add helper methods for accessing struct tail

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -1125,12 +1125,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     return Err(Unimplemented);
                 }
 
-                let tail_field = def
-                    .non_enum_variant()
-                    .fields
-                    .raw
-                    .last()
-                    .expect("expected unsized ADT to have a tail field");
+                let tail_field = def.non_enum_variant().tail();
                 let tail_field_ty = tcx.type_of(tail_field.did);
 
                 // Extract `TailField<T>` and `TailField<U>` from `Struct<T>` and `Struct<U>`,