be conservative in has_significant_drop

compiler/rustc_middle/src/ty/util.rs

@@ -816,6 +816,15 @@ impl<'tcx> ty::TyS<'tcx> {
                     [component_ty] => component_ty,
                     _ => self,
                 };
+
+                // FIXME: We should be canonicalizing, or else moving this to a method of inference
+                // context, or *something* like that, but for now just avoid passing inference
+                // variables to queries that can't cope with them. Instead, conservatively
+                // return "true" (may change drop order).
+                if query_ty.needs_infer() {
+                    return true;
+                }
+
                 // This doesn't depend on regions, so try to minimize distinct
                 // query keys used.
                 let erased = tcx.normalize_erasing_regions(param_env, query_ty);