Code shared from the Rust Playground

playground.rs

#![stable(feature = "playground", since = "0.0.0")]
#![feature(const_fn, staged_api)]
#![allow(dead_code)]

use core::marker::PhantomData;

#[stable(feature = "iter_empty", since = "1.2.0")]
pub struct Empty<T: 'static>(PhantomData<&'static T>);

impl<T> Empty<T> {
    #[stable(feature = "iter_empty", since = "1.2.0")]
    pub const fn new() -> Self {
        Empty(PhantomData)
    }
}

fn covariant<'a>(empty: Empty<&'static u8>) -> Empty<&'a u8> {
    empty
}

fn main() {
    fn assert_send_and_sync<T: Send + Sync>() {}
    let _ = assert_send_and_sync::<Empty<*mut u8>>;
}