Avoid unnecessary storage read can save gas

[code423n4]

Handle

WatchPug

Vulnerability details

https://github.com/code-423n4/2021-10-mochi/blob/8458209a52565875d8b2cefcb611c477cefb9253/projects/mochi-core/contracts/vault/MochiVault.sol#L85-L111

function accrueDebt(uint256 _id) public {
    // global debt for vault
    // first, increase gloabal debt;
    uint256 currentIndex = liveDebtIndex();
    uint256 increased = (debts * currentIndex) / debtIndex - debts;
    debts += increased;
    claimable += int256(increased);
    // update global debtIndex
    debtIndex = currentIndex;
    lastAccrued = block.timestamp;
    // individual debt
    if (_id != type(uint256).max && details[_id].debtIndex < debtIndex) {
        require(details[_id].status != Status.Invalid, "invalid");
        if (details[_id].debt != 0) {
            uint256 increasedDebt = (details[_id].debt * debtIndex) /
                details[_id].debtIndex -
                details[_id].debt;
            uint256 discountedDebt = increasedDebt.multiply(
                engine.discountProfile().discount(engine.nft().ownerOf(_id))
            );
            debts -= discountedDebt;
            claimable -= int256(discountedDebt);
            details[_id].debt += (increasedDebt - discountedDebt);
        }
        details[_id].debtIndex = debtIndex;
    }
}

At L93, debtIndex is set as currentIndex, therefore, at L96, L99, and L109 storage variable debtIndex can be changed to local variable currentIndex to avoid unnecessary storage read to save some gas.