rustc does not detect some recursive struct definitions

I think the impl is just necessary to get Rust to try to instantiate it (otherwise it assumes it's unused).  This may be a duplicate of issue #4363.

``` rust
use std::sync::Mutex;

struct Foo {
    foo: Mutex<Option<Foo>>,
}

impl Foo {
    fn bar(self) {}
}

fn main() {}
```

---

``` console
task 'rustc' has overflowed its stack
```
