error[E0658]: parenthetical notation is only stable when used with `Fn`-family traits --> $DIR/unboxed-closure-feature-gate.rs:13:20 | LL | let x: Box; | ^^^^^^^^^^ | = note: see issue #29625 for more information = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0658`.