In Rust, threads are the primary way to achieve concurrency and parallelism. However, working with threads can be challenging, especially when it comes to sharing data between them. This is because Rust’s ownership and borrowing rules make it difficult to ensure that multiple threads don’t modify the same memory at the same time, potentially causing unsafe behavior.

To address these challenges, Rust provides two traits: Sync and Send.

The Sync trait is implemented for types that can be safely shared between threads. In other words, Sync means that a type can be safely accessed (read from or written to) from multiple threads concurrently without causing data races. Most of the built-in types in Rust are Sync, including primitive types like i32, and commonly used types like String and Vec.

The Send trait is implemented for types that can be safely sent between threads. Send means that a type can be moved (sent) from one thread to another without causing any memory unsafety issues. This includes types that implement Sync, since sending a type between threads essentially requires a read operation on one thread and a write operation on another.

Together, these two traits enable safe and efficient sharing of data between threads in Rust. However, it’s important to note that not all types can be Sync or Send. For example, if a type contains a non-thread-safe reference or mutex, it cannot be Sync. Similarly, if a type contains a raw pointer, it cannot be Send. These restrictions ensure that only thread-safe types can be used in concurrent programming.