UnsafeCell now has no niches, ever.

compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs

@@ -96,13 +96,13 @@ impl Qualif for HasMutInterior {
     }
 
     fn in_adt_inherently<'tcx>(
-        cx: &ConstCx<'_, 'tcx>,
+        _cx: &ConstCx<'_, 'tcx>,
         adt: AdtDef<'tcx>,
         _: SubstsRef<'tcx>,
     ) -> bool {
         // Exactly one type, `UnsafeCell`, has the `HasMutInterior` qualif inherently.
         // It arises structurally for all other types.
-        Some(adt.did()) == cx.tcx.lang_items().unsafe_cell_type()
+        adt.is_unsafe_cell()
     }
 }