diff --git a/crates/tor-cell/src/relaycell/hs/ext.rs b/crates/tor-cell/src/relaycell/hs/ext.rs
index 2eb264674..3e8bbed14 100644
--- a/crates/tor-cell/src/relaycell/hs/ext.rs
+++ b/crates/tor-cell/src/relaycell/hs/ext.rs
@@ -127,6 +127,10 @@ impl<ID> UnrecognizedExt<ID> {
 }
 
 /// Declare an Extension group that takes a given identifier.
+//
+// TODO: This is rather similar to restrict_msg(), isn't it?  Also, We use this
+// pattern of (number, (cmd, length, body)*) a few of times in Tor outside the
+// hs module.  Perhaps we can extend and unify our code here...
 macro_rules! decl_extension_group {
     {
         $( #[$meta:meta] )*