Function derive_from_bytes_enum 

pub(crate) fn derive_from_bytes_enum(
    ast: &DeriveInput,
    enm: &DataEnum,
    zerocopy_crate: &Path,
) -> Result<TokenStream, Error>

An enum is FromBytes if:

• Every possible bit pattern must be valid, which means that every bit pattern must correspond to a different enum variant. Thus, for an enum whose layout takes up N bytes, there must be 2^N variants.
• Since we must know N, only representations which guarantee the layout’s size are allowed. These are repr(uN) and repr(iN) (repr(C) implies an implementation-defined size). usize and isize technically guarantee the layout’s size, but would require us to know how large those are on the target platform. This isn’t terribly difficult - we could emit a const expression that could call core::mem::size_of in order to determine the size and check against the number of enum variants, but a) this would be platform-specific and, b) even on Rust’s smallest bit width platform (32), this would require ~4 billion enum variants, which obviously isn’t a thing.
• All fields of all variants are FromBytes.