On Tue, 09 Dec 2025 09:52:13 +0900 "Alexandre Courbot" acourbot@nvidia.com wrote:
On Mon Dec 8, 2025 at 10:55 PM JST, Gary Guo wrote:
On Mon, 08 Dec 2025 11:47:01 +0900 Alexandre Courbot acourbot@nvidia.com wrote:
`build_assert` relies on the compiler to optimize out its error path. Functions using it with its arguments must thus always be inlined, otherwise the error path of `build_assert` might not be optimized out, triggering a build error.
Cc: stable@vger.kernel.org Fixes: c6af9a1191d0 ("rust: cpufreq: Extend abstractions for driver registration") Acked-by: Viresh Kumar viresh.kumar@linaro.org Reviewed-by: Daniel Almeida daniel.almeida@collabora.com Signed-off-by: Alexandre Courbot acourbot@nvidia.com
rust/kernel/cpufreq.rs | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/rust/kernel/cpufreq.rs b/rust/kernel/cpufreq.rs index f968fbd22890..0879a79485f8 100644 --- a/rust/kernel/cpufreq.rs +++ b/rust/kernel/cpufreq.rs @@ -1015,6 +1015,8 @@ impl<T: Driver> Registration<T> { ..pin_init::zeroed() };
- // Always inline to optimize out error path of `build_assert`.
- #[inline(always)] const fn copy_name(name: &'static CStr) -> [c_char; CPUFREQ_NAME_LEN] { let src = name.to_bytes_with_nul(); let mut dst = [0; CPUFREQ_NAME_LEN];
This change is not needed as this is a private function only used in const-eval only.
... for now. :)
I wonder if I should add another macro to assert that the function is only used in const eval instead? Do you think it might be useful to have something like:
#[const_only] const fn foo() {}
or
const fn foo() { const_only!(); }
? If so, I can send a patch that adds this feature.
Implementation-wise, this will behave similar to build_error, where a function is going to be added that is never-linked but has a body for const eval.
It could be useful in the general sense, but for this particular case the rule "if you do build_assert on a function argument, then always inline it" also covers us in case `copy_name` gets used outside of const context, so isn't it the preferable workaround?
In this particular case the `copy_name` shouldn't be used at all outside const eval. It's specificially for building a table during const eval. It's a bug if it's outside, hence I think `#[inline(always)]` adds confusion to the reader of this code.
I get that you want to have a general rule of "if you're using something with `build_assert!`, then use `#[inline(always)]`", but I think applying that rule here is detrimental.
Hence I suggested adding a marker to indicate const-eval only function, so we can either say const-eval-only functions are fine without inline markers, or perhaps just use normal panicking-assertion inside these functions (as `build_assert!` behave identical to just `assert!` in const-eval).
Best, Gary