From 1b18b37a2c30f6e6698205a06de393f7e626f5d2 Mon Sep 17 00:00:00 2001 From: Alexandre Courbot Date: Mon, 8 Dec 2025 11:46:59 +0900 Subject: [PATCH] rust: build_assert: add instructions for use with function arguments `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 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. The fix is to annotate functions passing their argument to `build_assert` with `#[inline(always)]`, telling the compiler to be as aggressive as possible with their inlining. This is also the correct behavior as inlining is mandatory for correct behavior in these cases. Add a paragraph instructing to annotate such functions with `#[inline(always)]` in `build_assert`'s documentation, and split its example to illustrate. Reviewed-by: Daniel Almeida Signed-off-by: Alexandre Courbot Link: https://patch.msgid.link/20251208-io-build-assert-v3-1-98aded02c1ea@nvidia.com Signed-off-by: Miguel Ojeda --- rust/kernel/build_assert.rs | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/rust/kernel/build_assert.rs b/rust/kernel/build_assert.rs index 6331b15d7c4de..f8124dbc663f7 100644 --- a/rust/kernel/build_assert.rs +++ b/rust/kernel/build_assert.rs @@ -61,8 +61,13 @@ macro_rules! build_error { /// build_assert!(N > 1); // Build-time check /// assert!(N > 1); // Run-time check /// } +/// ``` /// -/// #[inline] +/// When a condition depends on a function argument, the function must be annotated with +/// `#[inline(always)]`. Without this attribute, the compiler may choose to not inline the +/// function, preventing it from optimizing out the error path. +/// ``` +/// #[inline(always)] /// fn bar(n: usize) { /// // `static_assert!(n > 1);` is not allowed /// build_assert!(n > 1); // Build-time check -- 2.47.3