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

["Eliot Courtney" <ecourtney@nvidia.com>]

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.

[...]
> +#[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), "`")