rustc should not infinite-loop on nested data type

[pnkfelix]

Some languages support "nested data types" (aka non-uniform, aka non-regular). What this basically means are recursive structures S<A> where the recursive references to S within its definition do not take the form S<A>, but instead take the form S<T<A>> for some non-trivial type constructor T.

So, as an example:

enum Seq<A> {
Nil,
Cons{hd: A, tl:~Seq<(A,A)>}
}

impl<A> Seq<A> {
fn len(&self) -> int {
match self {
&Nil => 0,
&Cons{ hd: x, tl: ref s } => 1 + 2 * s.len()
}
}
}

fn main() {
io::println(fmt!("Hello World"));
}

But right now rustc seems to infinite loop on the above. I need to investigate more.

[catamorphism]

Dup of #7587