Function find_zero_variant 

pub(crate) fn find_zero_variant(enm: &DataEnum) -> Result<usize, bool>

Returns Ok(index) if variant index of the enum has a discriminant of zero. If Err(bool) is returned, the boolean is true if the enum has unknown discriminants (e.g. discriminants set to const expressions which we can’t evaluate in a proc macro). If the enum has unknown discriminants, then it might have a zero variant that we just can’t detect.