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

["Alexandre Courbot" <acourbot@nvidia.com>]

On Mon Nov 3, 2025 at 7:22 PM JST, Alice Ryhl wrote:
> 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.

I expected that doing so would clash with the methods of the same name
in `slice` on Rust >= 1.80, but instead I just got a warning that my
`AsFlattened` import was ignored - it looks like Rust did the right
thing and picked the non-trait method by default.

So all we need to do to make your proposal work is to make the import
conditional on Rust being < 1.80. And we can also make the whole trait
and its impl block that way.

That would actually be much cleaner! Thanks a lot.