Fix panic when handling intra doc links generated from macro

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 {

src/test/rustdoc-ui/auxiliary/module_macro_doc.rs

@@ -0,0 +1 @@
+//! [`long_cat`] is really long

src/test/rustdoc-ui/macro-docs.rs

@@ -0,0 +1,12 @@
+// check-pass
+
+macro_rules! m {
+    () => {
+        /// A
+        //~^ WARNING
+        #[path = "auxiliary/module_macro_doc.rs"]
+        pub mod mymodule;
+    }
+}
+
+m!();

src/test/rustdoc-ui/macro-docs.stderr

@@ -0,0 +1,20 @@
+warning: unresolved link to `long_cat`
+  --> $DIR/macro-docs.rs:5:9
+   |
+LL |         /// A
+   |         ^^^^^
+...
+LL | m!();
+   | ---- in this macro invocation
+   |
+   = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default
+   = note: the link appears in this line:
+
+           [`long_cat`] is really long
+            ^^^^^^^^^^
+   = note: no item named `long_cat` in scope
+   = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`
+   = note: this warning originates in the macro `m` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+warning: 1 warning emitted
+