error: unreachable expression
  --> $DIR/expr_box.rs:16:13
   |
LL |     let x = box return; //~ ERROR unreachable
   |             ^^^^^^^^^^
   |
note: lint level defined here
  --> $DIR/expr_box.rs:13:9
   |
LL | #![deny(unreachable_code)]
   |         ^^^^^^^^^^^^^^^^

error: aborting due to previous error