Function count_rows_from_slice_raw 

pub fn count_rows_from_slice_raw(
    bytes: &[u8],
    quote_char: Option<u8>,
    comment_prefix: Option<&CommentPrefix>,
    eol_char: u8,
) -> PolarsResult<usize>

Available on crate features csv or json only.

Read the number of rows without parsing columns, assuming bytes is at a newline starting point. Does not deal with start/header.