Message-Id: <20251101-b4-as-flattened-v1-1-860f2ebeedfd@nvidia.com>
Date: Sat, 01 Nov 2025 22:31:38 +0900
From: Alexandre Courbot <acourbot@...dia.com>
To: 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>, 
 Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, 
 Danilo Krummrich <dakr@...nel.org>, Nathan Chancellor <nathan@...nel.org>, 
 Nicolas Schier <nicolas.schier@...ux.dev>
Cc: linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, 
 linux-kbuild@...r.kernel.org, Alexandre Courbot <acourbot@...dia.com>
Subject: [PATCH RESEND] rust: enable slice_flatten feature and abstract it
 through an extension trait

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()
+        }
+    }
+}
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index d0ee33a487be..a84b9e3171a3 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -308,6 +308,7 @@ $(obj)/%.lst: $(obj)/%.c FORCE
 
 # The features in this list are the ones allowed for non-`rust/` code.
 #
+#   - Stable since Rust 1.80.0: `feature(slice_flatten)`.
 #   - Stable since Rust 1.81.0: `feature(lint_reasons)`.
 #   - Stable since Rust 1.82.0: `feature(asm_const)`,
 #     `feature(offset_of_nested)`, `feature(raw_ref_op)`.
@@ -317,7 +318,7 @@ $(obj)/%.lst: $(obj)/%.c FORCE
 #
 # Please see https://github.com/Rust-for-Linux/linux/issues/2 for details on
 # the unstable features in use.
-rust_allowed_features := asm_const,asm_goto,arbitrary_self_types,lint_reasons,offset_of_nested,raw_ref_op,used_with_arg
+rust_allowed_features := asm_const,asm_goto,arbitrary_self_types,lint_reasons,offset_of_nested,raw_ref_op,slice_flatten,used_with_arg
 
 # `--out-dir` is required to avoid temporaries being created by `rustc` in the
 # current working directory, which may be not accessible in the out-of-tree

---
base-commit: 9a3c2f8a4f84960a48c056d0da88de3d09e6d622
change-id: 20251101-b4-as-flattened-0b3e0f566836

Best regards,
-- 
Alexandre Courbot <acourbot@...dia.com>

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