False positive for unused-state-variables

[mrice32]

Not sure if this is technically something that should be raised as an issue, but we ran into this with some of our contracts. Feel free to close if this is working as intended. I simplified the false positive to this example:

/*
  Test.
*/
pragma solidity ^0.5.0;


contract Test {
    struct SwappableStruct {
        uint test;
    }

    SwappableStruct private a;
    SwappableStruct private b;

    function incrementChosenStruct(bool useA) external {
        SwappableStruct storage swappableStruct = getChosenStruct(useA);
        swappableStruct.test += 1;
    }

    function getChosenStruct(bool useA) private view returns (SwappableStruct storage swappableStruct) {
        return useA ? a : b;
    }
}

This causes slither to complain that we are not using a or b when the contract clearly makes use of both.

[dguido]

Not sure if this is technically something that should be raised as an issue

It is! Thanks for noting it. We'll see what we can do to refine the unused state variable check.

[montyly]

Hi @mrice32 , thanks for reporting these issues!

The issue is that slither does not handle currently storage reference pointers (see #70 , #82)

We are working to integrate their support (see #87).

[montyly]

For information, master now tracks alias for storage references but is not yet interprocedural.

It means that it will detect that

    function incrementChosenStruct(bool useA) external {
        SwappableStruct storage swappableStruct = useA ? a : b;
        swappableStruct.test += 1;
    }

Is a write to a or b, but is not yet able to do the same if the storage reference is a result of a function call (here getChosenStruct)

We are going to add the support for interprocedural alias analysis

[mrice32]

That sounds great! Thanks for taking the time to work on this!