struct PtrWrapper<'a> {
    ptr: u64,
    callback: Box<Fn(&'a PtrWrapper<'a>)>,
}

impl<'a> PtrWrapper<'a> {
    pub fn from(ptr: u64) -> Self {
        let cb = move |pw: &PtrWrapper| {
            println!("raw pointer: 0x{:x}", pw.ptr);
        };
        PtrWrapper {
            ptr: ptr,
            callback: Box::new(cb),
        }
    }
}

fn main() {
    let pw = PtrWrapper::from(1); // let's call its lifetime 'a
    {
        let pw2 = PtrWrapper::from(2); // and this one 'b
        (pw2.callback)(&pw);
                    // ↑ borrows for 'a because Box<Fn(&'a PtrWrapper<'a>)>
                                 // these two are equal ↑             ↑
        // which means pw2.callback must live for at least 'a
    }
    (pw.callback)(&pw);
    // same here pw.callback must live for at least as long as pw
    // but pw.callback is dropped before
}