Segfault after implicit conversion to ~Trait

```
//given this function...
fn func(t: ~Trait) {}

fn main() {
    // this causes some sort of memory unsafety
    func(~Struct {...});
    // and this doesn't
    func(~Struct {...} as ~Trait);
}
```

Full test case at https://gist.github.com/johnsoft/8482878
