This only works on 2015 edition. This code sample will fail to compile on [beta](https://play.rust-lang.org/?version=beta&mode=debug&edition=2015) but succeed on [stable](https://play.rust-lang.org/?version=stable&mode=debug&edition=2015). ```rust #![no_implicit_prelude] fn _foo() { assert_eq!(10, 20); } ``` cc @petrochenkov