UnionList

    pub fn UnionList(
        comptime flags_arg: @EnumLiteral(),
        comptime flag_arg: @EnumLiteral(),
        comptime UnionArg: type,
    ) type {
        return struct {
            /// When serializing it is UnionArg slice pointer.
            /// When deserializing it is extra index of first UnionArg element.
            data: ?*const anyopaque,
            len: usize,

            pub const storage: Storage = .union_list;
            pub const flags = flags_arg;
            pub const flag = flag_arg;
            pub const Union = UnionArg;

            pub const Tag = @typeInfo(Union).@"union".tag_type.?;
            pub const MetaInt = @Int(.unsigned, @bitSizeOf(Tag) + 1);
            pub const Meta = packed struct(MetaInt) {
                tag: Tag,
                last: bool,
            };

            /// Valid to call only when serializing.
            pub fn init(s: []const Union) @This() {
                return .{ .data = s.ptr, .len = s.len };
            }

            /// Valid to call only when deserializing.
            pub fn slice(this: *const @This(), extra: []const u32) []const u32 {
                return extra[@intFromPtr(this.data)..][0..this.len];
            }

            /// Valid to call only when deserializing.
            pub fn get(this: *const @This(), extra: []const u32, i: usize) Union {
                const elem = slice(this, extra)[i];
                return switch (this.tag(extra, i)) {
                    inline else => |comptime_tag| @unionInit(Union, @tagName(comptime_tag), @fromBackingInt(@intCast(elem))),
                };
            }

            /// Valid to call only when deserializing.
            pub fn tag(this: *const @This(), extra: []const u32, i: usize) Tag {
                const start = @intFromPtr(this.data);
                const meta_start = start - (this.len * @bitSizeOf(Meta) + 31) / 32;
                return loadBits(u32, extra[meta_start..], i * @bitSizeOf(Meta), Meta).tag;
            }

            fn extraLen(len: usize) usize {
                return len + (len * @bitSizeOf(Meta) + 31) / 32;
            }
        };
    }