foo.rs

pub enum Owned<T> {
    Empty,
    Mine(T),
}

impl<T> Owned<T> {
    pub fn take(&mut self) -> T {
        match std::mem::replace(self, Owned::Empty) {
            Owned::Mine(t) => t,
            Owned::Empty => panic!("Can't call take on Empty"),
        }
    }

    pub fn as_ref(&self) -> &T {
        match self {
            Owned::Empty => panic!("Can't call as_ref on Empty"),
            Owned::Mine(t) => &t,
        }
    }
}