Re: [PATCH v2 1/3] rust: num: add cv! macro to create values from constant expressions

["Gary Guo" <gary@garyguo.net>]

On Tue Sep 1, 2026 at 6:06 AM BST, Eliot Courtney wrote:
> Currently, using NonZero/Bounded constants is quite verbose. It's
> unfortunate because it disincentivizes using it in interface boundaries.
> Introduce a macro to make it nicer to use. The macro `cv!` (for constant
> value) takes a const integer expression and widens it to i128 (at build
> time only) before passing it as a const generic value to a new trait
> `FromConst`. The value is then converted and appears in the
> associated constant `FromConst::VALUE`. The trait is implemented by
> NonZero, Bounded, and Alignment and lets values of each be constructed
> from constants without a verbose turbofish syntax.
> For example, `const { NonZero::new(1).unwrap() }` can be written as
> `cv!(1)`.
>
> Suggested-by: Gary Guo <gary@garyguo.net>
> Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
> ---
>  rust/kernel/num.rs         | 133 +++++++++++++++++++++++++++++++++++++++++++++
>  rust/kernel/num/bounded.rs |  19 +++++++
>  rust/kernel/ptr.rs         |  14 +++++
>  3 files changed, 166 insertions(+)
>
> diff --git a/rust/kernel/num.rs b/rust/kernel/num.rs
> index dbe848e30efe..01ae2538b3e4 100644
> --- a/rust/kernel/num.rs
> +++ b/rust/kernel/num.rs
> @@ -2,6 +2,7 @@
>  
>  //! Additional numerical features for the kernel.
>  
> +use crate::const_assert;
>  use core::ops;
>  
>  pub mod bounded;
> @@ -9,6 +10,138 @@
>  
>  pub use bounded::*;
>  
> +/// Creates a value from an integer constant expression, with validity checked at build time.
> +///
> +/// This works for any type that implements [`FromConst`], with the target type inferred from
> +/// the context, or named explicitly with `cv!(value => Type)`.
> +///
> +/// # Examples
> +///
> +/// ```
> +/// use core::num::NonZero;
> +/// use kernel::num::Bounded;
> +/// use kernel::num::cv;
> +/// use kernel::ptr::Alignment;
> +///
> +/// let v: NonZero<usize> = cv!(8);
> +/// assert_eq!(v.get(), 8);
> +///
> +/// // Any integer constant expression works, not only literals.
> +/// let m: NonZero<usize> = cv!(usize::MAX);
> +/// assert_eq!(m.get(), usize::MAX);
> +///
> +/// let b: Bounded<u32, 4> = cv!(15);
> +/// assert_eq!(b.get(), 15);
> +///
> +/// let a: Alignment = cv!(4096);
> +/// assert_eq!(a.as_usize(), 4096);
> +///
> +/// // Checked narrowing of integer constants, including in `const` items.
> +/// const SMALL: u8 = cv!(200u32);
> +/// assert_eq!(SMALL, 200);
> +///
> +/// const N: NonZero<u8> = cv!(5);
> +/// assert_eq!(N.get(), 5);
> +///
> +/// // The target type can be given explicitly.
> +/// let e = cv!(200u32 => u8);
> +/// assert_eq!(e, 200);
> +///
> +/// // With an explicit primitive target, the expression can use generic parameters.
> +/// const fn as_u64<const KEY: u16>() -> u64 {
> +///     cv!(KEY => u64)
> +/// }
> +/// assert_eq!(as_u64::<0x40>(), 0x40);
> +/// ```
> +#[macro_export]
> +#[doc(hidden)]
> +macro_rules! cv {
> +    (@cast $v:expr => $t:ty) => {
> +        const {
> +            #[allow(unused_comparisons, unused_assignments, clippy::as_underscore)]
> +            {
> +                let v = $v;
> +                let r = v as $t;
> +                // Pin `back` to `v`'s type so `as _` casts back to the source type.
> +                let mut back = v;
> +                back = r as _;
> +
> +                ::core::assert!(
> +                    back == v && (v < 0) == (r < 0),
> +                    "value does not fit into the target type"
> +                );
> +
> +                r
> +            }
> +        }
> +    };
> +    ($v:expr => u8) => { $crate::cv!(@cast $v => u8) };
> +    ($v:expr => u16) => { $crate::cv!(@cast $v => u16) };
> +    ($v:expr => u32) => { $crate::cv!(@cast $v => u32) };
> +    ($v:expr => u64) => { $crate::cv!(@cast $v => u64) };
> +    ($v:expr => u128) => { $crate::cv!(@cast $v => u128) };
> +    ($v:expr => usize) => { $crate::cv!(@cast $v => usize) };
> +    ($v:expr => i8) => { $crate::cv!(@cast $v => i8) };
> +    ($v:expr => i16) => { $crate::cv!(@cast $v => i16) };
> +    ($v:expr => i32) => { $crate::cv!(@cast $v => i32) };
> +    ($v:expr => i64) => { $crate::cv!(@cast $v => i64) };
> +    ($v:expr => i128) => { $crate::cv!(@cast $v => i128) };
> +    ($v:expr => isize) => { $crate::cv!(@cast $v => isize) };
> +    ($v:expr => $t:ty) => {
> +        <$t as $crate::num::FromConst<{ $crate::cv!(@cast $v => i128) }>>::VALUE
> +    };
> +    ($v:expr) => {
> +        <_ as $crate::num::FromConst<{ $crate::cv!(@cast $v => i128) }>>::VALUE
> +    };
> +}
> +#[doc(inline)]
> +pub use cv;
> +
> +/// Types that can be created from an integer constant expression validated at build time.

