self types unsound with boxed ifaces

[nikomatsakis]

Currently, if a method on an iface mentions the type self, we forbid that method from being called via a boxed iface. But this restriction can be worked around by using generic types:

    iface i { fn foo() -> self<>; }
    fn foo<T:i>(v: T) -> T {
        ret v.foo();
    }
    impl of i for uint {
        fn foo() -> uint { ret self; }
    }
    fn main() {
        let x: i = 3u as i;
        let y = foo(x); // y has type i, but is in fact a uint
    }

A couple of solutions spring to mind, but on IRC we discussed saying that if an iface i includes a self type, then instances of i do not implement i.

[nikomatsakis]

A similar problem likely exists with monomorphization and calls to methods with type parameters. The same solution ought to prevent that bug as well.