[jacb]

If you want random access into an array of enums, you need them all to be the same size (there are also alignment concerns, which is why it's tough to use only 1 bit of overhead to discriminate an enum with two variants). I guess you could add indirection and have an enum with variants A(Box<AType>), B(Box<BType>), etc. This is good practice if you care about space and you have a variant with a type that's a few hundred bytes (not heap-allocated like with Vec). But it's not worth adding the indirection and potential cache miss to save a byte, when most cache lines can fit at least 32 bytes.

[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.