What is Send and Sync in async Rust

Send and Sync are unsafe marker traits that determine if a type can be transferred across threads or shared between threads safely. Send means ownership can move to another thread, while Sync means a reference (&T) can be shared across threads. Most standard types implement these automatically, but you must manually implement them for custom types using unsafe if they are thread-safe.

use std::thread;

struct MyData {
    value: i32,
}

// MyData implements Send and Sync by default because i32 does.
fn main() {
    let data = MyData { value: 42 };
    thread::spawn(move || {
        println!("Value: {}", data.value);
    }).join().unwrap();
}

Think of Send as a permission slip to move an item to a different room (thread), and Sync as a permission slip to let multiple people in different rooms look at the same item at once. These rules prevent your program from crashing due to data corruption when multiple threads access memory simultaneously. You usually don't need to worry about them because Rust handles them automatically for safe types.