Subsidized transactions for smart contracts.

Subsidized.sol

// SPDX-License-Identifier: AGPL-3.0-only
pragma solidity >=0.8.4;

import {SafeTransferLib} from "https://github.com/Rari-Capital/solmate/blob/main/src/utils/SafeTransferLib.sol";

/// @notice Subsidized transactions for smart contracts.
/// @author z0r0z.eth
abstract contract Subsidized {
    using SafeTransferLib for address;

    modifier subsidized() virtual {
        uint256 startGas = gasleft();

        _;

        unchecked {
            tx.origin.safeTransferETH(
                (
                    startGas - gasleft()
                ) 
                    * tx.gasprice
                );
        }
    }
}

contract SubsidizedTester is Subsidized {
    uint256 public counter;

    receive() external payable virtual {}

    function testCounter() public subsidized virtual {
        unchecked {
            ++counter;
        }
    }
}