// run-pass use std::fmt; struct Thingy { x: isize, y: isize } impl fmt::Debug for Thingy { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { write!(f, "{{ x: {:?}, y: {:?} }}", self.x, self.y) } } struct PolymorphicThingy { x: T } impl fmt::Debug for PolymorphicThingy { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { write!(f, "{:?}", self.x) } } pub fn main() { println!("{:?}", Thingy { x: 1, y: 2 }); println!("{:?}", PolymorphicThingy { x: Thingy { x: 1, y: 2 } }); }