feat: add flash allowance functionality

.forge-snapshots/batchTransferFrom.snap

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-61797
+40275

.forge-snapshots/batchTransferFromMultiToken.snap

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-81786
+60132

.forge-snapshots/lockdown.snap

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-28435
+6450

.forge-snapshots/permit2 + transferFrom2 with WETH9's mainnet address.snap

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-60346
+60343

.forge-snapshots/permit2 + transferFrom2 with a non EIP-2612 native token.snap

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-60811
+60784

.forge-snapshots/permit2 + transferFrom2 with an EIP-2612 native token.snap

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-46296
+46230

.forge-snapshots/permitBatchCleanWrite.snap

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-91924
+69636

.forge-snapshots/permitBatchDirtyWrite.snap

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+35436

.forge-snapshots/permitBatchTransferFromMultipleTokens.snap

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+120840

.forge-snapshots/permitBatchTransferFromSingleToken.snap

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-88867
+66552

.forge-snapshots/permitCleanWrite.snap

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+40904

.forge-snapshots/permitCompactSig.snap

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+40941

.forge-snapshots/permitDirtyNonce.snap

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+21799

.forge-snapshots/permitDirtyWrite.snap

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+23804

.forge-snapshots/permitInvalidSigner.snap

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+18168

.forge-snapshots/permitSetMaxAllowanceCleanWrite.snap

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-61114
+38899

.forge-snapshots/permitSetMaxAllowanceDirtyWrite.snap

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-44014
+21799

.forge-snapshots/permitSignatureExpired.snap

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+9332

.forge-snapshots/permitTransferFromBatchTypedWitness.snap

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+97318

.forge-snapshots/permitTransferFromCompactSig.snap

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+63987

.forge-snapshots/permitTransferFromSingleToken.snap

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-86092
+63951

.forge-snapshots/permitTransferFromTypedWitness.snap

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-87817
+65078

.forge-snapshots/safePermit + safeTransferFrom with an EIP-2612 native token.snap

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-48268
+48296

.forge-snapshots/simplePermit2 + transferFrom2 with a non EIP-2612 native token.snap

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-60811
+60784

.forge-snapshots/single recipient 2 tokens.snap

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-118525
+96094

.forge-snapshots/single recipient many tokens.snap

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+110164

.forge-snapshots/transferFrom with different owners.snap

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+40232

.forge-snapshots/transferFrom.snap

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+30322

foundry.toml

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 [profile.default]
-solc = "0.8.17"
+solc = "0.8.28"
 bytecode_hash = "none"
 optimizer = true
 via_ir = true

src/AllowanceTransfer.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity 0.8.17;
+pragma solidity 0.8.28;
 
 import {ERC20} from "solmate/src/tokens/ERC20.sol";
 import {SafeTransferLib} from "solmate/src/utils/SafeTransferLib.sol";

src/EIP712.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity 0.8.17;
+pragma solidity 0.8.28;
 
 import {IEIP712} from "./interfaces/IEIP712.sol";
 

src/FlashAllowanceTransfer.sol

@@ -0,0 +1,78 @@
+// SPDX-License-Identifier: MIT
+pragma solidity 0.8.28;
+
+import {ERC20} from "solmate/src/tokens/ERC20.sol";
+import {SafeTransferLib} from "solmate/src/utils/SafeTransferLib.sol";
+import {IAllowanceTransfer} from "./interfaces/IAllowanceTransfer.sol";
+import {IFlashAllowanceTransfer} from "./interfaces/IFlashAllowanceTransfer.sol";
+
+contract FlashAllowanceTransfer is IFlashAllowanceTransfer {
+    using SafeTransferLib for ERC20;
+
+    bytes32 private constant _TEMPORARY_ALLOWANCES_SLOT = bytes32(uint256(keccak256("Allowance.allowance")) - 1);
+
+    /// @notice Maps users to tokens to spender addresses and information about the approval on the token
+    /// @dev Indexed in the order of token owner address, token address, spender address
+    /// @dev The stored word saves the allowed amount. Defined and commented, but used equivalent in inline assembly.
+    //mapping(address => mapping(address => mapping(address => uint256))) private allowance;
+
+    /// @inheritdoc IFlashAllowanceTransfer
+    /// @dev Does not emit event as approval is reset at the end of txn, and an event is emitted by the transferFrom call
+    function flashApprove(address token, address spender, uint256 amount) external {
+        _setTemporaryAllowance(msg.sender, token, spender, amount);
+    }
+
+    /// @inheritdoc IFlashAllowanceTransfer
+    function flashTransferFrom(address from, address to, uint256 amount, address token) external {
+        _flashTransfer(from, to, amount, token);
+    }
+
+    /// @inheritdoc IFlashAllowanceTransfer
+    function flashTransferFrom(IAllowanceTransfer.AllowanceTransferDetails[] calldata transferDetails) external {
+        unchecked {
+            uint256 length = transferDetails.length;
+            for (uint256 i = 0; i < length; ++i) {
+                IAllowanceTransfer.AllowanceTransferDetails memory transferDetail = transferDetails[i];
+                _flashTransfer(transferDetail.from, transferDetail.to, uint256(transferDetail.amount), transferDetail.token);
+            }
+        }
+    }
+
+    /// @notice Internal function for transferring tokens using transient allowances
+    function _flashTransfer(address from, address to, uint256 amount, address token) private {
+        uint256 allowedAmount = _getTemporaryAllowance(from, token, msg.sender);
+
+        if (allowedAmount != type(uint256).max) {
+            if (amount > allowedAmount) {
+                revert IAllowanceTransfer.InsufficientAllowance(allowedAmount);
+            } else {
+                unchecked {
+                    allowedAmount -= amount;
+                }
+            }
+        }
+
+        // Transfer the tokens from the from address to the recipient.
+        ERC20(token).safeTransferFrom(from, to, amount);
+    }
+
+    function _setTemporaryAllowance(address owner, address token, address spender, uint256 value) internal {
+        bytes32 slot = _allowanceSlot(owner, token, spender);
+        assembly ("memory-safe") {
+            tstore(slot, value)
+        }
+    }
+
+    function _getTemporaryAllowance(address owner, address token, address spender) internal view returns (uint256 allowed) {
+        bytes32 slot = _allowanceSlot(owner, token, spender);
+        assembly ("memory-safe") {
+            allowed := tload(slot)
+        }
+    }
+
+    /// @notice Returns the position key for the allowance
+    /// @dev equivalent to mapping(address => mapping(address => mapping(address => uint256)))
+    function _allowanceSlot(address owner, address spender, address token) private pure returns (bytes32) {
+        return keccak256(abi.encode(spender, keccak256(abi.encode(token, keccak256(abi.encode(owner, _TEMPORARY_ALLOWANCES_SLOT))))));
+    }
+}

