An odd feature of Rust is that `Foo` is invalid, but `Bar` is okay:
type Foo<'a, 'b> = Box<dyn PartialEq<Foo<'a, 'b>>>;
type Bar<'a, 'b> = impl PartialEq<Bar<'a, 'b>>;
To get it right, track every time rustdoc descends into a type alias,
so if it shows up twice, it can be write the path instead of
infinitely expanding it.
