rust/src/test/run-pass/union/union-trait-impl.rs

// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

use std::fmt;

union U {
    a: u8
}

impl fmt::Display for U {
    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
        unsafe { write!(f, "Oh hai {}", self.a) }
    }
}

fn main() {
    assert_eq!(U { a: 2 }.to_string(), "Oh hai 2");
}