clone-on-write for Rc

rc-cow.rs

impl<T: Clone> Rc<T>
    #[inline(always)]
    fn inner_mut<'a>(&'a mut self) -> &'a mut RcBox<T> {
        unsafe { &mut (*self._ptr) }
    }

    /// Construct a mutable pointer by cloning the referent into a new
    /// reference-counted box, unless this is the sole reference.
    pub fn to_mut<'a>(&'a mut self) -> &'a mut T {
        // We hold a strong reference, which is also implicitly a weak
        // reference (so self.weak() >= 1).
        if self.weak() > 1 || self.strong() > 1 {
            self.dec_strong();
            self._ptr = transmute(box RcBox {
                value: self.inner().value.clone(),
                strong: Cell::new(1),
                weak: Cell::new(1)
            })
        }

        &mut self.inner_mut().value
    }
}