error[E0435]: attempt to use a non-constant value in a constant
  --> $DIR/issue-3668.rs:8:34
   |
LL |        static childVal: Box<P> = self.child.get();
   |        ---------------           ^^^^ non-constant value
   |        |
   |        help: consider using `let` instead of `static`: `let childVal`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0435`.