diff --git a/compiler/rustc_typeck/src/check/method/probe.rs b/compiler/rustc_typeck/src/check/method/probe.rs
index 88fd94931e1..e65e66c32f6 100644
--- a/compiler/rustc_typeck/src/check/method/probe.rs
+++ b/compiler/rustc_typeck/src/check/method/probe.rs
@@ -1463,6 +1463,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                 TraitCandidate(trait_ref) => {
                     if let Some(method_name) = self.method_name {
                         // Some trait methods are excluded for arrays before 2021.
+                        // (`array.into_iter()` wants a slice iterator for compatibility.)
                         if self_ty.is_array() && !method_name.span.rust_2021() {
                             let trait_def = self.tcx.trait_def(trait_ref.def_id);
                             if trait_def.skip_array_during_method_dispatch {