Re: [PATCH v2] rust: enable slice_flatten feature and provide it through an extension trait

[Alice Ryhl <aliceryhl@google.com>]

On Tue, Nov 04, 2025 at 11:15:03PM +0900, Alexandre Courbot wrote:
> On Tue Nov 4, 2025 at 11:10 PM JST, Alice Ryhl wrote:
> > On Tue, Nov 04, 2025 at 12:53:18PM +0900, Alexandre Courbot wrote:
> >> In Rust 1.80, the previously unstable `slice::flatten` family of methods
> >> have been stabilized and renamed to `slice::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 providing an
> >> `as_flattened` implementation through an extension trait for compiler
> >> versions where it is not available.
> >> 
> >> This lets code use `as_flattened` portably by just adding
> >> 
> >>     #[cfg(not(CONFIG_RUSTC_HAS_SLICE_AS_FLATTENED))]
> >>     use kernel::slice::AsFlattened;
> >> 
> >> 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/
> >> Acked-by: Danilo Krummrich <dakr@kernel.org>
> >> Acked-by: Miguel Ojeda <ojeda@kernel.org>
> >> 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.
> >> 
> >> For v2, the methods are aligned with the final names of the standard
> >> library, and the extension trait is only visible when needed. This
> >> simplifies both the patch, and the extra labor for user code.
> >> 
> >> [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/
> >
> > With the below concern verified, you may add:
> > Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> 
> Thanks! You may want to check the v3 that I just posted [1] which also
> addressed your comment about having the extensions trait in the prelude.
> 
> [1] https://lore.kernel.org/rust-for-linux/20251104-b4-as-flattened-v3-1-6cb9c26b45cd@nvidia.com/T/#u

Looks good thanks.

Alice