macro-substitute: Do not substitute non-repetition fragments in sub-maps

When creating a sub-map for repetitions, we need to be weary of not
accessing matched-fragments beyond the vector's size. For example, with
the following *fragments*

{ "e": [1], "es": [2, 3, 10]},

the sub-maps we want to create are the following:

{ "e": [1], "es": [2]},
{ "e": [1], "es": [3]},
{ "e": [1], "es": [10]},

Up until this point however, we were trying to access the second index
for the "e" metavar when creating the second submap, then the third, and
  so on... which is obviously outside of the vector's bounds.

We can simply check if the metavar only has one match and expand that
one in that case. We still need to work on checking that multiple
metavars in the same transcriber repetition pattern have the same amount
of matched fragments (ie the original vectors of matches in the
original map have the same size)

Author: CohenArthur

gcc/rust/expand/rust-macro-substitute-ctx.cc

@@ -17,8 +17,8 @@ SubstituteCtx::substitute_metavar (std::unique_ptr<AST::Token> &metavar)
   else
     {
       // Replace
-      // We only care about the vector when expanding repetitions. Just access
-      // the first element of the vector.
+      // We only care about the vector when expanding repetitions.
+      // Just access the first element of the vector.
       // FIXME: Clean this up so it makes more sense
       auto &frag = it->second[0];
       for (size_t offs = frag.token_offset_begin; offs < frag.token_offset_end;
@@ -103,7 +103,13 @@ SubstituteCtx::substitute_repetition (size_t pattern_start, size_t pattern_end)
       for (auto &kv_match : fragments)
 	{
 	  std::vector<MatchedFragment> sub_vec;
-	  sub_vec.emplace_back (kv_match.second[i]);
+
+	  // FIXME: Hack: If a fragment is not repeated, how does it fit in the
+	  // submap? Do we really want to expand it? Is this normal behavior?
+	  if (kv_match.second.size () == 1)
+	    sub_vec.emplace_back (kv_match.second[0]);
+	  else
+	    sub_vec.emplace_back (kv_match.second[i]);
 
 	  sub_map.insert ({kv_match.first, sub_vec});
 	}
@@ -177,6 +183,7 @@ std::vector<std::unique_ptr<AST::Token>>
 SubstituteCtx::substitute_tokens ()
 {
   std::vector<std::unique_ptr<AST::Token>> replaced_tokens;
+  rust_debug ("expanding tokens");
 
   for (size_t i = 0; i < macro.size (); i++)
     {