Re: [PATCH 1/2] kbuild: rust: preserve unreachable traps with inline helpers

On Sun, Aug 16, 2026 at 3:33 PM Miguel Ojeda <[email protected]> wrote:
>
> When `CONFIG_RUST_INLINE_HELPERS` is enabled, it is possible to hit
> `objtool` warnings like:
>
>     vmlinux.o: warning: objtool: _R..._4cmdq12CommandToGsp4init()
>     falls through to next function _R..._4core5array4iter8IntoIterRShKj3_EEEBa_()
>
> `rustc` normally emits traps for unreachable paths. However, under
> `CONFIG_RUST_INLINE_HELPERS=y`, `rustc` emits LLVM bitcode and Clang
> performs final code generation after the helper bitcode is linked,
> but Clang does not trap unreachable IR by default.
>
> In turn, this means `objtool` follows compiler-generated impossible Rust
> `enum` paths through alignment padding into the next function, resulting
> in fallthrough warnings.
>
> Thus pass the LLVM `trap-unreachable` option to the final Clang invocation
> and suppress traps immediately after `noreturn` calls, which `objtool`
> already recognizes as dead ends. The combination of both flags makes it
> match `rustc`'s behavior.
>
> Rust 1.85.0 (the minimum supported one) supports LLVM >= 18, and both
> flags are available in LLVM 18.
>
> Assisted-by: LLM
> Cc: Gary Guo <[email protected]>
> Cc: Boqun Feng <[email protected]>
> Cc: Alice Ryhl <[email protected]>
> Cc: Matthew Maurer <[email protected]>
> Cc: Josh Poimboeuf <[email protected]>
> Cc: Peter Zijlstra <[email protected]>
> Cc: [email protected]
> Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
> Signed-off-by: Miguel Ojeda <[email protected]>

Applied series to `rust-fixes` -- thanks everyone!

Cheers,
Miguel