Comment by andreabergia

The problem arises by the fact that `Class` needs to refer to other classes:

    pub struct Class<'a> {
        pub superclass: Option<ClassRef<'a>>,
    }
    pub type ClassRef<'a> = &'a Class<'a>;

Given that classes are managed by the arena, I know the reference will not be dangling thanks to the 'a lifetime.

Initially I had implemented this with raw pointers, without lifetimes, but then I switched to the reference because it felt more "idiomatic" and I had to put the lifetimes just about everywhere to make the compiler happy.

If there are better ways to do this, I would be really happy to learn, though!