add comment to visit_ct_substs

compiler/rustc_infer/src/infer/combine.rs

@@ -881,9 +881,7 @@ fn tys(&mut self, t: Ty<'tcx>, _t: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
                     }
                 }
             }
-            ty::Infer(ty::IntVar(_) | ty::FloatVar(_)) => {
-                Ok(t)
-            }
+            ty::Infer(ty::IntVar(_) | ty::FloatVar(_)) => Ok(t),
             _ => relate::super_relate_tys(self, t, t),
         }
     }

compiler/rustc_middle/src/ty/relate.rs

@@ -33,6 +33,11 @@ pub trait TypeRelation<'tcx>: Sized {
     /// relation. Just affects error messages.
     fn a_is_expected(&self) -> bool;
 
+    /// Whether we should look into the substs of unevaluated constants
+    /// even if `feature(const_evaluatable_checked)` is active.
+    ///
+    /// This is needed in `combine` to prevent accidentially creating
+    /// infinite types as we abuse `TypeRelation` to walk a type there.
     fn visit_ct_substs(&self) -> bool {
         false
     }