rustc infinite loop on recursive type (FingerTree, indirectly polymorphic/nonregular)

[pnkfelix]

Spawned off of #4363, specifically the example provided by @goffrie

This example code causes rustc to infinite loop (playground):

enum FingerTree<A> {
    Empty,
    Single(A),
    Deep(Node<A>)
}

struct Node<A> {
    count: i32,
    front: Digit<A>,
    inner: Box<FingerTree<(A,A)>>,
    back: Digit<A>
}

struct Digit<A> {
    count: i32,
    content: [Option<A>; 4]
}

fn FingerTree<A>() -> FingerTree<A> { FingerTree::Empty }

fn main() {
    let _ = FingerTree::Deep(Node { count: 0,
        front: Digit { count: 0, content: [None, None, None, None] },
        inner: Box::new(FingerTree::Single((1, 2))),
        back: Digit { count: 0, content: [None, None, None, None] }}
    );
}

[pnkfelix]

I wonder if we should have a specific I-diverges, I-hang, or I-infinite-loop label.

Such cases often cause stack overflows (which then usually get categorized as an ICE), but this particular instance is not overflowing; its just infinite looping AFAICT, which is in some ways much worse than an ICE, since infinite loops are often much more annoying to deal with in various environments.

(I kind of like the I-hang label, since that would presumably also cover cases of deadlock or livelock in stdlib routines that should not be allowed to do so, and thus be a more generally useful label.)