message: tweak and slightly extend box-box-trait test
diff: -#![feature(box_syntax)]
  -
  struct DroppableStruct;
  static mut DROPPED: bool = false;
  @@ -13,7 +11,6 @@ impl Drop for DroppableStruct {
  trait MyTrait { fn dummy(&self) { } }
  impl MyTrait for Box<DroppableStruct> {}
  -#[allow(dead_code)]
  struct Whatever { w: Box<dyn MyTrait+'static> }
  impl Whatever {
  @@ -24,8 +21,9 @@ impl Whatever {
  fn main() {
  {
  - let f: Box<_> = box DroppableStruct;
  - let _a = Whatever::new(box f as Box<dyn MyTrait>);
  + let f = Box::new(DroppableStruct);
  + let a = Whatever::new(Box::new(f) as Box<dyn MyTrait>);
  + a.w.dummy();
  }
  assert!(unsafe { DROPPED });
  }
  