feat: passthrough L1->L3 adapter to send messages to L3 via an L2 forwarder

contracts/chain-adapters/Rerouter_Adapter.sol

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+// SPDX-License-Identifier: BUSL-1.1
+pragma solidity ^0.8.0;
+
+import { AdapterInterface } from "./interfaces/AdapterInterface.sol";
+
+/**
+ * @notice Contract containing logic to send messages from L1 to a target (not necessarily a spoke pool) on L2.
+ * @notice The contract receiving messages on L2 will be "spoke pool like" functions, e.g. "relayRootBundle" and
+ * "relaySpokePoolAdminFunction".
+ * @dev Public functions calling external contracts do not guard against reentrancy because they are expected to be
+ * called via delegatecall, which will execute this contract's logic within the context of the originating contract.
+ * For example, the HubPool will delegatecall these functions, therefore its only necessary that the HubPool's methods
+ * that call this contract's logic guard against reentrancy.
+ * @custom:security-contact bugs@across.to
+ */
+
+// solhint-disable-next-line contract-name-camelcase
+contract Rerouter_Adapter is AdapterInterface {
+    address public immutable l1Adapter;
+    address public immutable l2Target;
+
+    error RelayMessageFailed();
+    error RelayTokensFailed(address l1Token);
+
+    /**
+     * @notice Constructs new Adapter for sending tokens/messages to an L2 target.
+     * @param _l1Adapter Address of the adapter contract on mainnet which implements message transfers
+     * and token relays.
+     * @param _l2Target Address of the L2 contract which receives the token and message relays.
+     */
+    constructor(address _l1Adapter, address _l2Target) {
+        l1Adapter = _l1Adapter;
+        l2Target = _l2Target;
+    }
+
+    /**
+     * @notice Send cross-chain message to a target on L2.
+     * @notice The original target field is omitted since messages are unconditionally sent to `l2Target`.
+     * @param message Data to send to target.
+     */
+    function relayMessage(address, bytes memory message) external payable override {
+        (bool success, ) = l1Adapter.delegatecall(abi.encodeCall(AdapterInterface.relayMessage, (l2Target, message)));
+        if (!success) revert RelayMessageFailed();
+    }
+
+    /**
+     * @notice Bridge tokens to a target on L2.
+     * @param l1Token L1 token to deposit.
+     * @param l2Token L2 token to receive.
+     * @param amount Amount of L1 tokens to deposit and L2 tokens to receive.
+     * @notice the "to" field is discarded since we unconditionally relay tokens to `l2Target`.
+     */
+    function relayTokens(
+        address l1Token,
+        address l2Token,
+        uint256 amount,
+        address
+    ) external payable override {
+        (bool success, ) = l1Adapter.delegatecall(
+            abi.encodeCall(AdapterInterface.relayTokens, (l1Token, l2Token, amount, l2Target))
+        );
+        if (!success) revert RelayTokensFailed(l1Token);
+    }
+}

