Message-Id: <20251104-as_casts-v1-1-0a0e95bd2a9f@nvidia.com>
Date: Tue, 04 Nov 2025 22:30:00 +0900
From: Alexandre Courbot <acourbot@...dia.com>
To: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
 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>
Cc: linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, 
 Alexandre Courbot <acourbot@...dia.com>
Subject: [PATCH RFC] rust: add functions and traits for lossless integer
 conversions

The core library's `From` implementations do not cover conversions
that are not portable or future-proof. For instance, even though it is
safe today, `From<usize>` is not implemented for `u64` because of the
possibility to support larger-than-64bit architectures in the future.

However, the kernel supports a narrower set of architectures, with a
considerable amount of code that is architecture-specific. This makes it
helpful and desirable to provide more infallible conversions, lest we
need to rely on the `as` keyword and carry the risk of silently losing
data.

Thus, introduce a new module `num::casts` that provides safe const
functions performing more conversions allowed by the build target, as
well as `FromSafeCast` and `IntoSafeCast` traits that are just
extensions of `From` and `Into` to conversions that are known to be
lossless.

Suggested-by: Danilo Krummrich <dakr@...nel.org>
Link: https://lore.kernel.org/rust-for-linux/DDK4KADWJHMG.1FUPL3SDR26XF@kernel.org/
Signed-off-by: Alexandre Courbot <acourbot@...dia.com>
---
This got already reviewed in the context of Nova [1][2][3] and I am
thinking of merging it soon there, but since it was suggested that this
would also be useful in the kernel crate, I am sending this RFC for more
visibility.

It might be a bit tight to get this in shape for 6.19 (thus the first
merge into Nova as we have a lot of incoming code that benefits from
this), but maybe we can consider the following cycle.

[1] https://lore.kernel.org/rust-for-linux/20251026-nova-as-v1-5-60c78726462d@nvidia.com/
[2] https://lore.kernel.org/rust-for-linux/20251027-nova-as-v2-5-a26bd1d067a4@nvidia.com/
[3] https://lore.kernel.org/rust-for-linux/20251029-nova-as-v3-4-6a30c7333ad9@nvidia.com/
---
 rust/kernel/lib.rs       |   1 +
 rust/kernel/num.rs       |   6 ++
 rust/kernel/num/casts.rs | 245 +++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 252 insertions(+)

