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When bootstrapping Rust with the llvm profile (both glibc and musl), the build fails at the mrustc stage due to two issues related to C++ standard library handling. 1. Compilation failure When llvm-core/llvm is built with clang, 'llvm-config --cxxflags' includes '-stdlib=libc++', but not when built with GCC. But under musl profile, GCC has a known bug (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122409) in handling the '-stdlib=libc++' option. It fails to search '/usr/include/c++/v1', causing standard headers such as <cstddef> to be missing unless the include path is manually specified via '-I/usr/include/c++/v1'. This patch fixes the issue by wrapping llvm-config calls and explicitly appending libc++'s header path whenever '-stdlib=libc++' is detected. Below is the relevant error log: > Calling /var/tmp/portage/dev-lang/rust-1.74.1-r101/work/bootstrap/mrustc-stage0/rustc-build/build_rustc_llvm_run failed (see /var/tmp/portage/dev-lang/rust-1.74.1-r101/work/bootstrap/mrustc-stage0/rustc-build/build_rustc_llvm.txt_failed.txt for stdout) /var/tmp/portage/dev-lang/rust-1.74.1-r101/work/bootstrap/mrustc-stage0/rustc-build/build_rustc_llvm.txt_failed.txt > running: "x86_64-pc-linux-musl-g++" "-O2" "-ffunction-sections" "-fdata-sections" "-fPIC" "-gdwarf-4" "-fno-omit-frame-pointer" "-m64" "-O2" "-pipe" "-I/usr/lib/llvm/17/include" "-std=c++17" "-stdlib=libc++" "-D__STDC_CONSTANT_MACROS" "-D__STDC_FORMAT_MACROS" "-D__STDC_LIMIT_MACROS" "-DLLVM_COMPONENT_AARCH64" "-DLLVM_COMPONENT_AMDGPU" "-DLLVM_COMPONENT_ARM" "-DLLVM_COMPONENT_ASMPARSER" "-DLLVM_COMPONENT_AVR" "-DLLVM_COMPONENT_BITREADER" "-DLLVM_COMPONENT_BITWRITER" "-DLLVM_COMPONENT_BPF" "-DLLVM_COMPONENT_COVERAGE" "-DLLVM_COMPONENT_HEXAGON" "-DLLVM_COMPONENT_INSTRUMENTATION" "-DLLVM_COMPONENT_IPO" "-DLLVM_COMPONENT_LINKER" "-DLLVM_COMPONENT_LOONGARCH" "-DLLVM_COMPONENT_LTO" "-DLLVM_COMPONENT_MIPS" "-DLLVM_COMPONENT_MSP430" "-DLLVM_COMPONENT_NVPTX" "-DLLVM_COMPONENT_POWERPC" "-DLLVM_COMPONENT_RISCV" "-DLLVM_COMPONENT_SPARC" "-DLLVM_COMPONENT_SYSTEMZ" "-DLLVM_COMPONENT_WEBASSEMBLY" "-DLLVM_COMPONENT_X86" "-o" "/var/tmp/portage/dev-lang/rust-1.74.1-r101/work/bootstrap/mrustc-stage0/rustc-build/build_rustc_llvm/llvm-wrapper/PassWrapper.o" "-c" "llvm-wrapper/PassWrapper.cpp" > cargo:warning=llvm-wrapper/PassWrapper.cpp:3:10: fatal error: cstddef: No such file or directory > cargo:warning= 3 | #include <cstddef> > cargo:warning= | ^~~~~~~~~ > cargo:warning=compilation terminated. > exit status: 1 2. Linker failure Found on llvm profile for both glibc and musl if llvm-core/llvm is built by GCC. The bundled llvm is always built by gcc because bootstrap with murst requires gcc. Under the llvm profile `LLVM_USE_LIBCXX=1` is defined, causing Rust build scripts to link with `-lc++` instead of `-lstdc++`. Since mrustc’s g++-compiled objects depend on libstdc++, linking fails with (glibc): > /.../x86_64-pc-linux-musl/bin/ld: /var/tmp/portage/dev-lang/rust-1.74.1-r101/work/bootstrap/mrustc-stage0/rustc-build/build_rustc_llvm/libllvm-wrapper.a(PassWrapper.o): undefined reference to symbol '_ZNSt6localeD1Ev@@GLIBCXX_3.4' > /.../x86_64-pc-linux-musl/bin/ld: /usr/lib/gcc/x86_64-pc-linux-musl/15/libstdc++.so.6: error adding symbols: DSO missing from command line or (musl): > /.../x86_64-pc-linux-musl/bin/ld: warning: x86_64.o: missing .note.GNU-stack section implies executable stack > /.../x86_64-pc-linux-musl/bin/ld: NOTE: This behaviour is deprecated and will be removed in a future version of the linker > /.../x86_64-pc-linux-musl/bin/ld: /var/tmp/portage/dev-lang/rust-1.74.1-r101/work/bootstrap/mrustc-stage0/rustc-build/build_rustc_llvm/libllvm-wrapper.a(PassWrapper.o): in function `std::_Rb_tree_iterator<std::pair<unsigned long const, llvm::GlobalValue::LinkageTypes> >::operator--()': > /usr/lib/gcc/x86_64-pc-linux-musl/14/include/g++-v14/bits/stl_tree.h:298:(.text._ZNSt8_Rb_treeImSt4pairIKmN4llvm11GlobalValue12LinkageTypesEESt10_Select1stIS5_ESt4lessImESaIS5_EE24_M_get_insert_unique_posERS1_.isra.0+0x93): undefined reference to `std::_Rb_tree_decrement(std::_Rb_tree_node_base*)' Fix by unsetting `LLVM_USE_LIBCXX` during bootstrap, ensuring that the build links against libstdc++ instead of libc++. See the relevant Rust code in `compiler/rustc_llvm/build.rs`. Note: llvm-config may refer to the bundled LLVM if USE="-system-llvm", so we avoid relying on tc-get-cxx-stdlib in this context. Signed-off-by: Z. Liu <zhixu.liu@gmail.com> Part-of: https://github.com/gentoo/gentoo/pull/44129 Closes: https://github.com/gentoo/gentoo/pull/44129 Signed-off-by: Sam James <sam@gentoo.org>