Re: [PATCH v4 10/14] rust: alloc: add `Box::into_pin`

[Danilo Krummrich <dakr@kernel.org>]

On Fri, Dec 06, 2024 at 08:33:02PM +0100, Andreas Hindborg wrote:
> Add an associated function to convert a `Box<T>` into a `Pin<Box<T>>`.

What do you need this function for?

There is an `impl<T, A> From<Box<T, A>> for Pin<Box<T, A>>` already.

> 
> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
> ---
>  rust/kernel/alloc/kbox.rs | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
> index 9ce414361c2c6dd8eea09b11041f6c307cbc7864..1a993ec8602b37041c192458d8b6acff30769a04 100644
> --- a/rust/kernel/alloc/kbox.rs
> +++ b/rust/kernel/alloc/kbox.rs
> @@ -245,6 +245,16 @@ pub fn pin(x: T, flags: Flags) -> Result<Pin<Box<T, A>>, AllocError>
>          Ok(Self::new(x, flags)?.into())
>      }
>  
> +    /// Convert a [`Box<T,A>`] to a [`Pin<Box<T,A>>`]. If `T` does not implement
> +    /// [`Unpin`], then `x` will be pinned in memory and can't be moved.
> +    pub fn into_pin(boxed: Self) -> Pin<Self> {
> +        // SAFETY: `Self` is guaranteed to be the only pointer to the boxed
> +        // value. Thus, if `T: !Unpin`, `T` is guaranteed to stay pinned; there
> +        // is no way to get rid of the `Pin` and move out of the returned
> +        // `Pin<Box<T>>`.
> +        unsafe { Pin::new_unchecked(boxed) }
> +    }
> +
>      /// Forgets the contents (does not run the destructor), but keeps the allocation.
>      fn forget_contents(this: Self) -> Box<MaybeUninit<T>, A> {
>          let ptr = Self::into_raw(this);
> 
> -- 
> 2.46.0
> 
>