Add some comments to QueryArenas

compiler/rustc_middle/src/query/plumbing.rs

@@ -342,12 +342,18 @@ pub fn provided_to_erased<'tcx>(
             })*
         }
 
+        /// Holds per-query arenas for queries with the `arena_cache` modifier.
         #[derive(Default)]
         pub struct QueryArenas<'tcx> {
-            $($(#[$attr])* pub $name: query_if_arena!([$($modifiers)*]
-                (TypedArena<<$V as $crate::query::arena_cached::ArenaCached<'tcx>>::Allocated>)
-                ()
-            ),)*
+            $(
+                $(#[$attr])*
+                pub $name: query_if_arena!([$($modifiers)*]
+                    // Use the `ArenaCached` helper trait to determine the arena's value type.
+                    (TypedArena<<$V as $crate::query::arena_cached::ArenaCached<'tcx>>::Allocated>)
+                    // No arena for this query, so the field type is `()`.
+                    ()
+                ),
+            )*
         }
 
         #[derive(Default)]