Adding Unchecked Directive will Save Gas for BurnMath.sol#getAsset and BurnMath.sol#getCollateral functions

[code423n4]

Handle

Rhynorater

Vulnerability details

In BurnMath.sol we have the following function defined;

    function getAsset(IPair.State memory state, uint256 liquidityIn) internal pure returns (uint128 assetOut) {
        if (state.reserves.asset <= state.totalClaims.bond) return assetOut; 
        uint256 _assetOut = state.reserves.asset;
        _assetOut -= state.totalClaims.bond;
        _assetOut = _assetOut.mulDiv(liquidityIn, state.totalLiquidity);
        assetOut = _assetOut.toUint128();
}

Since the above if statement ensures that state.reserves.asset is not less than or equal to state.totalClaims.bond, it is impossible for the _assetOut -= state.totalClaims.bond; line to underflow. As a result, adding the unchecked directive around this will save on gas.

By the same reasoning, in the getCollateral function:

deficit -= state.reserves.asset;

is already checked by the

if (state.reserves.asset >= state.totalClaims.bond) {

if statement. Surrounding this with unchecked will also save on gas.

Lastly, this also applies in WithdrawMath.sol#getCollateral:

        if (state.reserves.asset >= state.totalClaims.bond) return collateralOut;
        uint256 deficit = state.totalClaims.bond;
        deficit -= state.reserves.asset;

The deficit will never underflow here, so adding unchecked will save on gas.

##References
https://github.com/code-423n4/2022-01-timeswap/blob/5960e07d39f2b4a60cfabde1bd51f4b1e62e7e85/Timeswap/Timeswap-V1-Core/contracts/libraries/BurnMath.sol#L22
https://github.com/code-423n4/2022-01-timeswap/blob/5960e07d39f2b4a60cfabde1bd51f4b1e62e7e85/Timeswap/Timeswap-V1-Core/contracts/libraries/BurnMath.sol#L41
https://github.com/code-423n4/2022-01-timeswap/blob/5960e07d39f2b4a60cfabde1bd51f4b1e62e7e85/Timeswap/Timeswap-V1-Core/contracts/libraries/WithdrawMath.sol#L33

[Mathepreneur]

Timeswap-Labs/Timeswap-V1-Core#108