2018 borrow checker

[DonaldDriscoll]

https://doc.rust-lang.org/rust-by-example/scope/borrow.html shows an example that supposedly does not work, but actually does work with Rust 2018:

{
        // Take a reference to the data contained inside the box
        let _ref_to_i32: &i32 = &boxed_i32;

        // Error!
        // Can't destroy `boxed_i32` while the inner value is borrowed.
        eat_box_i32(boxed_i32);
        // FIXME ^ Comment out this line

        // `_ref_to_i32` goes out of scope and is no longer borrowed.
    }