expand: Tweak a comment in implementation of macro_rules

Author: petrochenkov

Diff for: 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(