fix for rustc master

Author: oli-obk

clippy_lints/src/len_zero.rs

@@ -276,7 +276,7 @@ fn has_is_empty(cx: &LateContext<'_, '_>, expr: &Expr) -> bool {
     let ty = &walk_ptrs_ty(cx.tables.expr_ty(expr));
     match ty.sty {
         ty::Dynamic(ref tt, ..) => cx.tcx
-            .associated_items(tt.principal().expect("trait impl not found").def_id())
+            .associated_items(tt.principal().def_id())
             .any(|item| is_is_empty(cx, &item)),
         ty::Projection(ref proj) => has_is_empty_impl(cx, proj.item_def_id),
         ty::Adt(id, _) => has_is_empty_impl(cx, id.did),