For the curious, this is technically possible in Rust using a dynamically sized type [1], but in practice is difficult and doesn't really play nice with the rest of the language. The nomicon entry concludes with "Yes, custom DSTs are a largely half-baked feature for now." [2]

[1] https://doc.rust-lang.org/reference/dynamically-sized-types....

[2] https://doc.rust-lang.org/nomicon/exotic-sizes.html