hang in fulfill due to exponential blowup

```rust
trait Trait {}

struct W<T>(T);

impl<T, U> Trait for W<(W<T>, W<U>)>
where
    W<T>: Trait,
    W<U>: Trait,
{}

fn impls<T: Trait>() {}

fn main() {
    impls::<W<_>>()
}
```
this program results in a hang in both rustc (fulfill) and in r-a (causing my PC to freeze).

if we prove this using a breadth first approach we try to prove `O(2^recursion_limit)` goals