Code shared from the Rust Playground

playground.rs

fn assert_copy<C: Copy>(_: C) {}
fn assert_fn<F: Fn()>(_: F) {}

struct Opaque(());

fn main() {
    let o = Opaque(());
    let f = move || { &o; };
    assert_copy(f);
    assert_fn(f);
    drop(f);
    drop(o);
}