Use when deciding whether a type is Send or Sync, or when rustc rejects a value at a thread or task boundary with "cannot be sent between threads safely", "cannot be shared between threads safely", or "future cannot be sent between threads safely". Also for auto trait rules of references and smart pointers (Arc vs Rc), Mutex vs RwLock payload bounds, why MutexGuard is not Send yet is Sync, PhantomData markers that remove Send or Sync, auto trait leakage from impl Trait and async fn, and E0321. Not for proving a manual unsafe impl Send or Sync; use `rust-unsafe`.
