expand: Tweak a comment in implementation of macro_rules

compiler/rustc_expand/src/mbe/transcribe.rs

@@ -232,17 +232,19 @@ pub(super) fn transcribe<'a>(
  // the meta-var.
  let ident = MacroRulesNormalizedIdent::new(orignal_ident);
  if let Some(cur_matched) = lookup_cur_matched(ident, interp, &repeats) {
- if let MatchedNonterminal(ref nt) = cur_matched {
- // FIXME #2887: why do we apply a mark when matching a token tree meta-var
- // (e.g. `$x:tt`), but not when we are matching any other type of token
- // tree?
- if let NtTT(ref tt) = **nt {
- result.push(tt.clone().into());
+ if let MatchedNonterminal(nt) = cur_matched {
+ let token = if let NtTT(tt) = &**nt {
+ // `tt`s are emitted into the output stream directly as "raw tokens",
+ // without wrapping them into groups.
+ tt.clone()
  } else {
+ // Other variables are emitted into the output stream as groups with
+ // `Delimiter::None` to maintain parsing priorities.
+ // `Interpolated` is currenty used for such groups in rustc parser.
  marker.visit_span(&mut sp);
- let token = TokenTree::token(token::Interpolated(nt.clone()), sp);
-  result.push(token.into());
- }
+ TokenTree::token(token::Interpolated(nt.clone()), sp)
+ };
+ result.push(token.into());
  } else {
  // We were unable to descend far enough. This is an error.
  return Err(cx.struct_span_err(