Function rustc_errors::push_trailing

pub(crate) fn push_trailing(
    buf: &mut String,
    line_opt: Option<&Cow<'_, str>>,
    lo: &Loc,
    hi_opt: Option<&Loc>
) -> usize

Append to a buffer the remainder of the line of existing source code, and return the count of lines that have been added for accurate highlighting.