Re: [PATCH 1/2] rust: num: casts: replace const type narrowing methods with a macro

["Alexandre Courbot" <acourbot@nvidia.com>]

On Tue Aug 25, 2026 at 4:18 PM JST, Eliot Courtney wrote:
> On Tue Aug 25, 2026 at 11:44 AM JST, Alexandre Courbot wrote:
>> The casts module features a series of const converters (e.g.
>> `u32_into_u16`) that narrow the type of a const expression provided that
>> its value can be proven to fit into the destination type at
>> compile-time.
>>
>> These functions are numerous (9 of them), generated by a macro and thus
>> not easily discoverable, and cumbersome to use as they require a
>> turbofish and const expression between `{` and `}` braces.
>>
>> Replace them all by a single `const_as!` macro that expands to a const
>> block verifying the lossless nature of the conversion at compile-time.
>> This turns e.g.:
>>
>>     const DMA_LEN: u32 = casts::usize_into_u32::<{ MEM_BLOCK_ALIGNMENT }>();
>>
>> into
>>
>>     const DMA_LEN: u32 = casts::const_as!(MEM_BLOCK_ALIGNMENT => u32);
>>
>> This makes things easier to read and understand, while shifting the
>> burden of checking the conversion's validity from reviewers (via a CAST
>> comment) to the compiler.
>>
>> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
>> ---
>>  rust/kernel/num/casts.rs | 129 +++++++++++++++++++++++++++++------------------
>>  1 file changed, 79 insertions(+), 50 deletions(-)
>>
>> diff --git a/rust/kernel/num/casts.rs b/rust/kernel/num/casts.rs
>> index 7e6c7dec747d..a4a18a6f2ba8 100644
>> --- a/rust/kernel/num/casts.rs
>> +++ b/rust/kernel/num/casts.rs
>> @@ -20,10 +20,8 @@
>>  //! - Two extension traits, [`FromSafeCast`] and [`IntoSafeCast`], providing conversion methods
>>  //!   similar to [`From`] and [`Into`] for conversions that are safe to perform in the kernel, 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, provided the value fits into the destination
>> -//!   type. This is useful if a constant is defined as a larger type, but needs to be used as a
>> -//!   smaller one.
>> +//! - A [`const_as!`] macro, losslessly casting a constant expression between any two integer
>> +//!   types, with conversions that would alter the value reported as build errors.
>>  //! - An [`arch`] sub-module, defining more conversion functions that are only guaranteed to be
>>  //!   lossless for a given pointer size. These can only be used in code that is specific to a
>>  //!   given pointer size.
>
> Can we add guidance somewhere in this file on when to use const_as! vs
> when to use the u8_as_usize etc ones, when both could work? e.g. use
> const_as! if you can, otherwise use the function version, or, use the
> function version if it's sufficient (types alone are enough to prove)
> otherwise use the macro.

Yes, that's a very good idea and the use we are currently doing in Nova
is also not consistent. Basically I think that the priority should be,
in order of preference:

- stdlib's `From`,
- `FromSafeCast`/`IntoSafeCast` (for non-const contexts)
- `const_as!` (for constant expressions including narrowing)
- `*_as_*` (for const fns with a runtime value)

>
> [...]
>> +#[macro_export]
>> +#[doc(hidden)]
>> +macro_rules! const_as {
>> +    ($v:expr => $into:ty) => {
>> +        const {
>> +            #[allow(unused_comparisons, unused_assignments, clippy::as_underscore)]
>> +            {
>> +                let v = $v;
>> +                let r = v as $into;
>> +                // Pin `back` to `v`'s type so `as _` casts back to the source type.
>> +                let mut back = v;
>> +                back = r as _;
>>  
>> -                N as $into
>> +                ::core::assert!(
>> +                    back == v && (v < 0) == (r < 0),
>> +                    "value does not fit into the target type"
>> +                );
>
> What about giving some text on what doesn't fit where? e.g.
> ::core::concat!("`", ::core::stringify!($v), "` does not fit into `", ::core::stringify!($into), "`")

Will do.