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// run-pass struct X { a: isize } trait Changer { fn change(self: Box<Self>) -> Box<Self>; } impl Changer for X { fn change(mut self: Box<X>) -> Box<X> { self.a = 55; self } } pub fn main() { let x: Box<_> = Box::new(X { a: 32 }); let new_x = x.change(); assert_eq!(new_x.a, 55); }