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 <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1d ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
0bff7711c1