Function write_dicts 

pub fn write_dicts<W: Write>(
    out: &mut W,
    cols: &[&str],
    rows: &[Map<String, Value>],
) -> Result<()>

DictWriter(fieldnames=cols): header, then each row’s values in column order. A missing key writes an empty cell (restval=""); extra keys are ignored (extrasaction="ignore").