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 3dd7bebe7888..235d0d8b1eff 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -109,6 +109,7 @@
 pub mod mm;
 #[cfg(CONFIG_NET)]
 pub mod net;
+pub mod num;
 pub mod of;
 #[cfg(CONFIG_PM_OPP)]
 pub mod opp;
diff --git a/rust/kernel/num.rs b/rust/kernel/num.rs
new file mode 100644
index 000000000000..3de1decb7779
--- /dev/null
+++ b/rust/kernel/num.rs
@@ -0,0 +1,6 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Numerical features for the kernel.
+
+pub mod casts;
+pub use casts::*;
diff --git a/rust/kernel/num/casts.rs b/rust/kernel/num/casts.rs
new file mode 100644
index 000000000000..c729778f64d4
--- /dev/null
+++ b/rust/kernel/num/casts.rs
@@ -0,0 +1,245 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Helpers for performing lossless integer casts.
+//!
+//! The `as` keyword can be used to perform casts between integer types, but it unfortunately makes
+//! no distinction between casts that are lossless, and casts from a larger type into a smaller one
+//! that might silently strip data away. Thus, its use in the kernel is discouraged in favor of
+//! [`From`] implementations.
+//!
+//! Concurrently, there are casts that are lossless depending on the build architecture (such as
+//! casting [`usize`] to [`u64`] on 32 or 64 bit archs), but not supported by [`From`]
+//! implementations in the standard library because they are not portable. It does however make
+//! sense for the kernel to support these, if only for code that is architecture-specific.
+//!
+//! This module provides ways to perform such conversions safely:
+//!
+//! - A series a const functions (e.g. [`usize_as_u64`] supporting safe conversions in const
+//!   context. Conversions supported by [`From`] implementations in the standard library are also
+//!   covered as the [`From`] trait cannot be used in const context.
+//! - Two extension traits, [`FromSafeCast`] and [`IntoSafeCast`], providing conversion methods
+//!   similar to [`From`] and [`Into`] for conversions that are safe to perform on the current
+//!   architecture, but not supported by the standard library.
+//! - Another series of const functions (e.g. [`u64_into_u8`]) supporting the conversion of a const
+//!   value from a larger type into a smaller one. This is useful if a constant is available in a
+//!   larger type, but needs to be used as a smaller one that can accommodate its value.
+//!
+//! # Examples
+//!
+//! ```
+//! use kernel::num::{self, FromSafeCast, IntoSafeCast};
+//!
+//! // Conversion from const context.
+//! const USIZED_CONST: usize = num::u8_as_usize(255u8);
+//!
+//! // Non-const conversions.``
+//! let a = u64::from_safe_cast(4096usize);
+//! let b: u64 = 4096usize.into_safe_cast();
+//! ```
+
+use kernel::macros::paste;
+use kernel::prelude::*;
+
+mod as_casts {}
+
+/// Implements safe `as` conversion functions from a given type into a series of target types.
+///
+/// These functions can be used in place of `as`, with the guarantee that they will be lossless.
+macro_rules! impl_safe_as {
+    ($from:ty as { $($into:ty),* }) => {
+        $(
+        paste! {
+            #[doc = ::core::concat!(
+                "Losslessly converts a [`",
+                ::core::stringify!($from),
+                "`] into a [`",
+                ::core::stringify!($into),
+                "`].")]
+            ///
+            /// This conversion is allowed as it is always lossless. Prefer this over the `as`
+            /// keyword to ensure no lossy casts are performed.
+            ///
+            /// This is for use from a `const` context. For non `const` use, prefer the
+            /// [`FromSafeCast`] and [`IntoSafeCast`] traits.
+            ///
+            /// # Examples
+            ///
+            /// ```
+            /// use kernel::num;
+            ///
+            #[doc = ::core::concat!(
+                "assert_eq!(num::",
+                ::core::stringify!($from),
+                "_as_",
+                ::core::stringify!($into),
+                "(1",
+                ::core::stringify!($from),
+                "), 1",
+                ::core::stringify!($into),
+                ");")]
+            /// ```
+            #[allow(unused)]
+            #[inline(always)]
+            pub const fn [<$from _as_ $into>](value: $from) -> $into {
+                kernel::static_assert!(size_of::<$into>() >= size_of::<$from>());
+
+                value as $into
+            }
+        }
+        )*
+    };
+}
+
+impl_safe_as!(u8 as { u16, u32, u64, usize });
+impl_safe_as!(u16 as { u32, u64, usize });
+impl_safe_as!(u32 as { u64, usize } );
+// `u64` and `usize` have the same size on 64-bit platforms.
+#[cfg(CONFIG_64BIT)]
+impl_safe_as!(u64 as { usize } );
+
+// A `usize` fits into a `u64` on 32 and 64-bit platforms.
+#[cfg(any(CONFIG_32BIT, CONFIG_64BIT))]
+impl_safe_as!(usize as { u64 });
+
+// A `usize` fits into a `u32` on 32-bit platforms.
