Mutability is a big one for correctness. In Python, any function you pass any (non-primitive) object to might be mutated right out from under you and you have

Mutability is a big one for correctness. In Python, any function you pass any (non-primitive) object to might be mutated right out from under you and you have no idea it's happening. In Rust, you have to explicitly provide mutable references, or you need to hand over ownership, in which case you don't care if the callee mutates its argument, because you no longer have access to it.