span_to_lines: account for DUMMY_SP

src/librustc_span/source_map.rs

@@ -499,14 +499,15 @@ pub fn span_to_lines(&self, sp: Span) -> FileLinesResult {
         // and to the end of the line. Be careful because the line
         // numbers in Loc are 1-based, so we subtract 1 to get 0-based
         // lines.
-        for line_index in lo.line - 1..hi.line - 1 {
+        let hi_line = hi.line.saturating_sub(1);
+        for line_index in lo.line.saturating_sub(1)..hi_line {
             let line_len = lo.file.get_line(line_index).map(|s| s.chars().count()).unwrap_or(0);
             lines.push(LineInfo { line_index, start_col, end_col: CharPos::from_usize(line_len) });
             start_col = CharPos::from_usize(0);
         }
 
         // For the last line, it extends from `start_col` to `hi.col`:
-        lines.push(LineInfo { line_index: hi.line - 1, start_col, end_col: hi.col });
+        lines.push(LineInfo { line_index: hi_line, start_col, end_col: hi.col });
 
         Ok(FileLines { file: lo.file, lines })
     }