+#[cfg(CONFIG_32BIT)]
+impl_safe_as!(usize as { u32 });
+
+/// Extension trait providing guaranteed lossless cast to `Self` from `T`.
+///
+/// The standard library's `From` implementations do not cover conversions that are not portable or
+/// future-proof. For instance, even though it is safe today, `From<usize>` is not implemented for
+/// [`u64`] because of the possibility to support larger-than-64bit architectures in the future.
+///
+/// The workaround is to either deal with the error handling of [`TryFrom`] for an operation that
+/// technically cannot fail, or to use the `as` keyword, which can silently strip data if the
+/// destination type is smaller than the source.
+///
+/// Both options are hardly acceptable for the kernel. It is also a much more architecture
+/// dependent environment, supporting only 32 and 64 bit architectures, with some modules
+/// explicitly depending on a specific bus width that could greatly benefit from infallible
+/// conversion operations.
+///
+/// Thus this extension trait that provides, for the architecture the kernel is built for, safe
+/// conversion between types for which such cast is lossless.
+///
+/// In other words, this trait is implemented if, for the current build target and with `t: T`, the
+/// `t as Self` operation is completely lossless.
+///
+/// Prefer this over the `as` keyword to ensure no lossy casts are performed.
+///
+/// If you need to perform a conversion in `const` context, use [`u64_as_usize`], [`u32_as_usize`],
+/// [`usize_as_u64`], etc.
+///
+/// # Examples
+///
+/// ```
+/// use kernel::num::FromSafeCast;
+///
+/// assert_eq!(usize::from_safe_cast(0xf00u32), 0xf00usize);
+/// ```
+pub trait FromSafeCast<T> {
+    /// Create a `Self` from `value`. This operation is guaranteed to be lossless.
+    fn from_safe_cast(value: T) -> Self;
+}
+
+impl FromSafeCast<usize> for u64 {
+    fn from_safe_cast(value: usize) -> Self {
+        usize_as_u64(value)
+    }
+}
+
+#[cfg(CONFIG_32BIT)]
+impl FromSafeCast<usize> for u32 {
+    fn from_safe_cast(value: usize) -> Self {
+        usize_as_u32(value)
+    }
+}
+
+impl FromSafeCast<u32> for usize {
+    fn from_safe_cast(value: u32) -> Self {
+        u32_as_usize(value)
+    }
+}
+
+#[cfg(CONFIG_64BIT)]
+impl FromSafeCast<u64> for usize {
+    fn from_safe_cast(value: u64) -> Self {
+        u64_as_usize(value)
+    }
+}
+
+/// Counterpart to the [`FromSafeCast`] trait, i.e. this trait is to [`FromSafeCast`] what [`Into`]
+/// is to [`From`].
+///
+/// See the documentation of [`FromSafeCast`] for the motivation.
+///
+/// # Examples
+///
+/// ```
+/// use kernel::num::IntoSafeCast;
+///
+/// assert_eq!(0xf00usize, 0xf00u32.into_safe_cast());
+/// ```
+pub trait IntoSafeCast<T> {
+    /// Convert `self` into a `T`. This operation is guaranteed to be lossless.
+    fn into_safe_cast(self) -> T;
+}
+
+/// Reverse operation for types implementing [`FromSafeCast`].
+impl<S, T> IntoSafeCast<T> for S
+where
+    T: FromSafeCast<S>,
+{
+    fn into_safe_cast(self) -> T {
+        T::from_safe_cast(self)
+    }
+}
+
+/// Implements lossless conversion of a constant from a larger type into a smaller one.
+macro_rules! impl_const_into {
+    ($from:ty => { $($into:ty),* }) => {
+        $(
+        paste! {
+            #[doc = ::core::concat!(
+                "Performs a build-time safe conversion of a [`",
+                ::core::stringify!($from),
+                "`] constant value into a [`",
+                ::core::stringify!($into),
+                "`].")]
+            ///
+            /// This checks at compile-time that the conversion is lossless, and triggers a build
+            /// error if it isn't.
+            ///
+            /// # Examples
+            ///
+            /// ```
+            /// use kernel::num;
+            ///
+            /// // Succeeds because the value of the source fits into the destination's type.
+            #[doc = ::core::concat!(
+                "assert_eq!(num::",
+                ::core::stringify!($from),
+                "_into_",
+                ::core::stringify!($into),
+                "::<1",
+                ::core::stringify!($from),
+                ">(), 1",
+                ::core::stringify!($into),
+                ");")]
+            /// ```
+            #[allow(unused)]
+            pub const fn [<$from _into_ $into>]<const N: $from>() -> $into {
+                build_assert!(N <= $into::MAX as $from);
+
+                N as $into
+            }
+        }
+        )*
+    };
+}
+
+impl_const_into!(usize => { u8, u16, u32 });
+impl_const_into!(u64 => { u8, u16, u32 });
+impl_const_into!(u32 => { u8, u16 });
+impl_const_into!(u16 => { u8 });

---
base-commit: 6553a8f168fb7941ae73d39eccac64f3a2b9b399
change-id: 20251104-as_casts-6a8882ac0192

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

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