Cannot move from individual upvars in a by-ref once closure

[nikomatsakis]

Example test that should work:

// 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.

// Test that in a by-ref once closure we move some variables even as
// we capture others by mutable reference.

fn call<F>(f: F) where F : FnOnce() {
    f();
}

fn main() {
    let mut x = vec!(format!("Hello"));
    let y = vec!(format!("World"));
    call(|| {
        // Here: `x` must be captured with a mutable reference in
        // order for us to append on it, and `y` must be captured by
        // value.
        for item in y.into_iter() {
            x.push(item);
        }
    });
    assert_eq!(x.len(), 2);
    assert_eq!(&*x[0], "Hello");
    assert_eq!(&*x[1], "World");
}