Well, rust does have pure functions (as most programming languages) you just can't tag a function as such to have the compiler enforce the purity for you (it us

Well, rust does have pure functions (as most programming languages) you just can't tag a function as such to have the compiler enforce the purity for you (it used to, though). That being said I'm not sure why the author mentioned that point in particular. Maybe he feels that having immutable-by-default semantics encourages writing pure functions?