Make it possible to create boxed iface instances without re-boxing

[marijnh]

A boxed iface is a pair of a regular box holding the self value and a vtable pointer. When creating such a thing from a box, one currently has to do (*my_box) as my_iface. It would be great if we could somehow just bump the refcount of the box and slap a vtable on it.

Proposed solution: make cast-to-iface, when no impl is found, autoderef the left hand side. If it finds a match for the box' contents, it'll resolve to that impl, and the trans code will notice this and skip the boxing. So my_box as my_iface would be a very cheap operation.

[catamorphism]

You don't have to do (*my_box) as my_iface anymore -- that is, this works:

struct Bird {
    x: int
}

trait Chirpy {
    fn tweet();
}

impl Bird: Chirpy {
    fn tweet() {}
}

fn main() {
    let x = @Bird{x: 5};
    let y = x as @Chirpy;
}

But looking at the generated code, it looks like the cast still involves copying x, so this is still an issue.

[nikomatsakis]

It should not be copying the contents of x but just bumping the ref count.

[pcwalton]

Fixed in 0.6.