error[E0658]: box expression syntax is experimental; you can call `Box::new` instead. (see issue #27779)
  --> $DIR/feature-gate-box_syntax.rs:14:13
   |
14 |     let x = box 3;
   |             ^^^^^
   |
   = help: add #![feature(box_syntax)] to the crate attributes to enable

error: aborting due to previous error

If you want more information on this error, try using "rustc --explain E0658"