Legacy Storage (slots assigned by compiler) to Namespace storage (slots explicitly assigned by us).

LegacyToNameSpaceConversion.t.sol

// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.13;

import {Test, console} from "forge-std/Test.sol";

contract LegacyStorage {
    uint256 public number;

    function setNumber(uint256 newNumber) public {
        number = newNumber;
    }
}

contract NamespaceStorage {
    struct State {
        uint256 number;
    }

    function state() internal pure returns (State storage s) {
        assembly {
            s.slot := 0
        }
    }

    function setNumber(uint256 newNumber) public {
        state().number = newNumber;
    }

    function number() public view returns (uint256) {
        return state().number;
    }
}


contract CounterTest is Test {
    LegacyStorage public l;
    NamespaceStorage public n;

    function setUp() public {
        l = new LegacyStorage();
        n = new NamespaceStorage();
    }

    function testFuzz_SetNumber(uint256 x) public {
        l.setNumber(x);
        n.setNumber(x);

        assertEq(l.number(), x);
        assertEq(n.number(), x);

        assertEq(uint256(vm.load(address(l), 0)), x);
        assertEq(uint256(vm.load(address(n), 0)), x);
    }
}