Re: [PATCH RESEND] rust: enable slice_flatten feature and abstract it through an extension trait

[Alice Ryhl <aliceryhl@google.com>]

On Sat, Nov 01, 2025 at 10:31:38PM +0900, Alexandre Courbot wrote:
> In Rust 1.80, the previously unstable `slice::flatten` family of methods
> have been stabilized and renamed to `as_flattened`.
> 
> This creates an issue as we want to use `as_flattened`, but need to
> support the MSRV (which at the moment is Rust 1.78) where it is named
> `flatten`.
> 
> Solve this by enabling the `slice_flatten` feature, and abstracting
> `as_flatten` behind an extension trait that calls the right method
> depending on the Rust version.
> 
> This extension trait can be removed once the MSRV passes 1.80.
> 
> Suggested-by: Miguel Ojeda <ojeda@kernel.org>
> Link: https://lore.kernel.org/all/CANiq72kK4pG=O35NwxPNoTO17oRcg1yfGcvr3==Fi4edr+sfmw@mail.gmail.com/
> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
> ---
> This patch was part of the Nova GSP boot series [1], but since it
> requires attention from the core Rust team (and possibly the build
> maintainers?) and is otherwise buried under Nova patches, I am taking
> the freedom to send it separately for visibility.
> 
> Hopefully it captures Miguel's suggestion [2] accurately, but please let
> me know if I missed something.
> 
> Since the Nova GSP boot series makes use of this, I hope to eventually
> merge it alongside the series, through the DRM tree.
> 
> Thanks!
> 
> [1] https://lore.kernel.org/all/20251029-gsp_boot-v7-0-34227afad347@nvidia.com/
> [2] https://lore.kernel.org/all/CANiq72kK4pG=O35NwxPNoTO17oRcg1yfGcvr3==Fi4edr+sfmw@mail.gmail.com/
> ---
>  init/Kconfig           |  3 +++
>  rust/kernel/lib.rs     |  4 ++++
>  rust/kernel/slice.rs   | 63 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  scripts/Makefile.build |  3 ++-
>  4 files changed, 72 insertions(+), 1 deletion(-)
> 
> diff --git a/init/Kconfig b/init/Kconfig
> index cab3ad28ca49..7da93c9cccc3 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -147,6 +147,9 @@ config LD_CAN_USE_KEEP_IN_OVERLAY
>  	# https://github.com/llvm/llvm-project/pull/130661
>  	def_bool LD_IS_BFD || LLD_VERSION >= 210000
>  
> +config RUSTC_HAS_SLICE_AS_FLATTENED
> +	def_bool RUSTC_VERSION >= 108000
> +
>  config RUSTC_HAS_COERCE_POINTEE
>  	def_bool RUSTC_VERSION >= 108400
>  
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index 3dd7bebe7888..2581a356d114 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -21,6 +21,9 @@
>  #![feature(inline_const)]
>  #![feature(pointer_is_aligned)]
>  //
> +// Stable since Rust 1.80.0.
> +#![feature(slice_flatten)]
> +//
>  // Stable since Rust 1.81.0.
>  #![feature(lint_reasons)]
>  //
> @@ -128,6 +131,7 @@
>  pub mod security;
>  pub mod seq_file;
>  pub mod sizes;
> +pub mod slice;
>  mod static_assert;
>  #[doc(hidden)]
>  pub mod std_vendor;
> diff --git a/rust/kernel/slice.rs b/rust/kernel/slice.rs
> new file mode 100644
> index 000000000000..7e837bec4bed
> --- /dev/null
> +++ b/rust/kernel/slice.rs
> @@ -0,0 +1,63 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Additional (and temporary) slice helpers.
> +
> +/// Extension trait providing a portable version of [`as_flattened`] and
> +/// [`as_flattened_mut`].
> +///
> +/// In Rust 1.80, the previously unstable `slice::flatten` family of methods
> +/// have been stabilized and renamed from `flatten` to `as_flattened`.
> +///
> +/// This creates an issue for as long as the MSRV is < 1.80, as the same functionality is provided
> +/// by different methods depending on the compiler version.
> +///
> +/// This extension trait solves this by abstracting `as_flatten` and calling the correct  method
> +/// depending on the Rust version.
> +///
> +/// This trait can be removed once the MSRV passes 1.80.
> +///
> +/// [`as_flattened`]: slice::as_flattened
> +/// [`as_flattened_mut`]: slice::as_flattened_mut
> +pub trait AsFlattened<T> {
> +    /// Takes an `&[[T; N]]` and flattens it to a `&[T]`.
> +    ///
> +    /// This is an portable layer on top of [`as_flattened`]; see its documentation for details.
> +    ///
> +    /// [`as_flattened`]: slice::as_flattened
> +    fn as_flattened_slice(&self) -> &[T];
> +
> +    /// Takes an `&mut [[T; N]]` and flattens it to a `&mut [T]`.
> +    ///
> +    /// This is an portable layer on top of [`as_flattened_mut`]; see its documentation for details.
> +    ///
> +    /// [`as_flattened_mut`]: slice::as_flattened_mut
> +    fn as_flattened_slice_mut(&mut self) -> &mut [T];
> +}
> +
> +impl<T, const N: usize> AsFlattened<T> for [[T; N]] {
> +    #[allow(clippy::incompatible_msrv)]
> +    fn as_flattened_slice(&self) -> &[T] {
> +        #[cfg(not(CONFIG_RUSTC_HAS_SLICE_AS_FLATTENED))]
> +        {
> +            self.flatten()
> +        }
> +
> +        #[cfg(CONFIG_RUSTC_HAS_SLICE_AS_FLATTENED)]
> +        {
> +            self.as_flattened()
> +        }
> +    }
> +
> +    #[allow(clippy::incompatible_msrv)]
> +    fn as_flattened_slice_mut(&mut self) -> &mut [T] {
> +        #[cfg(not(CONFIG_RUSTC_HAS_SLICE_AS_FLATTENED))]
> +        {
> +            self.flatten_mut()
> +        }
> +
> +        #[cfg(CONFIG_RUSTC_HAS_SLICE_AS_FLATTENED)]
> +        {
> +            self.as_flattened_mut()
> +        }

Hmm. Why not have this match the name that this was stabilized under?
That way, when bumping the MSRV, we can just remove this trait without
changing the callers.

Alice