Add a section on scoped threads

Author: Mark-Simulacrum

posts/2022-08-11-Rust-1.63.0.md

@@ -28,6 +28,50 @@ Please [report] any bugs you might come across!
 
 ## What's in 1.63.0 stable
 
+### Scoped threads
+
+Rust code could launch new threads with `std::thread::spawn` since 1.0, but this
+function bounds its closure with `'static`. Roughly, this means that threads
+currently must have ownership of any arguments passed into their closure; you
+can't pass borrowed data into a thread. In cases where the threads are expected
+to exit by the end of the function (by being `join()`'d), this isn't strictly
+necessary and can require workarounds like placing the data in an [`Arc`].
+
+Now, with 1.63.0, the standard library is adding *scoped* threads, which allow
+spawning a thread borrowing from the local stack frame. The
+[`std::thread::scope`] API provides the necessary guarantee that any spawned threads
+will have exited prior to itself returning, which allows for safely borrowing
+data. Here's an example:
+
+```rust
+let mut a = vec![1, 2, 3];
+let mut x = 0;
+
+std::thread::scope(|s| {
+    s.spawn(|| {
+        println!("hello from the first scoped thread");
+        // We can borrow `a` here.
+        dbg!(&a);
+    });
+    s.spawn(|| {
+        println!("hello from the second scoped thread");
+        // We can even mutably borrow `x` here,
+        // because no other threads are using it.
+        x += a[0] + a[2];
+    });
+    println!("hello from the main thread");
+});
+
+// After the scope, we can modify and access our variables again:
+a.push(4);
+assert_eq!(x, a.len());
+```
+
+[`std::thread::scope`]: https://doc.rust-lang.org/stable/std/thread/fn.scope.html
+[`std::thread::spawn`]: https://doc.rust-lang.org/stable/std/thread/fn.spawn.html
+[`Arc`]: https://doc.rust-lang.org/stable/std/sync/struct.Arc.html
+
+
 ### Rust ownership for raw file descriptors/handles (I/O Safety)
 
 Previously, Rust code working with platform APIs taking raw file descriptors (on
@@ -69,7 +113,7 @@ which code they can write.
 
 You can read more about non-lexical lifetimes in [this section of the 2018 edition announcement][nll].
 
-[this blog post]: https://github.com/rust-lang/blog.rust-lang.org/pull/989
+[this blog post]: https://blog.rust-lang.org/2022/08/05/nll-by-default.html
 [nll]: https://blog.rust-lang.org/2018/12/06/Rust-1.31-and-rust-2018.html#non-lexical-lifetimes
 
 ### Stabilized APIs