rust: kernel: add drop_contents to BoxExt

Sometimes (see [1]) it is necessary to drop the value inside of a
`Box<T>`, but retain the allocation. For example to reuse the allocation
in the future.
Introduce a new function `drop_contents` that turns a `Box<T>` into
`Box<MaybeUninit<T>>` by dropping the value.

Signed-off-by: Benno Lossin <[email protected]>
Link: https://lore.kernel.org/rust-for-linux/[email protected]/ [1]
Reviewed-by: Boqun Feng <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[boqun: apply some fixes on the Example]

Author: y86-dev

rust/kernel/alloc/box_ext.rs

@@ -5,6 +5,7 @@
 use super::{AllocError, Flags};
 use alloc::boxed::Box;
 use core::mem::MaybeUninit;
+use core::ptr;
 
 /// Extensions to [`Box`].
 pub trait BoxExt<T>: Sized {
@@ -17,6 +18,22 @@ pub trait BoxExt<T>: Sized {
     ///
     /// The allocation may fail, in which case an error is returned.
     fn new_uninit(flags: Flags) -> Result<Box<MaybeUninit<T>>, AllocError>;
+
+    /// Drops the contents, but keeps the allocation.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use kernel::alloc::{flags, box_ext::BoxExt};
+    ///
+    /// let value = Box::new([0; 32], flags::GFP_KERNEL)?;
+    /// let value = value.drop_contents();
+    /// // Now we can re-use `value`:
+    /// Box::write(value, [1; 32]);
+    ///
+    /// # Ok::<(), Error>(())
+    /// ```
+    fn drop_contents(self) -> Box<MaybeUninit<T>>;
 }
 
 impl<T> BoxExt<T> for Box<T> {
@@ -53,4 +70,12 @@ impl<T> BoxExt<T> for Box<T> {
         // zero-sized types, we use `NonNull::dangling`.
         Ok(unsafe { Box::from_raw(ptr) })
     }
+
+    fn drop_contents(self) -> Box<MaybeUninit<T>> {
+        let ptr = Box::into_raw(self);
+        // SAFETY: `ptr` is valid, because it came from `Box::into_raw`.
+        unsafe { ptr::drop_in_place(ptr) };
+        // SAFETY: `ptr` is valid, because it came from `Box::into_raw`.
+        unsafe { Box::from_raw(ptr.cast()) }
+    }
 }