Re: [PATCH v3 0/7] rust: build_assert: document and fix use with function arguments

[Alexandre Courbot]

On Mon Dec 8, 2025 at 10:49 PM JST, Gary Guo wrote:
> On Mon, 08 Dec 2025 11:46:58 +0900
> Alexandre Courbot <acourbot@xxxxxxxxxx> wrote:
>
>> `build_assert` relies on the compiler to optimize out its error path,
>> lest build fails with the dreaded error:
>> 
>>     ERROR: modpost: "rust_build_error" [path/to/module.ko] undefined!
>> 
>> It has been observed that very trivial code performing I/O accesses
>> (sometimes even using an immediate value) would seemingly randomly fail
>> with this error whenever `CLIPPY=1` was set. The same behavior was also
>> observed until different, very similar conditions [1][2].
>> 
>> The cause, as pointed out by Gary Guo [3], appears to be that the
>> failing function is eventually using `build_assert` with its argument,
>> but is only annotated with `#[inline]`. This gives the compiler freedom
>> to not inline the function, which it notably did when Clippy was active,
>> triggering the error.
>
> That's an interesting observation, so `#[inline]` is fine without
> clippy but `#[inline(always)]` is needed when Clippy is used?

Precisely. And sometimes just moving the code invoking the
not-always-inlined function that invokes build_assert is enough to fix
it, so I don't know of an accurately reproducible pattern. It also
occurs pretty rarely.