Feature request

It'd be nice to be able to achieve friend functions with PHPStan. An example of such usage that just popped up in our project:

class Product
{
    public function addCategory(Category $category): void;
}

class Category
{
    /**
     * @internal
     * @see Product::addCategory
     */
    public function addProduct(Product $product): void;
}

Category::addProduct should never be called directly but needs to be public so it might be called from Product::addCategory. It would be amazing to have a rule that forbids calls to methods marked as @internal unless a @friend FQCN::method (or something alike) is specified and the call is made from specified context.

What do you think?