Function count_append 

pub(crate) fn count_append(
    before: LineCount,
    open_tail: &[u8],
    at: u64,
    data: &[u8],
) -> LineCount

The counter after appending data at byte offset at to a file whose counter was before. open_tail is the file’s bytes from before.tail_start to at — the unterminated last line, empty when the file ends with \n — because appended bytes continue that line and can change whether it is blank.