Re: [PATCH v8 03/12] rust: num: add cv! macro to create values from constant expressions

[Eliot Courtney]

On Fri Aug 28, 2026 at 12:40 PM JST, Alexandre Courbot wrote:
[...]
>> Using an associated const by itself appears to work - I tried this which
>> is very similar to Alice's suggested approach above:
>>
>> ```
>> macro_rules! const_assert {
>>     ($condition:expr $(,$arg:literal)?) => {
>>         const { ::core::assert!($condition $(,$arg)?) };
>>     };
>> }
>
> Any reason this cannot use the `const_assert` already in the kernel
> crate?

No reason. I prototyped this in a single file so I had some misc shims.
This is just me copy pasting badly.

>
>>
>> trait FromConst<const V: i128>: Sized {
>>     const VALUE: Self;
>> }
>>
>> macro_rules! cv {
>>     (@widen $v:expr) => {{
>>         #[allow(unused_comparisons, unused_assignments)]
>>         {
>>             let v = $v;
>>             let r = v as i128;
>>             let mut back = v;
>>             back = r as _;
>>
>>             ::core::assert!(
>>                 back == v && (v < 0) == (r < 0),
>>                 "value cannot be losslessly widened to `i128`"
>>             );
>>
>>             r
>>         }
>>     }};
>>     ($v:expr => $t:ty) => {
>>         <$t as FromConst<{ cv!(@widen $v) }>>::VALUE
>
> nit for the actual posting: make sure to fully qualify `cv` when calling
> it recursively (and make sure all symbols are fully qualified).
>
>>     };
>>     ($v:expr) => {
>>         <_ as FromConst<{ cv!(@widen $v) }>>::VALUE
>>     };
>> }
>>
>> macro_rules! impl_from_const_int {
>>     ($($t:ty)*) => {$(
>>         impl<const V: i128> FromConst<V> for $t {
>>             const VALUE: Self = {
>>                 const_assert!(
>>                     V >= <$t>::MIN as i128 && V <= <$t>::MAX as i128,
>>                     "Constant cannot be represented by the target type."
>>                 );
>>                 V as $t
>>             };
>>         }
>>
>>         impl<const V: i128> FromConst<V> for NonZero<$t> {
>>             const VALUE: Self = {
>>                 const_assert!(
>>                     V >= <$t>::MIN as i128 && V <= <$t>::MAX as i128,
>>                     "Constant cannot be represented by the underlying type."
>>                 );
>
> Let's also have a `const_assert!(V != 0, ...)` to provide a better
> error message than "unwrap on None" if users call this with 0.

Sounds good~

>
>>                 NonZero::new(V as $t).unwrap()
>>             };
>>         }
>>     )*};
>> }
>> impl_from_const_int!(u8 u16 u32 u64 usize i8 i16 i32 i64 isize);
>>
>> impl<const V: i128> FromConst<V> for Alignment {
>>     const VALUE: Self = {
>>         const_assert!(V > 0 && V <= usize::MAX as i128);
>>         // The unwrap fails the build if `V` is not a power of two.
>>         Alignment::new_checked(V as usize).unwrap()
>>     };
>> }
>>
>> const A: u8 = cv!(200u32);
>> const B: NonZero<u8> = cv!(5);
>> const C: Alignment = cv!(4096);
>> const D: u8 = cv!(200u32 => u8);
>> ```
>
> That looks like it could work! IIUC it even supports something like
> `cv!(x => NonZero<u8>)`. The only limitation is see is that this cannot
> take expressions using generic parameters, but we can probably work
> around that.

Yeah agreed. One workaround is just to list each primitive type in the
macro, although it doesn't work if you define a type alias or something.
I think that's reasonable and not too complex. FWIW I found one
potential user that would want to be able to reference generics in cv!.
In drm::Device:
`num_ioctls: T::IOCTLS.len() as i32,`
wants to be `cv!(T::IOCTLS.len() => i32),` which needs this.

>
> I guess you'll want to split this out into its own series so it doesn't
> remain hidden within the ranges/bitmap work. Basically as a replacement
> for the `const_as` I was driving [1]. I'll recycle the `const_as` series
> to just switch to the kernel converters, and will follow-up with using
> `cv!` once it lands.
>
> Since this is going to be a multi-cycle effort we should probably keep
> the legacy `*_into_*` functions around for now and remove them in
> another patch once all users are converted.
>
> [1] https://lore.kernel.org/20260825-const_as-v1-0-1ce712225fe2@xxxxxxxxxx

Yes I'll split it out. Thanks!