warning: unused generator that must be used
  --> $DIR/yield-in-box.rs:11:5
   |
LL | /     || {
LL | |         let y = 2u32;
LL | |         {
LL | |             let _t = box (&x, yield 0, &y);
...  |
LL | |         }
LL | |     };
   | |______^
   |
   = note: `#[warn(unused_must_use)]` on by default
   = note: generators are lazy and do nothing unless resumed

warning: 1 warning emitted