Data Races

Data races occur when multiple threads try to access the same memory segment, trying to write to it, and they can cause undefined behavior. Safe Rust guarantees that no data races will occur, and a big player for this is the ownership model. By definition, if a value can only have one owner (can make changes), then it can only be written to by its single owner. However, general race conditions are not prevented in Rust. They simply can’t be prevented from a mathematical standpoint, due to the way the scheduler works in different operating systems. This is something that is out of the developer's' control. This means that while a program may get deadlocked, or have incorrect synchronization, a Rust program will still be safe.

To learn more about subtyping and variance, it is recommended to read the official Rust resource The Rustonomicon chapter 8.1.