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

[Josh Poimboeuf <jpoimboe@kernel.org>]

On Sun, Aug 16, 2026 at 03:32:32PM +0200, Miguel Ojeda 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.

FWIW, I've been working on some patches that should hopefully let us get
rid of all the objtool hard-coded noreturn lists altogether.  I'll try to post
those soon.

-- 
Josh