fix(contracts-rfq): TokenZapV1 native gas token behaviour [SLT-389]

packages/contracts-rfq/contracts/zaps/TokenZapV1.sol

@@ -15,37 +15,51 @@ import {IERC20, SafeERC20} from "@openzeppelin/contracts/token/ERC20/utils/SafeE
 import {Address} from "@openzeppelin/contracts/utils/Address.sol";
 
 /// @title TokenZapV1
-/// @notice Facilitates atomic token operations known as "Zaps," allowing to execute predefined actions
-/// on behalf of users like deposits or swaps. Supports ERC20 tokens and native gas tokens (e.g., ETH).
-/// @dev Tokens must be pre-transferred to the contract for execution, with native tokens sent as msg.value.
+/// @notice Facilitates atomic token operations known as "Zaps", allowing the execution of predefined actions
+/// on behalf of users, such as deposits or swaps. Supports ERC20 tokens and native gas tokens (e.g., ETH).
+/// @dev Tokens must be transferred to the contract before execution, native tokens could be provided as `msg.value`.
 /// This contract is stateless and does not hold assets between Zaps; leftover tokens can be claimed by anyone.
-/// Ensure Zaps fully utilize tokens or revert to prevent fund loss.
+/// Ensure that Zaps fully utilize tokens or revert to prevent the loss of funds.
 contract TokenZapV1 is IZapRecipient {
     using SafeERC20 for IERC20;
     using ZapDataV1 for bytes;
 
     address public constant NATIVE_GAS_TOKEN = 0xEeeeeEeeeEeEeeEeEeEeeEEEeeeeEeeeeeeeEEeE;
 
-    error TokenZapV1__AmountIncorrect();
     error TokenZapV1__PayloadLengthAboveMax();
+    error TokenZapV1__TargetZeroAddress();
 
-    /// @notice Performs a Zap action using the specified token and amount. This amount must be previously
-    /// transferred to this contract (or supplied as msg.value if the token is native gas token).
+    /// @notice Allows the contract to receive ETH.
+    /// @dev Leftover ETH can be claimed by anyone. Ensure the full balance is spent during Zaps.
+    receive() external payable {}
+
+    /// @notice Performs a Zap action using the specified token and amount. This amount must have previously been
+    /// transferred to this contract (could also be supplied as msg.value if the token is a native gas token).
+    /// Zap action will be performed forwarding full `msg.value` for ERC20s or `amount` for native gas tokens.
+    /// Note: all funds remaining after the Zap action is performed can be claimed by anyone.
+    /// Make sure to spend the full balance during the Zaps and avoid sending extra funds if a single Zap is performed.
     /// @dev The provided ZapData contains the target address and calldata for the Zap action, and must be
-    /// encoded using the encodeZapData function.
+    /// encoded using the encodeZapData function. Native gas token transfers could be done by using empty `payload`,
+    /// this is the only case where target could be an EOA.
     /// @param token        Address of the token to be used for the Zap action.
     /// @param amount       Amount of the token to be used for the Zap action.
-    ///                     Must match msg.value if the token is a native gas token.
     /// @param zapData      Encoded Zap Data containing the target address and calldata for the Zap action.
     /// @return selector    Selector of this function to signal the caller about the success of the Zap action.
     function zap(address token, uint256 amount, bytes calldata zapData) external payable returns (bytes4) {
         // Validate the ZapData format and extract the target address.
         zapData.validateV1();
         address target = zapData.target();
+        if (target == address(0)) revert TokenZapV1__TargetZeroAddress();
+        // Note: we don't check the amount that was transferred to TokenZapV1 (or msg.value for native gas tokens).
+        // Transferring more than `amount` will lead to remaining funds in TokenZapV1, which can be claimed by anyone.
+        // Ensure that you send the exact amount for a single Zap or spend the full balance for multiple `zap()` calls.
+        uint256 msgValue = msg.value;
         if (token == NATIVE_GAS_TOKEN) {
-            // For native gas token (e.g., ETH), verify msg.value matches the expected amount.
-            // No approval needed since native token doesn't use allowances.
-            if (msg.value != amount) revert TokenZapV1__AmountIncorrect();
+            // For native gas tokens, we forward the requested amount to the target contract during the Zap action.
+            // Similar to ERC20s, we allow using pre-transferred native tokens for the Zap.
+            msgValue = amount;
+            // No approval is needed since native tokens don't use allowances.
+            // Note: balance check is performed within `Address.sendValue` or `Address.functionCallWithValue` below.
         } else {
             // For ERC20 tokens, grant unlimited approval to the target if the current allowance is insufficient.
             // This is safe since the contract doesn't custody tokens between zaps.
@@ -57,9 +71,16 @@ contract TokenZapV1 is IZapRecipient {
         // Construct the payload for the target contract call with the Zap action.
         // The payload is modified to replace the placeholder amount with the actual amount.
         bytes memory payload = zapData.payload(amount);
-        // Perform the Zap action, forwarding full msg.value to the target contract.
-        // Note: this will bubble up any revert from the target contract.
-        Address.functionCallWithValue({target: target, data: payload, value: msg.value});
+        if (payload.length == 0 && token == NATIVE_GAS_TOKEN) {
+            // Zap Action in a form of native gas token transfer to the target is requested.
+            // Note: we avoid using `functionCallWithValue` because the target might be an EOA. This will
+            // revert with a generic custom error should the target contract revert on incoming transfer.
+            Address.sendValue({recipient: payable(target), amount: msgValue});
+        } else {
+            // Perform the Zap action, forwarding the requested native value to the target contract.
+            // Note: this will bubble up any revert from the target contract, and revert if target is EOA.
+            Address.functionCallWithValue({target: target, data: payload, value: msgValue});
+        }
         // Return function selector to indicate successful execution
         return this.zap.selector;
     }

packages/contracts-rfq/test/mocks/WETHMock.sol

@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.20;
+
+import {ERC20} from "@openzeppelin/contracts/token/ERC20/ERC20.sol";
+import {Address} from "@openzeppelin/contracts/utils/Address.sol";
+
+// solhint-disable no-empty-blocks
+/// @notice WETH mock for testing purposes. DO NOT USE IN PRODUCTION.
+contract WETHMock is ERC20 {
+    constructor() ERC20("Mock Wrapped Ether", "Mock WETH") {}
+
+    receive() external payable {
+        deposit();
+    }
+
+    /// @notice We include an empty "test" function so that this contract does not appear in the coverage report.
+    function testWETHMock() external {}
+
+    function withdraw(uint256 amount) external {
+        _burn(msg.sender, amount);
+        Address.sendValue(payable(msg.sender), amount);
+    }
+
+    function deposit() public payable {
+        _mint(msg.sender, msg.value);
+    }
+}