implement Copy/Clone for generators

compiler/rustc_ty_utils/src/instance.rs

@@ -263,7 +263,10 @@ fn resolve_associated_item<'tcx>(
                     let is_copy = self_ty.is_copy_modulo_regions(tcx.at(DUMMY_SP), param_env);
                     match self_ty.kind() {
                         _ if is_copy => (),
-                        ty::Closure(..) | ty::Tuple(..) => {}
+                        ty::Generator(..) |
+                        ty::GeneratorWitness(..) |
+                        ty::Closure(..) |
+                        ty::Tuple(..) => {},
                         _ => return Ok(None),
                     };