Re: [PATCH] rust: num: bounded: Always inline fits_within and from_expr

[Miguel Ojeda]

On Sat, Nov 22, 2025 at 5:01 AM Alexandre Courbot <acourbot@xxxxxxxxxx> wrote:
>
> `from_expr` relies on `build_assert` to infer that the passed expression
> fits the type's boundaries at build time. That inference can only be
> successful its code (and that of `fits_within`, which performs the
> check) is inlined, as a dedicated function would need to work with a
> variable and cannot verify that property.
>
> While inlining happens as expected in most cases, it is not guaranteed.
> In particular, kernel options that optimize for size like
> `CONFIG_CC_OPTIMIZE_FOR_SIZE` can result in `from_expr` not being
> inlined.
>
> Add `#[inline(always)]` attributes to both `fits_within` and `from_expr`
> to make the compiler inline these functions more aggressively, as it
> does not make sense to use them non-inlined anyway.
>
> Fixes: 01e345e82ec3 ("rust: num: add Bounded integer wrapping type")
> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> Closes: https://lore.kernel.org/oe-kbuild-all/202511210055.RUsFNku1-lkp@xxxxxxxxx/
> Suggested-by: Gary Guo <gary@xxxxxxxxxxx>
> Signed-off-by: Alexandre Courbot <acourbot@xxxxxxxxxx>

Applied to `rust-next` -- thanks!

    [ For reference, the errors look like:

        ld.lld: error: undefined symbol: rust_build_error
        >>> referenced by build_assert.rs:83 (rust/kernel/build_assert.rs:83)
        >>>
rust/doctests_kernel_generated.o:(<kernel::num::bounded::Bounded<u8,
1>>::from_expr) in archive vmlinux.a

          - Miguel ]

I could have rebased, but since you sent this nice patch with a good
commit message and kernel test robot reported it, let's add it :)

Cheers,
Miguel