Using rules_rust toolchain lld for linking Rust + C++ (cross-language LTO with older clang bitcode than Rust)?

[sthornington]

Hi folks,

I'm trying to do Rust-C++ interop with cross-language ThinLTO - in our environment, clang is only at 18 but Rust is all the way current at 1.90.0. The only way I have found that I can link these things is by using clang++ as the linker front-end, but with -fuse-ld=<path-to-that-lld> but I am running into difficulty programmatically getting a label of the path to this binary so that it can be passed to the CC toolchain.

Has anyone been able to do this? It does seem to work as long as the linker is newer than all the bitcode in the objects, but I'm just kinda stuck on the bazel toolchain part, of finding a query or location or something that can be evaluated early enough in the repo/toolchain steps.

Thanks!

Simon.

[sthornington]

I believe I got this working in the hackyest way imaginable, by synthesizing the rust toolchain repo name like rust_linux_x86_64__x86_64_unknown_linux_gnu__stable_tools so I can run rustc to --print target-libdir in order to find the bin/gcc-ld/ld.lld there but honestly it's super gross - I need some way to get the path of the lld that the rust toolchain uses, in order to pass that to the CC toolchain to use as its -fuse-ld=<it> so that I can ensure that the linker can handle Rust's bitcode, on the assumption that it is going to be the newer one of the two.

[sthornington]

Ok I still haven't found a better way to do this. Trying to query the rust toolchain more directly results in a circular dependency between the rust toolchain depending on the C++ toolchain for ffi/cc/global_allocator_library and my CC toolchains depending on the rust toolchain for lld.

Any ideas for cleaner ways of doing this would be appreciated! I wonder if there is a tiny lld-only LLVM repo that they could both use.