Re: [PATCH v7 03/10] rust: num: add nz! macro for compile time NonZero values

[Gary Guo]

On Mon Aug 17, 2026 at 8:04 AM BST, Eliot Courtney wrote:
> Currently, using NonZero constants is quite verbose. It's unfortunate
> because it disincentivizes using it in interface boundaries. Introduce a
> macro to make it nicer to use.
>
> Link: https://lore.kernel.org/DKO414SNC0B2.248FUBGJGSPLU@xxxxxxxxxx
> Signed-off-by: Eliot Courtney <ecourtney@xxxxxxxxxx>
> ---
>  rust/kernel/num.rs | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
>
> diff --git a/rust/kernel/num.rs b/rust/kernel/num.rs
> index 8532b511384c..055d23ca7fd1 100644
> --- a/rust/kernel/num.rs
> +++ b/rust/kernel/num.rs
> @@ -7,6 +7,29 @@
>  pub mod bounded;
>  pub use bounded::*;
>  
> +/// Infallibly creates a [`NonZero`] value from a constant expression.
> +///
> +/// [`NonZero`]: core::num::NonZero
> +///
> +/// # Examples
> +///
> +/// ```
> +/// use core::num::NonZero;
> +/// use kernel::nz;
> +///
> +/// let v: NonZero<usize> = nz!(8);
> +/// assert_eq!(v.get(), 8);
> +///
> +/// const N: NonZero<u32> = nz!(0x10);
> +/// assert_eq!(N.get(), 0x10);
> +/// ```
> +#[macro_export]
> +macro_rules! nz {
> +    ($v:expr) => {
> +        const { ::core::num::NonZero::new($v).unwrap() }
> +    };
> +}

I wonder if if we can have a generic macro for creating types from literals,
given that this is needed for bounded too. Something like

    trait FromLiteral {
        fn from_literal<const N: i64>() -> Self;
    }

    impl FromLiteral for NonZero<u32> {
        #[inline]
        fn from_literal<const N: i64>() -> Self {
            const_assert!(N > 0 && N < u32::MAX as i64);
            const { NonZero::new(N as u32).unwrap() }
        }
    }

    macro_rules! lit {
        ($x:expr) => {
            FromLiteral::from_literal::<{$x}>()
        }
    }

    let nz: NonZero<u32> = lit!(1);
    let bounded: Bounded<..> = lit!(1);

I think the only downside is that this expression isn't const expr anymore. We
can change this to be

    const trait FromLiteral {
        fn from_literal(v: i64) -> Self;
    }

when const trait impl is stable upstream and we have MSRV bumped high enough to
have an impl of it.

Best,
Gary