> a lack of proper sum types

Do you consider Rust enums 'proper sum types'? If yes what are Zig's tagged unions missing?

E.g.:

    const Variant = union(enum) {
        int: i32,
        boolean: bool,
        none,

        fn truthy(self: Variant) bool {
            return switch (self) {
                Variant.int => |x_int| x_int != 0,
                Variant.boolean => |x_bool| x_bool,
                Variant.none => false,
            };
        }
    };