A Solidity contract to estimate the used mana via the `manaleft()` syntax for dedicated function calls.

ManaTesting.sol

// SPDX-License-Identifier: WTFPL
pragma solidity ^0.8.19;

contract ManaTesting {
    function test_iWeak() public view returns (uint256 manaUsed) {
        uint256 startMana = manaleft();
        uint256 i = 0;
        i++;
        manaUsed = startMana - manaleft();
    }

    function test_iStrong() public view returns (uint256 manaUsed) {
        uint256 startMana = manaleft();
        uint256 i = 0;
         ++
        i  ;
        manaUsed = startMana - manaleft();
    }

    function manaleft() internal view returns (uint256) {
        return gasleft();
    }
}

If you use Foundry, you can invoke a function like that:

forge script --optimize --optimizer-runs 200 --use 0.8.19 --via-ir ManaTesting.sol --target-contract ManaTesting --sig "test_iWeak()"