Sosa.Make_native_conversions.to_native_string_knowing

let to_native_string_knowing_size
      ~future_size ~iter ~write_char_to_native_string l =
    let length = future_size l in
    let buf = String.make length 'B' in
    let index = ref 0 in
    iter l ~f:begin fun c ->
      match write_char_to_native_string c ~buf ~index:!index with
      | `Ok siz ->  index := !index + siz
      | `Error `out_of_bounds ->
        failwith "Bug in Make_native_conversions.to_native_string"
    end;
    buf