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Message-ID: <aQm7LNwPwxm8HZ_I@google.com>
Date: Tue, 4 Nov 2025 08:37:00 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Alexandre Courbot <acourbot@...dia.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
"Björn Roy Baron" <bjorn3_gh@...tonmail.com>, Benno Lossin <lossin@...nel.org>,
Andreas Hindborg <a.hindborg@...nel.org>, Trevor Gross <tmgross@...ch.edu>,
Danilo Krummrich <dakr@...nel.org>, Nathan Chancellor <nathan@...nel.org>,
Nicolas Schier <nicolas.schier@...ux.dev>, linux-kernel@...r.kernel.org,
rust-for-linux@...r.kernel.org, linux-kbuild@...r.kernel.org
Subject: Re: [PATCH RESEND] rust: enable slice_flatten feature and abstract it
through an extension trait
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 <ojeda@...nel.org>
> >> Link: https://lore.kernel.org/all/CANiq72kK4pG=O35NwxPNoTO17oRcg1yfGcvr3==Fi4edr+sfmw@mail.gmail.com/
> >> Signed-off-by: Alexandre Courbot <acourbot@...dia.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.
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
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