Lisp is not necessarily functional. There's a lot of mutation in Common Lisp even if the community favors a functional style.

I think the question is: how does Lisp function without garbage collecting cons cells?

For one, I'm not sure they even rely on cons cells like "real" Lisp. Clojure doesn't either. They cite ML as inspiration.

Carp language guide states object lifetimes are statically inferred [1] which my guess is they allocate on stack (or malloc/free by scope) and detect use-after-free at compile time.

Another, more theoretical approach is using linear types which require all values to be "consumed" [2]

1: https://github.com/carp-lang/Carp/blob/master/docs/LanguageG...

2: https://www.cs.utexas.edu/users/hunt/research/hash-cons/hash...