Rust: What is the difference between Send and !Send?

Send — can be sent between threads. !Send — cannot.

Send:

• Most types.
• Thread-safe.
• Can move across threads.

!Send:

• Rc, raw pointers.
• Not thread-safe.
• Cannot move.

1use std::rc::Rc;
2
3// Send
4let v = vec![1, 2, 3];
5std::thread::spawn(move || println!("{:?}", v));
6
7// !Send
8let r = Rc::new(5);
9// std::thread::spawn(move || println!("{}", r));  // Error

Key: Send for threads, !Send for single-thread.