openzeppelin-contracts/contracts/token/ERC20/extensions/ERC4626.sol

// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts (last updated v4.8.0) (token/ERC20/extensions/ERC4626.sol)

pragma solidity ^0.8.0;

import "../ERC20.sol";
import "../utils/SafeERC20.sol";
import "../../../interfaces/IERC4626.sol";
import "../../../utils/math/Math.sol";

/**
 * @dev Implementation of the ERC4626 "Tokenized Vault Standard" as defined in
 * https://eips.ethereum.org/EIPS/eip-4626[EIP-4626].
 *
 * This extension allows the minting and burning of "shares" (represented using the ERC20 inheritance) in exchange for
 * underlying "assets" through standardized {deposit}, {mint}, {redeem} and {burn} workflows. This contract extends
 * the ERC20 standard. Any additional extensions included along it would affect the "shares" token represented by this
 * contract and not the "assets" token which is an independent contract.
 *
 * [CAUTION]
 * ====
 * In empty (or nearly empty) ERC-4626 vaults, deposits are at high risk of being stolen through frontrunning
 * with a "donation" to the vault that inflates the price of a share. This is variously known as a donation or inflation
 * attack and is essentially a problem of slippage. Vault deployers can protect against this attack by making an initial
 * deposit of a non-trivial amount of the asset, such that price manipulation becomes infeasible. Withdrawals may
 * similarly be affected by slippage. Users can protect against this attack as well as unexpected slippage in general by
 * verifying the amount received is as expected, using a wrapper that performs these checks such as
 * https://github.com/fei-protocol/ERC4626#erc4626router-and-base[ERC4626Router].
 *
 * Since v4.9, this implementation uses virtual assets and shares to mitigate that risk. The `_decimalsOffset()`
 * corresponds to an offset in the decimal representation between the underlying asset's decimals and the vault
 * decimals. This offset also determines the rate of virtual shares to virtual assets in the vault, which itself
 * determines the initial exchange rate. While not fully preventing the attack, analysis shows that the default offset
 * (0) makes it non-profitable, as a result of the value being captured by the virtual shares (out of the attacker's
 * donation) matching the attacker's expected gains. With a larger offset, the attack becomes orders of magnitude more
 * expensive than it is profitable. More details about the underlying math can be found
 * xref:erc4626.adoc#inflation-attack[here].
 *
 * The drawback of this approach is that the virtual shares do capture (a very small) part of the value being accrued
 * to the vault. Also, if the vault experiences losses, the users try to exit the vault, the virtual shares and assets
 * will cause the first user to exit to experience reduced losses in detriment to the last users that will experience
 * bigger losses. Developers willing to revert back to the pre-v4.9 behavior just need to override the
 * `_convertToShares` and `_convertToAssets` functions.
 *
 * To learn more, check out our xref:ROOT:erc4626.adoc[ERC-4626 guide].
 * ====
 *
 * _Available since v4.7._
 */
abstract contract ERC4626 is ERC20, IERC4626 {
    using Math for uint256;

    IERC20 private immutable _asset;
    uint8 private immutable _underlyingDecimals;

    /**
     * @dev Set the underlying asset contract. This must be an ERC20-compatible contract (ERC20 or ERC777).
     */
    constructor(IERC20 asset_) {
        (bool success, uint8 assetDecimals) = _tryGetAssetDecimals(asset_);
        _underlyingDecimals = success ? assetDecimals : 18;
        _asset = asset_;
    }

    /**
     * @dev Attempts to fetch the asset decimals. A return value of false indicates that the attempt failed in some way.
     */
    function _tryGetAssetDecimals(IERC20 asset_) private view returns (bool, uint8) {
        (bool success, bytes memory encodedDecimals) = address(asset_).staticcall(
            abi.encodeWithSelector(IERC20Metadata.decimals.selector)
        );
        if (success && encodedDecimals.length >= 32) {
            uint256 returnedDecimals = abi.decode(encodedDecimals, (uint256));
            if (returnedDecimals <= type(uint8).max) {
                return (true, uint8(returnedDecimals));
            }
        }
        return (false, 0);
    }

    /**
     * @dev Decimals are read from the underlying asset in the constructor and cached. If this fails (e.g., the asset
     * has not been created yet), the cached value is set to a default obtained by `super.decimals()` (which depends on
     * inheritance but is most likely 18). Override this function in order to set a guaranteed hardcoded value.
     * See {IERC20Metadata-decimals}.
     */
    function decimals() public view virtual override(IERC20Metadata, ERC20) returns (uint8) {
        return _underlyingDecimals + _decimalsOffset();
    }

    /** @dev See {IERC4626-asset}. */
    function asset() public view virtual override returns (address) {
        return address(_asset);
    }

    /** @dev See {IERC4626-totalAssets}. */
    function totalAssets() public view virtual override returns (uint256) {
        return _asset.balanceOf(address(this));
    }

    /** @dev See {IERC4626-convertToShares}. */
    function convertToShares(uint256 assets) public view virtual override returns (uint256) {
        return _convertToShares(assets, Math.Rounding.Down);
    }

    /** @dev See {IERC4626-convertToAssets}. */
    function convertToAssets(uint256 shares) public view virtual override returns (uint256) {
        return _convertToAssets(shares, Math.Rounding.Down);
    }

    /** @dev See {IERC4626-maxDeposit}. */
    function maxDeposit(address) public view virtual override returns (uint256) {
        return type(uint256).max;
    }

