zig/doc/langref/test_packed_structs.zig

const std = @import("std");
const expectEqual = std.testing.expectEqual;

const Full = packed struct {
    number: u16,
};
const Divided = packed struct {
    half1: u8,
    quarter3: u4,
    quarter4: u4,
};

test "@bitCast between packed structs" {
    try doTheTest();
    try comptime doTheTest();
}

fn doTheTest() !void {
    try expectEqual(2, @sizeOf(Full));
    try expectEqual(2, @sizeOf(Divided));
    const full = Full{ .number = 0x1234 };
    const divided: Divided = @bitCast(full);
    try expectEqual(0x34, divided.half1);
    try expectEqual(0x2, divided.quarter3);
    try expectEqual(0x1, divided.quarter4);

    const ordered: [2]u8 = @bitCast(full);
    try expectEqual(0x34, ordered[0]);
    try expectEqual(0x12, ordered[1]);
}

// test