Re: [PATCH v2 1/2] rust: kernel: add `drop_contents` to `BoxExt`

[Boqun Feng <boqun.feng@gmail.com>]

On Mon, Jul 08, 2024 at 02:14:50PM -0700, Boqun Feng wrote:
> On Mon, Jul 08, 2024 at 08:53:38PM +0000, Benno Lossin wrote:
> > 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 <benno.lossin@proton.me>
> > Link: https://lore.kernel.org/rust-for-linux/20240418-b4-rbtree-v3-5-323e134390ce@google.com/ [1]
> > ---
> >  rust/kernel/alloc/box_ext.rs | 21 +++++++++++++++++++++
> >  1 file changed, 21 insertions(+)
> > 
> > diff --git a/rust/kernel/alloc/box_ext.rs b/rust/kernel/alloc/box_ext.rs
> > index cdbb5ad166d9..6cf79f96d6c7 100644
> > --- a/rust/kernel/alloc/box_ext.rs
> > +++ b/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;
> >  use core::result::Result;
> >  
> >  /// Extensions to [`Box`].
> > @@ -18,6 +19,18 @@ 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.
> > +    ///

(I spoke too soon ;-))

> > +    /// # Examples
> > +    ///
> > +    /// ```

need a `use` here:

	use kernel::alloc::{flags, box_ext::BoxExt};

> > +    /// let value = Box::new([0; 32], flags::GFP_KERNEL)

missing a '?' and a ';' at the end of this line.

> > +    /// let value = value.drop_contents();
> > +    /// // Now we can re-use `value`:
> > +    /// Box::write(value, [1; 32]);

Need a line:

	# Ok::<(), Error>(())

here.
> > +    /// ```
> > +    fn drop_contents(self) -> Box<MaybeUninit<T>>;
> >  }
> >  
[...]

I queued this patch in rust-dev with these fixes applied, FYI.

Regards,
Boqun