Make the ordering of non-terminal binds in ambiguity error messages deterministic

compiler/rustc_expand/src/mbe/macro_parser.rs

@@ -696,11 +696,15 @@ impl TtParser {
     }
 
     fn ambiguity_error<'matcher, T: Tracker<'matcher>>(
-        &self,
+        &mut self,
         parser: &Parser<'_>,
         matcher: &'matcher [MatcherLoc],
         track: &mut T,
     ) -> NamedParseResult<T::Failure> {
+        // Use a reasonable and deterministic ordering for data in the error message.
+        self.bb_mps.sort_unstable_by_key(|mp| mp.idx);
+        self.next_mps.sort_unstable_by_key(|mp| mp.idx);
+
         let bb_locs = self.bb_mps.iter().map(|mp| &matcher[mp.idx]);
         let next_locs = self.next_mps.iter().map(|mp| &matcher[mp.idx]);
         track.ambiguity(parser, bb_locs, next_locs);

tests/ui/macros/local-ambiguity-option-ordering.rs

@@ -0,0 +1,49 @@
+// Test the order in which meta-variable options causing ambiguity are presented in error messages.
+
+#![crate_type = "lib"]
+
+macro_rules! ambiguity_1 {
+    ($($i:ident)* $j:ident) => {};
+}
+
+ambiguity_1!(error); //~ ERROR local ambiguity
+
+macro_rules! ambiguity_2 {
+    ( $( $( ; $i:ident )? $( $j:ident )? ; )* ) => {};
+}
+
+ambiguity_2!( ; error ); //~ ERROR local ambiguity
+//
+// Parse tree:
+// - `$(...)*`:
+// - Try one repetition
+//   - `$(; $i)?`:
+//   - Try entering
+//     - Parse `;`
+// ----> `$i` can parse `error`
+//   - Try ignoring
+//     - `$($j)?`:
+//     - Try entering
+//       - `$j` cannot parse `;`
+//       - failure
+//     - Try ignoring
+//       - Parse `;`
+//       - `$(...)*`:
+//       - Try another repetition
+//         - `$(; $i)?`:
+//         - Try entering
+//           - Cannot parse `;`
+//           - failure
+//         - Try ignoring
+//           - `$($j)?`:
+//           - Try entering
+// ------------> `$j` can parse `error`
+//           - Try ignoring
+//             - Cannot parse `;`
+//             - failure
+//       - Try ignoring
+//         - EOF vs `error`
+//         - failure
+// - Try no repetitions
+//   - EOF vs `;`
+//   - failure

tests/ui/macros/local-ambiguity-option-ordering.stderr

@@ -0,0 +1,14 @@
+error: local ambiguity when calling macro `ambiguity_1`: multiple parsing options: built-in NTs ident ('i') or ident ('j').
+  --> $DIR/local-ambiguity-option-ordering.rs:9:14
+   |
+LL | ambiguity_1!(error);
+   |              ^^^^^
+
+error: local ambiguity when calling macro `ambiguity_2`: multiple parsing options: built-in NTs ident ('i') or ident ('j').
+  --> $DIR/local-ambiguity-option-ordering.rs:15:17
+   |
+LL | ambiguity_2!( ; error );
+   |                 ^^^^^
+
+error: aborting due to 2 previous errors
+