tools depending on rustc crates can't build with `rust.download-rustc=true` and `llvm.download-ci-llvm=false` options

[onur-ozkan]

When trying to build tools that rely on rustc crates using the rust.download-rustc=true and llvm.download-ci-llvm=false options, ld can not find the LLVM it needs.

$ x build miri
...
...
...
  = note: /usr/bin/ld: cannot find -lLLVM-18-rust-1.79.0-nightly: No such file or directory
          collect2: error: ld returned 1 exit status


error: could not compile `rustdoc-tool` (bin "rustdoc_tool_binary") due to 1 previous error
Build completed unsuccessfully in 0:01:02

I have also attempted to build rustdoc and clippy and they too failed due to the same issue. So I am quite sure this happens on any tool that uses rustc crates.

[onur-ozkan]

I am unsure whether this is a bug in bootstrap or a limitation of using precompiled CI rustc.. 🤔

cc @nikic

[bjorn3]

Building tools that link against rustc internals requires both the rustc-dev and llvm-tools-preview components to be installed. The former for all crate metadata and librustc_driver.so in the sysroot, while the latter is for libLLVM.so. Linking against the libLLVM.so in the rustc component that rustc itself is linked against doesn't work as it isn't in the sysroot for the target. It is in lib/ rather than lib/rustlib/some-target-triple/lib.

[onur-ozkan]

I am unsure whether this is a bug in bootstrap or a limitation of using precompiled CI rustc.. 🤔

Even if llvm.download-ci-llvm is set to true, stage > 0 rustc will always use the prebuilt LLVM library which comes with ci-rustc. So I tried to use locally-built LLVM libraries in the ci-rustc by replacing the existing LLVM libraries with the locally built ones, and it appears that this is indeed a limitation of using rust.download-rustc=true as it fails with the following error:

$ ./build/host/ci-rustc/bin/rustc --version
./build/host/ci-rustc/bin/rustc: symbol lookup error: /home/nimda/devspace/.other/rustc-builds/build/x86_64-unknown-linux-gnu/ci-rustc/bin/../lib/librustc_driver-a03ea465d8e03db1.so: undefined symbol: LLVMInitializeARMTargetInfo, version LLVM_18.1

So, if rust.download-rustc is set to true and llvm.download-ci-llvm is false, I believe bootstrap should terminate the process (as it always uses prebuilt LLVM libraries from ci-rustc, there is no point to build LLVM locally) while parsing the configuration.