parse_stmt_mac: add a comment

src/librustc_parse/parser/stmt.rs

@@ -180,6 +180,7 @@ impl<'a> Parser<'a> {
             self.warn_missing_semicolon();
             StmtKind::Mac(P((mac, style, attrs)))
         } else {
+            // Since none of the above applied, this is an expression statement macro.
             let e = self.mk_expr(lo.to(hi), ExprKind::Mac(mac), ThinVec::new());
             let e = self.maybe_recover_from_bad_qpath(e, true)?;
             let e = self.parse_dot_or_call_expr_with(e, lo, attrs)?;