Introducing Scoped Threads — The New Addition to Rust 1.63.0

A peek into the awesome update

Let the crab threads go rampant

Rust 1.63.0 has been released and this release comes with one of the most awaited features in recent times. After all, this capability of crossbeam was one of the main reasons to implement multi-threading in Rust with this crate instead of using the standard library. Let’s see what the new scoped threads allow us.

What we used to have

The following code is a fairly basic multi-thread implementation. Something that could be part of any exercise of introduction to parallel programming.

We are reading the list of numbers and calculating two statistics of that list on two different threads. Finally, we wait for both results and print the results.

So, what happens if we try to run this code? We get two errors like the following one, one for each thread that we spawned:

error[E0373]: closure may outlive the current function, but it borrows `numbers`, which is owned by the current function
 --> src/main.rs:7:28
  |
7 | std::thread::spawn(|| numbers.iter().sum::<i32>() as f32...);
  |                    ^^ ------- `numbers` is borrowed here
  |                    |
  |                    may outlive borrowed value `numbers`
  |
note: function requires argument type to outlive `'static`
 --> src/main.rs:7:9
  |
7 | std::thread::spawn(|| numbers.iter().sum::<i32>() as f32...);
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: to force the closure to take ownership of `numbers` (and any other referenced variables), use the `move` keyword
  |
7 | std::thread::spawn(move || numbers.iter().sum::<i32>() as ...);

It says that the closure invoked with the thread can outlive our reference. It gives the tip of using move to give the ownership of the reference to the thread, that way we know that it will not be outlived for it. The thing is that we want to keep using our list in the current scope and we also want to pass it to the other thread…