error: unnecessary allocation, use `&` instead
  --> $DIR/unused-allocation.rs:4:9
   |
LL |     _ = Box::new([1]).len();
   |         ^^^^^^^^^^^^^
   |
note: the lint level is defined here
  --> $DIR/unused-allocation.rs:1:9
   |
LL | #![deny(unused_allocation)]
   |         ^^^^^^^^^^^^^^^^^

error: aborting due to 1 previous error