BufferIterator can be Send + Sync (#8873)

BufferIterator is Send and Sync just like Vec IntoIter, fix #8869

Signed-off-by: Robert Kruszewski <github@robertk.io>

Author: robert3005

vortex-buffer/src/buffer.rs

@@ -713,6 +713,11 @@ pub struct BufferIterator<T: Copy> {
     end: *const T,
 }
 
+// SAFETY: `BufferIterator` is a `Buffer<T>` plus two cursors into it, so it can safely be
+// `Send`/`Sync` exactly when `Buffer<T>` is. Same bounds as `std::vec::IntoIter`.
+unsafe impl<T: Copy + Send> Send for BufferIterator<T> {}
+unsafe impl<T: Copy + Sync> Sync for BufferIterator<T> {}
+
 impl<T: Copy> Iterator for BufferIterator<T> {
     type Item = T;
 
@@ -777,6 +782,17 @@ mod test {
         assert_eq!(aligned.as_slice(), &[0, 1, 2]);
     }
 
+    #[test]
+    fn buffer_iterator_send_sync() {
+        fn assert_send_sync<T: Send + Sync>(_: &T) {}
+
+        let mut iter = buffer![0i32, 1, 2, 3].into_iter();
+        assert_send_sync(&iter);
+        iter.next();
+        let remaining: Vec<i32> = std::thread::spawn(move || iter.collect()).join().unwrap();
+        assert_eq!(remaining, vec![1, 2, 3]);
+    }
+
     #[test]
     fn slice() {
         let buf = buffer![0, 1, 2, 3, 4];