No locking (which provides mutable `Sync` in rust terms) is generally required, but things must be Send (i.e., allowed to move between threads, but not shared). In my experience is mostly an issue when dealing with references, because `&T` is only Send if `T` is sync.

However, `&mut T` is Send if `T` is Send. So if you can avoid aliasing your references, you're also ok.