What are auto traits in Rust?

In Rust, an auto trait is a special kind of trait that is automatically implemented for types satisfying certain conditions. The most common examples are the Send and Sync traits:

• The Send trait indicates that a value of a type can be safely moved between threads.
• The Sync trait indicates that a value of a type can be safely shared across multiple threads, meaning it is safe to access concurrently from multiple threads. These traits do not require explicit implementation; instead, they are automatically derived based on their constituent parts. If all components of a type are Send, then the type is automatically Send. Similarly, if all components of a type are Sync, then the type is automatically Sync. A key feature of auto traits is that they utilize negative reasoning, meaning by default all types implement these traits unless explicitly opted out via opt-out mechanism (for example, by marking a custom type as not Send or not Sync using the std::marker::PhantomData type). Overall, auto traits provide an efficient way to handle thread safety, allowing developers to focus more on logical implementation rather than the thread safety details of each type.