Function build_enum_reader 

fn build_enum_reader(
    row_name: &Ident,
    variant_infos: &[VariantInfo],
    columns: &[ResolvedColumn],
    attrs: &DataFrameAttrs,
    _impl_generics: &ImplGenerics<'_>,
    _ty_generics: &TypeGenerics<'_>,
    _where_clause: Option<&WhereClause>,
) -> Option<TokenStream>

Build the try_from_dataframe / try_from_dataframe_par methods for a tagged enum.

Returns None if the enum is not reader-capable (tagless, has skipped fields, has conflicts = "string", or has a field type that can’t be read back from R). When None, the enum keeps the DataFrameRowConvert trait default (rows_from_dataframe → None), which surfaces as a clear DataFrameError::Conversion at runtime.