rust: enable slice_flatten feature and provide it through an extension trait

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.

The trait is then exported from the prelude, making the `as_flattened`
family of methods transparently available for all supported compiler
versions.

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>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Message-ID: <20251110-gsp_boot-v9-8-8ae4058e3c0e@nvidia.com>
Message-ID: <20251104-b4-as-flattened-v3-1-6cb9c26b45cd@nvidia.com>

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
 

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;

rust/kernel/prelude.rs

@@ -51,3 +51,6 @@
 pub use super::current;
 
 pub use super::uaccess::UserPtr;
+
+#[cfg(not(CONFIG_RUSTC_HAS_SLICE_AS_FLATTENED))]
+pub use super::slice::AsFlattened;

rust/kernel/slice.rs

@@ -0,0 +1,49 @@
+// 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
+#[cfg(not(CONFIG_RUSTC_HAS_SLICE_AS_FLATTENED))]
+pub trait AsFlattened<T> {
+    /// Takes a `&[[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(&self) -> &[T];
+
+    /// Takes a `&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_mut(&mut self) -> &mut [T];
+}
+
+#[cfg(not(CONFIG_RUSTC_HAS_SLICE_AS_FLATTENED))]
+impl<T, const N: usize> AsFlattened<T> for [[T; N]] {
+    #[allow(clippy::incompatible_msrv)]
+    fn as_flattened(&self) -> &[T] {
+        self.flatten()
+    }
+
+    #[allow(clippy::incompatible_msrv)]
+    fn as_flattened_mut(&mut self) -> &mut [T] {
+        self.flatten_mut()
+    }
+}