src/test/run-pass/variance-intersection-of-ref-and-opt-ref.rs

// Copyright 2015 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.

// Elaborated version of the opening example from RFC 738. This failed
// to compile before variance because invariance of `Option` prevented
// us from approximating the lifetimes of `field1` and `field2` to a
// common intersection.

#![allow(dead_code)]

struct List<'l> {
    field1: &'l i32,
    field2: Option<&'l i32>,
}

fn foo(field1: &i32, field2: Option<&i32>) -> i32 {
    let list = List { field1: field1, field2: field2 };
    *list.field1 + list.field2.cloned().unwrap_or(0)
}

fn main() {
    let x = 22;
    let y = Some(3);
    let z = None;
    assert_eq!(foo(&x, y.as_ref()), 25);
    assert_eq!(foo(&x, z.as_ref()), 22);
}