Understanding Send and Sync traits

In the previous neural network example, we used a static data structure that was shared between threads without being wrapped in a counter or lock. It contained locks, but why was the outer data structure permitted to be shared?

To answer this question, let's first review the rules of ownership:

• Each value in Rust has a variable that's called its owner
• There can only be one owner at a time
• When the owner goes out of scope, the value will be dropped

With these rules in mind, let's try to share a variable across threads, as follows:

use std::thread;

fn main() {
   let a = vec![1, 2, 3];

   thread::spawn(|| {
      println!("a = {:?}", a);
   });
}

If we try to compile this, then we will get an error complaining of the following:

closure may outlive the current function, but it borrows `a`, which is owned by the current function

This error indicates the following:

• Referencing variable a from inside the closure is okay
• The closure lives longer than variable a

Closures...