give ItemSliceSend a constructor and hide its field

As suggested by @cramertj in our review of unsafe code at Google.

Author: martinvonz

gix-pack/src/cache/delta/traverse/mod.rs

@@ -130,18 +130,14 @@ where
         };
         size_progress.init(None, progress::bytes());
         let size_counter = size_progress.counter();
-        let child_items = self.child_items.as_mut_slice();
         let object_progress = OwnShared::new(Mutable::new(object_progress));
 
         let start = std::time::Instant::now();
         in_parallel_with_slice(
             &mut self.root_items,
             thread_limit,
             {
-                let child_items = ItemSliceSend(std::ptr::slice_from_raw_parts_mut(
-                    child_items.as_mut_ptr(),
-                    child_items.len(),
-                ));
+                let child_items = ItemSliceSend::new(&mut self.child_items);
                 {
                     let object_progress = object_progress.clone();
                     move |thread_index| {

gix-pack/src/cache/delta/traverse/util.rs

@@ -1,9 +1,18 @@
 use crate::cache::delta::Item;
 
-pub struct ItemSliceSend<T>(pub *mut [T])
+pub struct ItemSliceSend<T>(*mut [T])
 where
     T: Send;
 
+impl<T> ItemSliceSend<T>
+where
+    T: Send,
+{
+    pub fn new(items: &mut [T]) -> Self {
+        ItemSliceSend(std::ptr::slice_from_raw_parts_mut(items.as_mut_ptr(), items.len()))
+    }
+}
+
 /// SAFETY: This would be unsafe if this would ever be abused, but it's used internally and only in a way that assure that the pointers
 ///         don't violate aliasing rules.
 impl<T> Clone for ItemSliceSend<T>