Change tokenApproval slot calculation to avoid compiler error

[emo-eth]

This should mostly be a nit/cleanup thing, but I was running into errors when compiling with --via-ir in a project I'm working on.

Some combination of factors led the IR pipeline to fail, presumably when trying to put the slot pointer onto the stack outside of the assembly block:

❯ forge build
[⠆] Compiling...
[⠊] Compiling 25 files with 0.8.15
[⠒] Solc 0.8.15 finished in 1.61s
Error: 
Compiler run failed
InternalCompilerError: Invalid stack item name: slot

Referencing the slot directly in the mstore seemed to fix it on my end, and shouldn't cost any extra gas.

[Vectorized]

This will maintain the slot position for diamonds.

Copypasta the following into Remix and test

// SPDX-License-Identifier: MIT
pragma solidity >=0.8.0;

contract TokenApprovalTest {
    struct TokenApprovalRef {
        address value;
    }

    struct Layout {
        mapping(uint256 => TokenApprovalRef) _tokenApprovals;
    }

    bytes32 internal constant STORAGE_SLOT = keccak256('LoremIpsum');

    function layout() internal pure returns (Layout storage l) {
        bytes32 s = STORAGE_SLOT;
        assembly {
            l.slot := s
        }
    }

    function getSlot(uint256 tokenId) external view returns (uint256 result) {
        TokenApprovalRef storage tokenApproval = layout()._tokenApprovals[tokenId];
        assembly {
            result := tokenApproval.slot
        }
        // 0: 91113508023499702123240479191097072207067519098954698186524668019222587556952
        // 1: 109446771143514877113816426284965643011704570302985655154505988876143917244871
    }
}

// SPDX-License-Identifier: MIT
pragma solidity >=0.8.0;

contract TokenApprovalTest {
    struct TokenApprovalRef {
        address value;
    }

    struct Layout {
        mapping(uint256 => address) _tokenApprovals;
    }

    bytes32 internal constant STORAGE_SLOT = keccak256('LoremIpsum');

    function layout() internal pure returns (Layout storage l) {
        bytes32 s = STORAGE_SLOT;
        assembly {
            l.slot := s
        }
    }

    function getSlot(uint256 tokenId) external view returns (uint256 result) {
        mapping(uint256 => address) storage tokenApprovalsPtr = layout()._tokenApprovals;
        assembly {
            mstore(0x00, tokenId)
            mstore(0x20, tokenApprovalsPtr.slot)
            result := keccak256(0x00, 0x40)
        }
        // 0: 91113508023499702123240479191097072207067519098954698186524668019222587556952
        // 1: 109446771143514877113816426284965643011704570302985655154505988876143917244871
    }
}

[Vectorized]

Thanks for alerting us to this. @jameswenzel

[ahbanavi]

I like to mention that we could move this to the interface, but for private use, I think it's ok to be here.

[Vectorized]

Definitely only for private use.

Gave it to a more obscure name like TokenApprovalRef instead of TokenApproval to prevent namespace collision.