pub fn region_with_subregion(
        &'a self,
        region: LineColumnRegion,
        sub_region: LineColumnRegion,
    ) -> DocBuilder<'a, Self, Annotation> {
...
        if error_highlight_line {
            let highlight_text = ERROR_UNDERLINE
                .repeat((sub_region.end().column - sub_region.start().column) as usize);

            let highlight_line = self
                .line()
                // Omit the gutter bar when we know there are no further
                // line numbers to be printed after this!
                .append(self.text(" ".repeat(max_line_number_length + GUTTER_BAR_WIDTH)))
                .append(if highlight_text.is_empty() {
                    self.nil()
                } else {
                    self.text(" ".repeat(sub_region.start().column as usize))
                        .indent(indent)
                        .append(self.text(highlight_text).annotate(Annotation::Error))
                });

            result = result.append(highlight_line);
        }
...
}