Function header_names 

pub fn header_names(
    source: impl BufRead,
    rows: &[usize],
    join: &str,
    separator: u8,
    comment: Option<&str>,
) -> Result<Vec<String>>

The names of the columns, from the lines rows names (1-based, counted from the top of the file before anything is skipped), each split on separator and trimmed.

A column’s name is its pieces from those lines, in the order rows gives them, joined with join; a blank piece adds nothing. A line that starts with comment is a header line all the same, since the user named it, and loses the prefix. A file that ends before the last line named is an error: it has no header there.