Monomorphizing an infinitely expanding fn generates an ICE

[nikomatsakis]

This example causes an ICE, which I believe is due to the fact that it requires an infinite series of functions to compile monomorphically:

iface to_opt {
    fn to_option() -> option<self>;
}

impl of to_opt for uint {
    fn to_option() -> option<uint> {
        some(self)
    }
}

impl<T:copy> of to_opt for option<T> {
    fn to_option() -> option<option<T>> {
        some(self)
    }
}

fn function<T:to_opt>(counter: uint, t: T) {
    if counter > 0u {
        function(counter - 1u, t.to_option());
    }
}

fn main() {
    function(22u, 22u);
}

Clearly, I don't think we should compile such a thing. But we should give a better error. It's somewhat unclear to me where would be the best place to detect this pattern.