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]
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Matt Gilbride <mattgilbride@google.com>
Link: https://lore.kernel.org/r/20240904144229.18592-1-ojeda@kernel.org
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

@@ -1059,15 +1059,14 @@ 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 }
     }
 }