Test case: <struct> storage

Solidity tightly packs struct storage words starting from the right-hand side. This test ensures that relative offsets are computed properly for a struct with a few small fields (struct Record { uint8 x; uint8 y; bytes4 salt; }).

Test source

const structStorageTest: ObserveTraceTest<{
  x: number;
  y: number;
  salt: string;
}> = {
  pointer: findExamplePointer("struct-storage-contract-variable-slot"),
  compileOptions: singleSourceCompilation({
    path: "StructStorage.sol",
    contractName: "StructStorage",
    content: `contract StructStorage {
      Record record;

      uint8 step;

      constructor() {
        record = Record({
          x: 5,
          y: 8,
          salt: 0xdeadbeef
        });

        // trick the optimizer maybe (otherwise the first record assignment
        // will get optimized out)
        //
        // compiler might be smarter in the future and cause this test to fail
        step = 1;

        record = Record({
          x: 1,
          y: 2,
          salt: 0xfeedface
        });

        step = 2;
      }
    }

    struct Record {
      uint8 x;
      uint8 y;
      bytes4 salt;
    }
    `
  }),

  expectedValues: [
    { x: 0, y: 0, salt: "0x" },
    { x: 5, y: 8, salt: "0xdeadbeef" },
    { x: 1, y: 2, salt: "0xfeedface" },
  ],

  async observe({ regions, read }) {
    const x = Number(
      (await read(regions.lookup["x"])).asUint()
    );

    const y = Number(
      (await read(regions.lookup["y"])).asUint()
    );

    const salt = (await read(regions.lookup["salt"])).toHex();

    return { x, y, salt };
  },

  equals(a, b) {
    return a.x === b.x && a.y === b.y && a.salt === b.salt;
  }
};

Tested pointer

# example `struct Record { uint8 x; uint8 y; bytes4 salt; }` in storage
define:
  "struct-storage-contract-variable-slot": 0
in:
  group:
    - name: "x"
      location: storage
      slot: "struct-storage-contract-variable-slot"
      offset:
        $difference:
          - $wordsize
          - .length: $this
      length: 1 # uint8
    - name: "y"
      location: storage
      slot: "struct-storage-contract-variable-slot"
      offset:
        $difference:
          - .offset: "x"
          - .length: $this
      length: 1 # uint8
    - name: "salt"
      location: storage
      slot: "struct-storage-contract-variable-slot"
      offset:
        $difference:
          - .offset: "y"
          - .length: $this
      length: 4 # bytes4