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<dyn Foo(isize)>;
   |                    ^^^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/29625
   = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0658`.