// run-pass
use std::ops::{Deref, DerefMut};

struct CheckedDeref<T, F> {
    value: T,
    check: F
}

impl<F: Fn(&T) -> bool, T> Deref for CheckedDeref<T, F> {
    type Target = T;
    fn deref(&self) -> &T {
        assert!((self.check)(&self.value));
        &self.value
    }
}

impl<F: Fn(&T) -> bool, T> DerefMut for CheckedDeref<T, F> {
    fn deref_mut(&mut self) -> &mut T {
        assert!((self.check)(&self.value));
        &mut self.value
    }
}


fn main() {
    let mut v = CheckedDeref {
        value: vec![0],
        check: |v: &Vec<_>| !v.is_empty()
    };
    v.push(1);
    assert_eq!(*v, vec![0, 1]);
}