src/test/ui/feature-gate-box-expr.stderr

error: box expression syntax is experimental; you can call `Box::new` instead. (see issue #27779)
  --> $DIR/feature-gate-box-expr.rs:22:13
   |
22 |     let x = box 'c'; //~ ERROR box expression syntax is experimental
   |             ^^^^^^^
   |
   = help: add #![feature(box_syntax)] to the crate attributes to enable

error: aborting due to previous error