Re: [OE-core] [PATCH V3 1/2] rust: Use clang instead of rust-llvm

["Mathieu Dubois-Briand" <mathieu.dubois-briand@bootlin.com>]

On Fri Sep 26, 2025 at 12:24 PM CEST, Deepesh via lists.openembedded.org Varatharajan wrote:
> From: Deepesh Varatharajan <Deepesh.Varatharajan@windriver.com>
>
> Updated the Rust build to depend on Clang instead.
>
> *Summary of discussion with the rust upstream about using latest LLVM instead of Rust maintained LLVM fork.
> https://internals.rust-lang.org/t/can-we-use-proper-clang-instead-of-llvm-fork-what-rust-uses/23489
>
> *Upstream LLVM is generally compatible:
> - Rust does support building with upstream (vanilla) LLVM, especially the latest
> major release and the one or two preceding ones.
> https://rustc-dev-guide.rust-lang.org/backend/updating-llvm.html#updating-llvm
>
> *Impact on Yocto Rust upgrades:
> - Rust upgrades shall always check for updates on rust forked llvm and backport
> the relevant patches to clang's llvm.
>
> *Regarding the rust forked llvm local patches:
> - There are no local patches on rust forked llvm other than the backported fixes
> from llvm master.
>
> *We now add these flags "-Clink-arg=-lz -Clink-arg=-lzstd" because of this following
> diff otherwise we will get errors during link time.
>
> Setup in rust-llvm
> -DLLVM_ENABLE_ZLIB=OFF \
> -DLLVM_ENABLE_ZSTD=OFF \
> -DLLVM_ENABLE_FFI=OFF \
>
> Setup in clang
> -DLLVM_ENABLE_FFI=ON \
> -DLLVM_ENABLE_ZSTD=ON \
>
> *When multilibs enabled:
>
> llvm-config expects static libraries to be located in the lib directory rather than
> lib64. However, since LLVM is built as a non-multilib component, the lib directory
> doesn't contain any library files. To accommodate this without breaking multilib
> behavior, we copy the required library files appropriately.
>
> Previously, when we depended on rust-llvm, this worked because we specified:
> -DCMAKE_INSTALL_PREFIX:PATH=${libdir}/llvm-rust
>
> With this setup, llvm-config was installed inside ${libdir}/llvm-rust, which included
> its own bin and lib directories. Thus, llvm-config located in bin would correctly find
> the libraries in the adjacent lib directory.
>
> Even when multilib was enabled or not, llvm-config would still look for libraries under
> lib in this structure, so everything functioned as expected.
>
> *Changes needs to be done when llvm splits from clang:
> In rust recipe:
> Update the dependency from:
> DEPENDS += "ninja-native clang" to DEPENDS += "ninja-native llvm"
>
> In llvm recipe:
> Apply the same changes that were made in the Clang recipe, as those
> configurations have now been moved to the LLVM recipe after the split.
>
> Signed-off-by: Deepesh Varatharajan <Deepesh.Varatharajan@windriver.com>
> ---

Hi Deepesh,

Thanks for your patch.

It looks like this is breaking some build, strangely only the arm64 one
with musl.

ERROR: rust-1.90.0-r0 do_install: Execution of '/srv/pokybuild/yocto-worker/musl-qemuarm64/build/build/tmp/work/cortexa57-poky-linux-musl/rust/1.90.0/temp/run.do_install.3194366' failed with exit code 1
...
| Building libunwind.a for aarch64-poky-linux-musl
| cargo:warning=/srv/pokybuild/yocto-worker/musl-qemuarm64/build/build/tmp/work/cortexa57-poky-linux-musl/rust/1.90.0/sources/rustc-1.90.0-src/src/llvm-project/libunwind/src/UnwindLevel1.c:191:1: error: arch extension 'gcs' should be prefixed by '+'
| cargo:warning=  191 | unwind_phase2(unw_context_t *uc, unw_cursor_t *cursor, _Unwind_Exception *exception_object) {
| cargo:warning=      | ^~~~~~~~~~~~~
| cargo:warning=/srv/pokybuild/yocto-worker/musl-qemuarm64/build/build/tmp/work/cortexa57-poky-linux-musl/rust/1.90.0/sources/rustc-1.90.0-src/src/llvm-project/libunwind/src/UnwindLevel1.c:337:22: error: arch extension 'gcs' should be prefixed by '+'
| cargo:warning=  337 |                      _Unwind_Stop_Fn stop, void *stop_parameter) {
| cargo:warning=      |                      ^~~~~~~~~~~~~~~
|

Can you have a look at this failure please?

Thanks,
Mathieu

-- 
Mathieu Dubois-Briand, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com