// 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]); }