diff --git a/src/token.rs b/src/token.rs
index 8827f0b1..3e853635 100644
--- a/src/token.rs
+++ b/src/token.rs
@@ -819,6 +819,7 @@ macro_rules! export_token_macro {
 #[cfg(syn_can_match_trailing_dollar)]
 export_token_macro![($)];
 
+// Old rustc does not support ($) => {...} as a macro rule.
 #[cfg(not(syn_can_match_trailing_dollar))]
 export_token_macro![];