Implement Deref for Box

[kennytm]

As titled. Currently Box does not implement Deref:

fn f<G: Deref<int>>(g: &G) -> int { **g }
fn main() {
    f(&std::rc::Rc::new(1i));   // ok
    f(&box 1i); // error. Deref<int> not impl for Box<int>
}

But Box should be able to implement Deref.

impl<Sized? T> Deref<T> for Box<T> {
    fn deref(&self) -> &T { &**self }  // or any equivalent like `self.0.as_ref().unwrap()`
}
impl<Sized? T> DerefMut<T> for Box<T> {
    fn deref_mut(&mut self) -> &mut T { &mut **self }
}

[huonw]

(Tagged E-easy in the hope that it's as simple as adding the impls @kennytm gives in the issue. There is the chance that there is some compiler peculiarities that make this not-easy.)

[huonw]

@nikomatsakis has kindly volunteered to mentor; catch him here or as nmatsakis on IRC.

[vojtechkral]

Can I have a go at this? Looks easy enough... @nikomatsakis

[huonw]

@vojtechkral it's already been done/attempted in #19023. 😄

[vojtechkral]

Oh well, never mind, I must've overlooked the cross-ref...

[darnuria]

@huonw This issue seems resolved due to #19023 close and merge.

[huonw]

@darnuria unfortunately it wasn't merged.