On Fri Nov 14, 2025 at 3:49 PM JST, Mikko Perttunen wrote:
> 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 <[email protected]>
>> ---
>> 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.
Ah, nice catch, thanks! I missed that because the doccomments are not
built yet for code outside of `kernel`.
Fixing it by taking the example from the non-Nova RFC [1] of this patch.
[1] https://lore.kernel.org/all/[email protected]/
