rust: avoid box_uninit_write feature

Like commit 0903b9e ("rust: alloc: eschew
`Box<MaybeUninit<T>>::write`"), but for the new `rbtree` and `alloc` code.

That is, `feature(new_uninit)` [1] got partially stabilized [2]
for Rust 1.82.0 (expected to be released on 2024-10-17), but it
did not include `Box<MaybeUninit<T>>::write`, which got split into
`feature(box_uninit_write)` [3].

To avoid relying on a new unstable feature, rewrite the `write` +
`assume_init` pair manually.

Link: rust-lang/rust#63291 [1]
Link: rust-lang/rust#129401 [2]
Link: rust-lang/rust#129397 [3]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/alloc/box_ext.rs

@@ -26,9 +26,11 @@ pub trait BoxExt<T>: Sized {
  /// use kernel::alloc::{flags, box_ext::BoxExt};
  /// let value = Box::new([0; 32], flags::GFP_KERNEL)?;
  /// assert_eq!(*value, [0; 32]);
- /// let value = Box::drop_contents(value);
+ /// let mut value = Box::drop_contents(value);
  /// // Now we can re-use `value`:
- /// let value = Box::write(value, [1; 32]);
+ /// value.write([1; 32]);
+ /// // SAFETY: We just wrote to it.
+ /// let value = unsafe { value.assume_init() };
  /// assert_eq!(*value, [1; 32]);
  /// # Ok::<(), Error>(())
  /// ```

rust/kernel/rbtree.rs

@@ -1062,15 +1062,14 @@ impl<K, V> RBTreeNodeReservation<K, V> {
  /// Initialises a node reservation.
  ///
  /// It then becomes an [`RBTreeNode`] that can be inserted into a tree.
- pub fn into_node(self, key: K, value: V) -> RBTreeNode<K, V> {
- let node = Box::write(
- self.node,
- Node {
- key,
- value,
- links: bindings::rb_node::default(),
- },
- );
+ pub fn into_node(mut self, key: K, value: V) -> RBTreeNode<K, V> {
+ self.node.write(Node {
+ key,
+ value,
+ links: bindings::rb_node::default(),
+ });
+ // SAFETY: We just wrote to it.
+ let node = unsafe { self.node.assume_init() };
  RBTreeNode { node }
  }
 }