unmovable.rs

use std::cell::Cell;
use std::kinds::marker;

struct Unmovable<'a> {
    lock: Cell<marker::ContravariantLifetime<'a>>,
    marker: marker::NoCopy
}

impl<'a> Unmovable<'a> {
    fn new() -> Unmovable<'a> {
        Unmovable { 
            lock: Cell::new(marker::ContravariantLifetime),
            marker: marker::NoCopy
        }
    }

    fn lock(&'a self) {
        self.lock.set(marker::ContravariantLifetime);
    }
    
    fn new_in(self_: &'a mut Option<Unmovable<'a>>) {
        *self_ = Some(Unmovable::new());
        self_.as_ref().unwrap().lock();
    }
}

fn main(){
    let x = Unmovable::new();
    x.lock();
    
    // error: cannot move out of `x` because it is borrowed
    // let z = x; 
    
    let mut y = None;
    Unmovable::new_in(&mut y);
    
    // error: cannot move out of `y` because it is borrowed
    // let z = y; 
    
    assert_eq!(std::mem::size_of::<Unmovable>(), 0)
}