Can you add a comment referencing the `cv!` macro and advice against using this
directly?

> +#[diagnostic::on_unimplemented(message = "`{Self}` cannot be converted from a constant")]
> +pub trait FromConst<const V: i128>: Sized {
> +    /// The value that corresponds to the constant `V`.
> +    ///
> +    /// Fails the build if `V` is not a valid value for `Self`.
> +    const VALUE: Self;
> +}
> +
> +/// Implements [`FromConst`] for primitive integer types and their [`NonZero`](core::num::NonZero)
> +/// versions.
> +macro_rules! impl_from_const {
> +    ($($type:ty)*) => {
> +        $(
> +        impl<const V: i128> FromConst<V> for $type {
> +            const VALUE: Self = {
> +                const_assert!(
> +                    V >= <$type>::MIN as i128 && V <= <$type>::MAX as i128,
> +                    "Constant cannot be represented by the target type."
> +                );

nit: as this forms part of compiler error message, this should start with lower case and
not have ending with period, to be consistent with other Rust error messages. I
probably should have mentioned why I made the change in my alt version.

Also, `const_assert!` would have an additional `const {}` wrapping which won't
be necessary because we're already in a definitively const context. Probably not
matter though.

> +
> +                V as $type
> +            };
> +        }
> +
> +        impl<const V: i128> FromConst<V> for core::num::NonZero<$type> {
> +            const VALUE: Self = {
> +                const_assert!(V != 0, "Constant cannot be zero.");
> +                const_assert!(
> +                    V >= <$type>::MIN as i128 && V <= <$type>::MAX as i128,
> +                    "Constant cannot be represented by the underlying type."
> +                );
> +
> +                core::num::NonZero::new(V as $type).unwrap()

This unwrap duplicates the zero check, which is why my alt version does a match
here instead.

Best,
Gary

> +            };
> +        }
> +        )*
> +    };
> +}
> +
> +impl_from_const!(
> +    u8 u16 u32 u64 usize
> +    i8 i16 i32 i64 isize
> +);
> +
>  /// Designates unsigned primitive types.
>  pub enum Unsigned {}
>  
> diff --git a/rust/kernel/num/bounded.rs b/rust/kernel/num/bounded.rs
> index 2a2b0a4bca5e..0cfb3ef16a6d 100644
> --- a/rust/kernel/num/bounded.rs
> +++ b/rust/kernel/num/bounded.rs
> @@ -14,6 +14,7 @@
>  
>  use kernel::{
>      num::{
> +        FromConst,
>          Integer,
>          Unsigned, //
>      },
> @@ -272,6 +273,24 @@ pub const fn new<const VALUE: $type>() -> Self {
>                  unsafe { Self::__new(VALUE) }
>              }
>          }
> +
> +        impl<const N: u32, const V: i128> FromConst<V> for Bounded<$type, N> {
> +            const VALUE: Self = {
> +                const_assert!(
> +                    V >= <$type>::MIN as i128 && V <= <$type>::MAX as i128,
> +                    "Constant cannot be represented by the underlying type."
> +                );
> +                // Statically assert that `V` fits within the set number of bits.
> +                const_assert!(
> +                    fits_within!(V as $type, $type, N),
> +                    "Constant cannot be represented within the given number of bits."
> +                );
> +
> +                // SAFETY: the asserts above confirmed that `V` can be represented within `N`
> +                // bits.
> +                unsafe { Self::__new(V as $type) }
> +            };
> +        }
>          )*
>      };
>  }
> diff --git a/rust/kernel/ptr.rs b/rust/kernel/ptr.rs
> index 82acb531b17b..ecb73bdb0c5f 100644
> --- a/rust/kernel/ptr.rs
> +++ b/rust/kernel/ptr.rs
> @@ -166,6 +166,20 @@ pub const fn mask(self) -> usize {
>      }
>  }
>  
> +impl<const V: i128> crate::num::FromConst<V> for Alignment {
> +    const VALUE: Self = {
> +        const_assert!(
> +            V > 0 && V <= usize::MAX as i128,
> +            "Constant cannot be represented as an Alignment."
> +        );
> +
> +        match Alignment::new_checked(V as usize) {
> +            Some(alignment) => alignment,
> +            None => panic!("Constant is not a power of two."),
> +        }
> +    };
> +}
> +
>  /// Trait for items that can be aligned against an [`Alignment`].
>  pub trait Alignable: Sized {
>      /// Aligns `self` down to `alignment`.