Function derive_into_bytes_union 

pub(crate) fn derive_into_bytes_union(
    ast: &DeriveInput,
    unn: &DataUnion,
    zerocopy_crate: &Path,
) -> Result<TokenStream, Error>

A union is IntoBytes if:

• all fields are IntoBytes
• repr(C), repr(transparent), or repr(packed)
• no padding (size of union equals size of each field type)