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build: Fix build for variety of musl-based Linux distributions

vadrequested to merge
vad.sol/llvm-sys.rs:fix-musl into main

Before this change, build.rs had two problems which were making it impossible to build on different musl-based Linux environments:

  • It was assuming that the C++ library on all musl-based systems is LLVM libc++.
    • In the most cases, it's the opposite. LLVM packaged in Alpine is linked against GNU libstdc++, as well as LLVM tarballs in Rust CI.
    • Gentoo with musl-llvm profile, even though it links LLVM against libc++, has a different name for the system-wide static library (/usr/lib/libc++_static.a instead of /usr/lib/libc++.a, which is a link script).
  • When linking runtime libraries statically (crt-static), Cargo doesn't look for the system-wide static libraries in /usr/lib, unless that directory is added to the link flags with -L.

The first point is addressed by:

  • Linking to libstdc++ by default, even for musl systems.
  • Ability to override that choice with LLVM_SYS_LIBCPP environment variable for environments which use LLVM libc++.

The second point is addressed by:

  • Adding /usr/lib and other system dirs to cargo::rustc-link-search on Linux with crt-static builds.
  • Exposing LLVM_SYS_SYSTEM_LIBRARY_DIRS environment variable, where callers can add additional paths.

After this change, building llvm-sys on Alpine can be done just with the usual cargo build and LLVM_SYS_191_PREFIX variable pointing to the LLVM 19 prefix. In CI, we are using Rust CI tarballs for that.

On any Linux environment where LLVM is linked against libc++, llvm-sys can be built with:

LLVM_SYS_LIBCPP=c++ cargo build --no-default-features

Building llvm-sys Gentoo with llvm-musl profile, where libc++ static library is called libc++_static.a and it needs libc++abi.a to work, can be done with:

LLVM_SYS_LIBCPP=c++_static RUSTFLAGS="-lc++abi" cargo test --no-default-features

Unfortunately, the latter configurations are not tested in CI - as for now, there are no publicly available libLLVM binary blobs linked against LLVM libc++ - such configuration requires compilation of LLVM.

Edited by vad

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