rust/src/test/compile-fail/assignment-operator-unimplemented.rs

// Copyright 2014 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.

struct Foo;

fn main() {
  let mut a = Foo;
  let ref b = Foo;
  a += *b; //~ Error: binary assignment operation `+=` cannot be applied to type `Foo`
}