For the uninitiated, this is a completely harmless choice. But it suggests a potential lack of understanding of some pretty fundamental parts of Rust.

Although I think the implementation of the gensym function is broken, the article does explain that it wasn’t possible to use &mut u32 because multiple references to the value were required. It would be more idiomatic to use Cell rather than RefCell, but there’s nothing really wrong with using RefCell as far as I can see.