    /** @dev See {IERC4626-maxMint}. */
    function maxMint(address) public view virtual override returns (uint256) {
        return type(uint256).max;
    }

    /** @dev See {IERC4626-maxWithdraw}. */
    function maxWithdraw(address owner) public view virtual override returns (uint256) {
        return _convertToAssets(balanceOf(owner), Math.Rounding.Down);
    }

    /** @dev See {IERC4626-maxRedeem}. */
    function maxRedeem(address owner) public view virtual override returns (uint256) {
        return balanceOf(owner);
    }

    /** @dev See {IERC4626-previewDeposit}. */
    function previewDeposit(uint256 assets) public view virtual override returns (uint256) {
        return _convertToShares(assets, Math.Rounding.Down);
    }

    /** @dev See {IERC4626-previewMint}. */
    function previewMint(uint256 shares) public view virtual override returns (uint256) {
        return _convertToAssets(shares, Math.Rounding.Up);
    }

    /** @dev See {IERC4626-previewWithdraw}. */
    function previewWithdraw(uint256 assets) public view virtual override returns (uint256) {
        return _convertToShares(assets, Math.Rounding.Up);
    }

    /** @dev See {IERC4626-previewRedeem}. */
    function previewRedeem(uint256 shares) public view virtual override returns (uint256) {
        return _convertToAssets(shares, Math.Rounding.Down);
    }

    /** @dev See {IERC4626-deposit}. */
    function deposit(uint256 assets, address receiver) public virtual override returns (uint256) {
        require(assets <= maxDeposit(receiver), "ERC4626: deposit more than max");

        uint256 shares = previewDeposit(assets);
        _deposit(_msgSender(), receiver, assets, shares);

        return shares;
    }

    /** @dev See {IERC4626-mint}.
     *
     * As opposed to {deposit}, minting is allowed even if the vault is in a state where the price of a share is zero.
     * In this case, the shares will be minted without requiring any assets to be deposited.
     */
    function mint(uint256 shares, address receiver) public virtual override returns (uint256) {
        require(shares <= maxMint(receiver), "ERC4626: mint more than max");

        uint256 assets = previewMint(shares);
        _deposit(_msgSender(), receiver, assets, shares);

        return assets;
    }

    /** @dev See {IERC4626-withdraw}. */
    function withdraw(uint256 assets, address receiver, address owner) public virtual override returns (uint256) {
        require(assets <= maxWithdraw(owner), "ERC4626: withdraw more than max");

        uint256 shares = previewWithdraw(assets);
        _withdraw(_msgSender(), receiver, owner, assets, shares);

        return shares;
    }

    /** @dev See {IERC4626-redeem}. */
    function redeem(uint256 shares, address receiver, address owner) public virtual override returns (uint256) {
        require(shares <= maxRedeem(owner), "ERC4626: redeem more than max");

        uint256 assets = previewRedeem(shares);
        _withdraw(_msgSender(), receiver, owner, assets, shares);

        return assets;
    }

    /**
     * @dev Internal conversion function (from assets to shares) with support for rounding direction.
     *
     * Will revert if assets > 0, totalSupply > 0 and totalAssets = 0. That corresponds to a case where any asset
     * would represent an infinite amount of shares.
     */
    function _convertToShares(uint256 assets, Math.Rounding rounding) internal view virtual returns (uint256) {
        return assets.mulDiv(totalSupply() + 10 ** _decimalsOffset(), totalAssets() + 1, rounding);
    }

    /**
     * @dev Internal conversion function (from shares to assets) with support for rounding direction.
     */
    function _convertToAssets(uint256 shares, Math.Rounding rounding) internal view virtual returns (uint256) {
        return shares.mulDiv(totalAssets() + 1, totalSupply() + 10 ** _decimalsOffset(), rounding);
    }

    /**
     * @dev Deposit/mint common workflow.
     */
    function _deposit(address caller, address receiver, uint256 assets, uint256 shares) internal virtual {
        // If _asset is ERC777, `transferFrom` can trigger a reenterancy BEFORE the transfer happens through the
        // `tokensToSend` hook. On the other hand, the `tokenReceived` hook, that is triggered after the transfer,
        // calls the vault, which is assumed not malicious.
        //
        // Conclusion: we need to do the transfer before we mint so that any reentrancy would happen before the
        // assets are transferred and before the shares are minted, which is a valid state.
        // slither-disable-next-line reentrancy-no-eth
        SafeERC20.safeTransferFrom(_asset, caller, address(this), assets);
        _mint(receiver, shares);

        emit Deposit(caller, receiver, assets, shares);
    }

    /**
     * @dev Withdraw/redeem common workflow.
     */
    function _withdraw(
        address caller,
        address receiver,
        address owner,
        uint256 assets,
        uint256 shares
    ) internal virtual {
        if (caller != owner) {
            _spendAllowance(owner, caller, shares);
        }

        // If _asset is ERC777, `transfer` can trigger a reentrancy AFTER the transfer happens through the
        // `tokensReceived` hook. On the other hand, the `tokensToSend` hook, that is triggered before the transfer,
        // calls the vault, which is assumed not malicious.
        //
        // Conclusion: we need to do the transfer after the burn so that any reentrancy would happen after the
        // shares are burned and after the assets are transferred, which is a valid state.
        _burn(owner, shares);
        SafeERC20.safeTransfer(_asset, receiver, assets);

        emit Withdraw(caller, receiver, owner, assets, shares);
    }

    function _decimalsOffset() internal view virtual returns (uint8) {
        return 0;
    }
}