// run-pass #![allow(non_camel_case_types)] trait double { fn double(self: Box<Self>) -> usize; } impl double for usize { fn double(self: Box<usize>) -> usize { *self * 2 } } pub fn main() { let x: Box<Box<_>> = Box::new(Box::new(3)); assert_eq!(x.double(), 6); }