Mastering Rust - Second Edition

Unsafe traits and implementations

Apart from functions, traits can also be marked as unsafe. It isn't obvious why we would need unsafe traits. One of the primary motivations for unsafe traits existing in the first place is to mark types that cannot be sent to or shared between threads. This is achieved via the unsafe Send and Sync marker traits. These types are also auto traits, which means that they are implemented for most types in the standard library whenever appropriate. However, they are also explicitly opted out for certain types, for instance, the Rc<T>. An Rc<T> does not have an atomic reference counting mechanism and if it were to implement Sync and later be used in multiple threads, then we might end up with the wrong reference ...