector.isContract()) revert FeeCollectorCannotBeContract();
         if (_newFeeCollector == address(0)) revert FeeCollectorCannotBeZero();
         address oldFeeCollector = feeCollector;
         assembly {
@@ -360,19 +362,4 @@ contract SponsorshipPaymaster is
                 maxPriorityFeePerGas + block.basefee
             );
     }
-
-     /**
-     * @notice Will return true if provided address is a contract
-     * @param account Address to verify if contract or not
-     * @dev This contract will return false if called within the constructor of
-     *      a contract's deployment, as the code is not yet stored on-chain.
-     */
-    function isContract(address account) internal view returns (bool) {
-        uint256 csize;
-        // solhint-disable-next-line no-inline-assembly
-        assembly {
-            csize := extcodesize(account)
-        }
-        return csize != 0;
-    }
 }