expansion: Only add fragments if the matcher succeeded

gcc/rust/ChangeLog:

	* expand/rust-macro-expand.cc (MacroExpander::match_n_matches): Do not
	insert fragments and substack fragments if the matcher failed.

gcc/testsuite/ChangeLog:

	* rust/compile/macros/mbe/macro-issue3708.rs: New test.

1 files changed, 8 insertions, 7 deletions

diff --git a/gcc/rust/expand/rust-macro-expand.cc b/gcc/rust/expand/rust-macro-expand.cc
index 7829757..c22db1c 100644
--- a/gcc/rust/expand/rust-macro-expand.cc
+++ b/gcc/rust/expand/rust-macro-expand.cc
@@ -621,9 +621,10 @@ MacroExpander::match_n_matches (Parser<MacroInvocLexer> &parser,
 		// matched fragment get the offset in the token stream
 		size_t offs_end = source.get_offs ();
 
-		sub_stack.insert_metavar (
-		  MatchedFragment (fragment->get_ident ().as_string (),
-				   offs_begin, offs_end));
+		if (valid_current_match)
+		  sub_stack.insert_metavar (
+		    MatchedFragment (fragment->get_ident ().as_string (),
+				     offs_begin, offs_end));
 	      }
 	      break;
 
@@ -650,15 +651,15 @@ MacroExpander::match_n_matches (Parser<MacroInvocLexer> &parser,
 	}
       auto old_stack = sub_stack.pop ();
 
-      // nest metavars into repetitions
-      for (auto &ent : old_stack)
-	sub_stack.append_fragment (ent.first, std::move (ent.second));
-
       // If we've encountered an error once, stop trying to match more
       // repetitions
       if (!valid_current_match)
 	break;
 
+      // nest metavars into repetitions
+      for (auto &ent : old_stack)
+	sub_stack.append_fragment (ent.first, std::move (ent.second));
+
       match_amount++;
 
       // Break early if we notice there's too many expressions already