Example

struct Foo {
    data: *const Bar,
}
struct Bar {
    data: *const Foo,
}

Possible output

struct Bar;
struct Foo {
    const Bar* data;
}
struct Bar {
    const Foo* data;
}

This would require the dependency ordering algorithm to understand when a reference needs a declaration and when it needs a definition. If in the previous example, the data members were not pointers then it wouldn't work.