src/Permit2.sol

@@ -1,11 +1,12 @@
 // SPDX-License-Identifier: MIT
-pragma solidity 0.8.17;
+pragma solidity 0.8.28;
 
 import {SignatureTransfer} from "./SignatureTransfer.sol";
 import {AllowanceTransfer} from "./AllowanceTransfer.sol";
+import {FlashAllowanceTransfer} from "./FlashAllowanceTransfer.sol";
 
 /// @notice Permit2 handles signature-based transfers in SignatureTransfer and allowance-based transfers in AllowanceTransfer.
 /// @dev Users must approve Permit2 before calling any of the transfer functions.
-contract Permit2 is SignatureTransfer, AllowanceTransfer {
-// Permit2 unifies the two contracts so users have maximal flexibility with their approval.
+contract Permit2 is SignatureTransfer, AllowanceTransfer, FlashAllowanceTransfer {
+// Permit2 unifies the three contracts so users have maximal flexibility with their approval.
 }

src/PermitErrors.sol

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 // SPDX-License-Identifier: MIT
-pragma solidity 0.8.17;
+pragma solidity 0.8.28;
 
 /// @notice Shared errors between signature based transfers and allowance based transfers.
 

src/SignatureTransfer.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity 0.8.17;
+pragma solidity 0.8.28;
 
 import {ISignatureTransfer} from "./interfaces/ISignatureTransfer.sol";
 import {SignatureExpired, InvalidNonce} from "./PermitErrors.sol";

src/interfaces/IFlashAllowanceTransfer.sol

@@ -0,0 +1,31 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.0;
+
+import "./IAllowanceTransfer.sol";
+
+/// @title AllowanceTransfer
+/// @notice Handles ERC20 token permissions through signature based allowance setting and ERC20 token transfers by checking allowed amounts
+/// @dev Requires user's token approval on the Permit2 contract
+interface IFlashAllowanceTransfer {
+    /// @notice Approves the spender to use up to amount of the specified token
+    /// @param token The token to approve
+    /// @param spender The spender address to approve
+    /// @param amount The approved amount of the token
+    /// @dev Setting amount to type(uint160).max sets an unlimited approval
+    function flashApprove(address token, address spender, uint256 amount) external;
+
+    /// @notice Transfer approved tokens from one address to another
+    /// @param from The address to transfer from
+    /// @param to The address of the recipient
+    /// @param amount The amount of the token to transfer
+    /// @param token The token address to transfer
+    /// @dev Requires the from address to have approved at least the desired amount
+    /// of tokens to msg.sender.
+    function flashTransferFrom(address from, address to, uint256 amount, address token) external;
+
+    /// @notice Transfer approved tokens in a batch
+    /// @param transferDetails Array of owners, recipients, amounts, and tokens for the transfers
+    /// @dev Requires the from addresses to have approved at least the desired amount
+    /// of tokens to msg.sender.
+    function flashTransferFrom(IAllowanceTransfer.AllowanceTransferDetails[] calldata transferDetails) external;
+}

test/AllowanceTransferInvariants.t.sol

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-pragma solidity 0.8.17;
+pragma solidity 0.8.28;
 
 import {Test} from "forge-std/Test.sol";
 import {StdInvariant} from "forge-std/StdInvariant.sol";

test/actors/Permitter.sol

@@ -1,4 +1,4 @@
-pragma solidity 0.8.17;
+pragma solidity 0.8.28;
 
 import {Vm} from "forge-std/Vm.sol";
 import {Permit2} from "../../src/Permit2.sol";

test/actors/Spender.sol

@@ -1,4 +1,4 @@
-pragma solidity 0.8.17;
+pragma solidity 0.8.28;
 
 import {Test} from "forge-std/Test.sol";
 import {Permit2} from "../../src/Permit2.sol";