Your rust example isn't considered interior mutability, since it needs `&mut`. Since `&mut` references are exclusive, they're quite similar to immutable values

Your rust example isn't considered interior mutability, since it needs `&mut`. Since `&mut` references are exclusive, they're quite similar to immutable values in functional languages, despite the API appearing mutable. In Rust the 'interor mutability' term is used for types like `Mutex`/`RefCell`, `Cell`, or atomics, which are mutable even through shared references. These come with the same problems as interor mutability in scala.