#![allow(unused)]
fn main() {
use std::pin::Pin;
fn swap_pins<T>(x: Pin<&mut T>, y: Pin<&mut T>) {
    // `mem::swap` needs `&mut T`, but we cannot get it.
    // We are stuck, we cannot swap the contents of these references.
    // We could use `Pin::get_unchecked_mut`, but that is unsafe for a reason:
    // we are not allowed to use it for moving things out of the `Pin`.
}
}