Memory-layout-compatible enum/struct subsets

[demurgos]

For the enum case, this could be covered by refined types / pattern types:

• Implement minimal, internal-only pattern types in the type system by oli-obk · Pull Request #120131 · rust-lang/rust · GitHub
• Thoughts on pattern types and subtyping - #35

With your example, EncodingError would be pattern_type!(Error is Error::A(_) | Error::E).