Function encode_copy_text_cells_opts 

pub fn encode_copy_text_cells_opts(
    cells: &[Option<String>],
    delimiter: char,
    null_str: &str,
) -> String

Encode one row’s cells as a COPY text-format line with a custom delimiter and NULL marker (PG COPY … WITH (FORMAT text, DELIMITER 'c', NULL 'str')). The named C-escapes (\t \n \r \b \f \v \\) are always applied; a delimiter character that is not itself one of those gets a literal \<char> escape so it round-trips.