Implement Deref for &T

[kennytm]

As titled. Add the following trait implementations:

impl<'a, Sized? T> Deref<T> for &'a T {
    fn deref(&self) -> &T { *self }
}
impl<'a, Sized? T> Deref<T> for &'a mut T {
    fn deref(&self) -> &S { *self }
}
impl<'a, Sized? T> DerefMut<T> for &'a mut T {
    fn deref_mut(&mut self) -> &mut T { *self }
}

This allows generic code that accepts smart pointer can take references too, e.g.

fn sum_pointers<V: Deref<int>>(v: &[V]) -> int;
sum_pointers([Rc::new(1i), Rc::new(2i)]);
sum_pointers([&1i, &2i, &3i]);

fn sum_2d<V: Deref<[int]>>(v: &[V]) -> int;
sum_2d(vec![vec![2,3], vec![4,5,6]].as_slice());
sum_2d([[2,3].as_slice(), [4,5,6].as_slice()].as_slice());

[aturon]

cc @nikomatsakis

[nikomatsakis]

yes I want this 👍

[Gankra]

This would also allow code to be polymorphic over &T and &mut T by accepting Deref<T>. A huge win imho.

[aturon]

The branch here creates some trouble.