So, I've decided to do my own GC. Have now reached the hash table chapter. Rust already has its own HashMap. I could use that as a shortcut. Or I could try to roll with the book and make my own.
At the moment, the Hash in the book seems somewhat disconnected from everything else. And I don't recall Lox as having HashMap as a kind of value that you could use in user code. Somewhat inclined to use Rust HashMap<> for this until I run into a reason not to.