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charset="utf-8" On Tue, Nov 04, 2025 at 12:06:54AM +0900, Alexandre Courbot wrote: > 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 > >> Link: https://lore.kernel.org/all/CANiq72kK4pG=O35NwxPNoTO17oRcg1yfGcvr3==Fi4edr+sfmw@mail.gmail.com/ > >> Signed-off-by: Alexandre Courbot > >> --- > >> 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 { > >> + /// 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 AsFlattened 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. One option is to put the trait in the prelude on relevant rustc versions. That way, users of the method don't need any conditional logic. Alice