Fix panic when intra-doc link comes from a generated doc comment

Author: GuillaumeGomez

src/librustdoc/passes/mod.rs

@@ -178,7 +178,7 @@ crate fn source_span_for_markdown_range(
 
     'outer: for (line_no, md_line) in md_lines.enumerate() {
         loop {
-            let source_line = src_lines.next().expect("could not find markdown in source");
+            let source_line = src_lines.next()?;
             match source_line.find(md_line) {
                 Some(offset) => {
                     if line_no == starting_line {