[comex]

But you don't always need random access; often you only need sequential access, in which case you can pack differently-sized values into a single buffer. However, Rust enums don't support that use case. A macro could probably do it without too much loss of ergonomics, but I'm not sure if there's a good crate for it.

[jacb]

That's a good point! I guess you'd still have to make sure that variants with alignment requirements don't get unaligned. One way to do that would be to add leading padding if they follow an unaligned variant. Adds some complexity to the representation, I guess, and I suspect there's other ways of doing it. Neat problem.