On Monday, November 10, 2025 10:34 PM Alexandre Courbot wrote:
> There are times where we need to store a constant value defined as a
> larger type (e.g. through a binding) into a smaller type, knowing
> that the value will fit. Rust, unfortunately, only provides us with the
> `as` operator for that purpose, the use of which is discouraged as it
> silently strips data.
>
> Extend the `num` module with functions allowing to perform the
> conversion infallibly, at compile time.
>
> Example:
>
> const FOO_VALUE: u32 = 1;
>
> // `FOO_VALUE` fits into a `u8`, so the conversion is valid.
> let foo = num::u32_to_u8::<{ FOO_VALUE }>();
>
> We are going to use this feature extensively in Nova.
>
> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
> ---
> drivers/gpu/nova-core/num.rs | 51 ++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 51 insertions(+)
>
> diff --git a/drivers/gpu/nova-core/num.rs b/drivers/gpu/nova-core/num.rs
> index 92a91b9e30de..f3740ab6cb9d 100644
> --- a/drivers/gpu/nova-core/num.rs
> +++ b/drivers/gpu/nova-core/num.rs
> @@ -163,3 +163,54 @@ fn into_safe_cast(self) -> T {
> T::from_safe_cast(self)
> }
> }
> +
> +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!($into),
> + ");")]
This expands to e.g. assert_eq!(num::u32_to_u8(1u32), 1u8), i.e. not with a generic parameter.
With that fixed,
Reviewed-by: Mikko Perttunen <mperttunen@nvidia.com>
> + /// ```
> + #[allow(unused)]
> + pub(crate) const fn [<$from _into_ $into>]<const N: $from>() -> $into {
> + // Make sure that the target type is smaller than the source one.
> + static_assert!($from::BITS >= $into::BITS);
> + // CAST: we statically enforced above that `$from` is larger than `$into`, so the
> + // `as` conversion will be lossless.
> + build_assert!(N >= $into::MIN as $from && 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 });
>
>