diff --git a/compiler/rustc_query_impl/src/plumbing.rs b/compiler/rustc_query_impl/src/plumbing.rs
index 0d9d6d6f582..244f0e84b43 100644
--- a/compiler/rustc_query_impl/src/plumbing.rs
+++ b/compiler/rustc_query_impl/src/plumbing.rs
@@ -700,6 +700,8 @@ macro_rules! define_queries {
             }
         }
 
+        // These arrays are used for iteration and can't be indexed by `DepKind`.
+
         const TRY_COLLECT_ACTIVE_JOBS: &[for<'tcx> fn(TyCtxt<'tcx>, &mut QueryMap<DepKind>)] =
             &[$(query_impl::$name::try_collect_active_jobs),*];