Add string and bytes support to the StorageSlots library

[Amxx]

String manipulation usefull for an upcoming ShortString library.

See #3969

PR Checklist

• Tests
• Documentation
• Changeset entry (run npx changeset add)

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🦋 Changeset detected

Latest commit: 26db62a

The changes in this PR will be included in the next version bump.

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Name	Type
openzeppelin-solidity	Minor

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[frangio]

Overall this is good. Minor comment below.

[frangio]

I think we can make the assumption that a string will always have offset zero. But I would consider adding an assertion/require just to make sure. Do you know what I mean?

[Amxx]

I don't

[frangio]

Storage pointers have an .offset field in addition to the .slot field. The offset indicates where in the slot the value begins. It can be non-zero when multiple values are packed in a slot. What I'm saying is that for a string pointer we can probably assume offset 0 but it's something we need to be aware of.

[Amxx]

I was not able to create a storage string pointer with an offset.

contract Test {
    struct MyStruct {
        uint256 a;
        bool b;
        string c;
    }

    uint256 private constant KEY = uint256(keccak256("some.key.for.the.mapping"));
    mapping(uint256 => MyStruct) private mymapping;


    function getMyStruct() public view returns (uint256 slot, uint256 offset) {
        MyStruct storage ptr = mymapping[KEY];
        assembly {
            slot := ptr.slot
            offset := ptr.offset
        }
    }

    function getMyStructString() public view returns (uint256 slot, uint256 offset) {
        string storage ptr = mymapping[KEY].c;
        assembly {
            slot := ptr.slot
            offset := ptr.offset
        }
    }
}

Offset is 0 in both cases,
slot for getMyStructString is 2 more than getMyStruct

[frangio]

I think we can assume offset is zero. We've already asked anyway and should have confirmation soon.