test/evm/foundry/local/Arbitrum_L3_Adapter.t.sol

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+// SPDX-License-Identifier: BUSL-1.1
+pragma solidity ^0.8.0;
+
+import { Test } from "forge-std/Test.sol";
+import { MockERC20 } from "forge-std/mocks/MockERC20.sol";
+import { ERC20, IERC20 } from "@openzeppelin/contracts/token/ERC20/ERC20.sol";
+import { IERC20Upgradeable } from "@openzeppelin/contracts-upgradeable/token/ERC20/IERC20Upgradeable.sol";
+import { IL1StandardBridge } from "@eth-optimism/contracts/L1/messaging/IL1StandardBridge.sol";
+import { Rerouter_Adapter } from "../../../../contracts/chain-adapters/Rerouter_Adapter.sol";
+import { Optimism_Adapter } from "../../../../contracts/chain-adapters/Optimism_Adapter.sol";
+import { WETH9Interface } from "../../../../contracts/external/interfaces/WETH9Interface.sol";
+import { ITokenMessenger } from "../../../../contracts/external/interfaces/CCTPInterfaces.sol";
+
+// We normally delegatecall these from the hub pool, which has receive(). In this test, we call the adapter
+// directly, so in order to withdraw Weth, we need to have receive().
+contract Mock_Rerouter_Adapter is Rerouter_Adapter {
+    constructor(address _l1Adapter, address _l2Target) Rerouter_Adapter(_l1Adapter, _l2Target) {}
+
+    receive() external payable {}
+}
+
+contract Token_ERC20 is ERC20 {
+    constructor(string memory name, string memory symbol) ERC20(name, symbol) {}
+
+    function mint(address to, uint256 value) public virtual {
+        _mint(to, value);
+    }
+
+    function burn(address from, uint256 value) public virtual {
+        _burn(from, value);
+    }
+}
+
+contract MinimalWeth is Token_ERC20 {
+    constructor(string memory name, string memory symbol) Token_ERC20(name, symbol) {}
+
+    function withdraw(uint256 amount) public {
+        _burn(msg.sender, amount);
+        (bool success, ) = payable(msg.sender).call{ value: amount }("");
+        require(success);
+    }
+}
+
+contract CrossDomainMessenger {
+    event MessageSent(address indexed target);
+
+    function sendMessage(
+        address target,
+        bytes calldata,
+        uint32
+    ) external {
+        emit MessageSent(target);
+    }
+}
+
+contract StandardBridge {
+    event ETHDepositInitiated(address indexed to, uint256 amount);
+
+    function depositERC20To(
+        address l1Token,
+        address l2Token,
+        address to,
+        uint256 amount,
+        uint32,
+        bytes calldata
+    ) external {
+        Token_ERC20(l1Token).burn(msg.sender, amount);
+        Token_ERC20(l2Token).mint(to, amount);
+    }
+
+    function depositETHTo(
+        address to,
+        uint32,
+        bytes calldata
+    ) external payable {
+        emit ETHDepositInitiated(to, msg.value);
+    }
+}
+
+contract ArbitrumL3AdapterTest is Test {
+    Rerouter_Adapter l3Adapter;
+    Optimism_Adapter optimismAdapter;
+
+    Token_ERC20 l1Token;
+    Token_ERC20 l2Token;
+    Token_ERC20 l1Weth;
+    Token_ERC20 l2Weth;
+    CrossDomainMessenger crossDomainMessenger;
+    StandardBridge standardBridge;
+
+    address l2Target;
+
+    function setUp() public {
+        l2Target = makeAddr("l2Target");
+
+        l1Token = new Token_ERC20("l1Token", "l1Token");
+        l2Token = new Token_ERC20("l2Token", "l2Token");
+        l1Weth = new MinimalWeth("l1Weth", "l1Weth");
+        l2Weth = new MinimalWeth("l2Weth", "l2Weth");
+
+        crossDomainMessenger = new CrossDomainMessenger();
+        standardBridge = new StandardBridge();
+
+        optimismAdapter = new Optimism_Adapter(
+            WETH9Interface(address(l1Weth)),
+            address(crossDomainMessenger),
+            IL1StandardBridge(address(standardBridge)),
+            IERC20(address(0)),
+            ITokenMessenger(address(0))
+        );
+        l3Adapter = new Mock_Rerouter_Adapter(address(optimismAdapter), l2Target);
+    }
+
+    // Messages should be indiscriminately sent to the l2Forwarder.
+    function testRelayMessage(address target, bytes memory message) public {
+        vm.expectEmit(address(crossDomainMessenger));
+        emit CrossDomainMessenger.MessageSent(l2Target);
+        l3Adapter.relayMessage(target, message);
+    }
+
+    // Sending Weth should call depositETHTo().
+    function testRelayWeth(uint256 amountToSend, address random) public {
+        vm.deal(address(l1Weth), amountToSend);
+        l1Weth.mint(address(l3Adapter), amountToSend);
+        assertEq(amountToSend, l1Weth.totalSupply());
+        vm.expectEmit(address(standardBridge));
+        emit StandardBridge.ETHDepositInitiated(l2Target, amountToSend);
+        l3Adapter.relayTokens(address(l1Weth), address(l2Weth), amountToSend, random);
+        assertEq(0, l1Weth.totalSupply());
+    }
+
+    // Sending any random token should call depositERC20To().
+    function testRelayToken(uint256 amountToSend, address random) public {
+        l1Token.mint(address(l3Adapter), amountToSend);
+        assertEq(amountToSend, l1Token.totalSupply());
+        l3Adapter.relayTokens(address(l1Token), address(l2Token), amountToSend, random);
+        assertEq(amountToSend, l2Token.balanceOf(l2Target));
+        assertEq(amountToSend, l2Token.totalSupply());
+        assertEq(0, l1Token.totalSupply());
+    }
+}