Macro rust_2021 

macro_rules! rust_2021 {
    ($($t:tt)*) => { ... };
}

Switch to Rust 2021.

rust_2021! {
    // Arrays now implement IntoIter.
    let _: i32 = [1, 2, 3].into_iter().next().unwrap();

    // But Box<[T]> doesn't yet. (Note the &i32 instead of i32.)
    let _: &i32 = vec![1, 2, 3].into_boxed_slice().into_iter().next().unwrap();
}