linker error when building rustc with clang+lld 21 and `-C linker-plugin-lto`

Summary

Originally reported in https://bugs.gentoo.org/964380

Occurs starting from Rust 1.90.0. Everyone facing this error tries to build Rust 1.90.0 from sources, using Rust upstream binary to bootstrap it, on a system where:

• LLVM 21 is the default C toolchain, clang and lld used for building rustc.
• LTO is enabled globally by RUSTFLAGS="${RUSTFLAGS} -C target-cpu=native -C linker-plugin-lto" system-wide.

Such combination used to work just fine for older versions of Rust.

Command used

RUST_BACKTRACE=1 ./x.py build -v --config=bootstrap.toml

Expected behaviour

I would expect the build to succeed.

I'm still trying to understand whether we could make rustc_driver_impl build with -C linker-plugin-lto, or whether it's impossible and we should conditionally strip out the flag for that part of the build.

For now, the issue can be worked around by temporarily setting RUSTFLAGS="${RUSTFLAGS} -C target-cpu=native" just for the moment of building rustc. However, I would like to avoid doing that, if possible. Enforcing LTO flags globally is a standard practice in Gentoo and used to work.

Actual behaviour

     Running `CARGO=/usr/lib/rust/1.90.0/bin/cargo CARGO_BIN_NAME=rustc-main CARGO_CRATE_NAME=rustc_main CARGO_MANIFEST_DIR=/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/compiler/rustc CARGO_MANIFEST_PATH=/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/compiler/rustc/Cargo.toml CARGO_PKG_AUTHORS='' CARGO_PKG_DESCRIPTION='' CARGO_PKG_HOMEPAGE='' CARGO_PKG_LICENSE='' CARGO_PKG_LICENSE_FILE='' CARGO_PKG_NAME=rustc-main CARGO_PKG_README='' CARGO_PKG_REPOSITORY='' CARGO_PKG_RUST_VERSION='' CARGO_PKG_VERSION=0.0.0 CARGO_PKG_VERSION_MAJOR=0 CARGO_PKG_VERSION_MINOR=0 CARGO_PKG_VERSION_PATCH=0 CARGO_PKG_VERSION_PRE='' CARGO_PRIMARY_PACKAGE=1 CARGO_SBOM_PATH='' LD_LIBRARY_PATH=/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/release/deps OUT_DIR=/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/x86_64-unknown-linux-musl/release/build/rustc-main-630efcf6209d93df/out /var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/bootstrap/debug/rustc /var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/bootstrap/debug/rustc --crate-name rustc_main --edition=2024 compiler/rustc/src/main.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --crate-type bin --emit=dep-info,link -C opt-level=3 -C embed-bitcode=no --cfg 'feature="llvm"' --cfg 'feature="max_level_info"' --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values("check_only", "jemalloc", "llvm", "max_level_info", "rustc_randomized_layouts"))' -C metadata=7a6468985b0b531f -C extra-filename=-bbfd9d6158d1a34d --out-dir /var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/x86_64-unknown-linux-musl/release/deps --target x86_64-unknown-linux-musl -C linker=clang-21 -L dependency=/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/x86_64-unknown-linux-musl/release/deps -L dependency=/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/release/deps --extern rustc_codegen_ssa=/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/x86_64-unknown-linux-musl/release/deps/librustc_codegen_ssa-13d457041e1f2354.rlib --extern rustc_driver=/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/x86_64-unknown-linux-musl/release/deps/librustc_driver-d2380b03692a4f0a.so --extern rustc_driver_impl=/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/x86_64-unknown-linux-musl/release/deps/librustc_driver_impl-254b9ca3ac44ae12.rlib --extern rustc_public=/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/x86_64-unknown-linux-musl/release/deps/librustc_public-57fa484b4e3c29ff.rlib --extern rustc_public_bridge=/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/x86_64-unknown-linux-musl/release/deps/librustc_public_bridge-7d273d7c58e4934f.rlib -C target-cpu=native -C linker-plugin-lto -Lnative=/usr/lib/llvm/20/lib -C target-feature=-crt-static --cfg=windows_raw_dylib -Csymbol-mangling-version=v0 -Zunstable-options '--check-cfg=cfg(bootstrap)' '--check-cfg=cfg(llvm_enzyme)' -Zmacro-backtrace -Csplit-debuginfo=off -Ctarget-feature=-crt-static -Clink-args=-Wl,-z,origin '-Clink-args=-Wl,-rpath,$ORIGIN/../lib' -Clinker-features=+lld -Zunstable-options -Alinker-messages -Zon-broken-pipe=kill -Zdefault-visibility=protected -Zdylib-lto -Clto=thin -Cembed-bitcode=yes -Clink-args=-Wl,--icf=all -Z binary-dep-depinfo -L native=/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/x86_64-unknown-linux-musl/release/build/psm-4c52a2a752ad411a/out -L native=/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/x86_64-unknown-linux-musl/release/build/blake3-4b710a6cade225a4/out -L native=/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/x86_64-unknown-linux-musl/release/build/blake3-4b710a6cade225a4/out -L native=/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/x86_64-unknown-linux-musl/release/build/rustc_llvm-53d79c3d485dbcab/out -L native=/usr/lib/llvm/20/lib`
error: linking with `clang-21` failed: exit status: 1
  |
  = note:  "clang-21" "-m64" "/var/tmp/portage/dev-lang/rust-1.90.0-r1/temp/rustccUYUa9/symbols.o" "<1 object files omitted>" "-Wl,--as-needed" "-Wl,-Bdynamic" "<sysroot>-rustc/x86_64-unknown-linux-musl/release/deps/librustc_driver-d2380b03692a4f0a.so" "-Wl,-Bstatic" "<sysroot>/lib/rustlib/x86_64-unknown-linux-musl/lib/libcompiler_builtins-*.rlib" "-Wl,-Bdynamic" "-lLLVM-20+libcxx" "-lc++" "-ldl" "-lunwind" "-lc" "-L" "/var/tmp/portage/dev-lang/rust-1.90.0-r1/temp/rustccUYUa9/raw-dylibs" "-fuse-ld=lld" "-Wl,--eh-frame-hdr" "-Wl,-z,noexecstack" "-Wl,-plugin-opt=O3,-plugin-opt=mcpu=znver3" "-L" "/usr/lib/llvm/20/lib" "-L" "<sysroot>-rustc/x86_64-unknown-linux-musl/release/build/psm-4c52a2a752ad411a/out" "-L" "<sysroot>-rustc/x86_64-unknown-linux-musl/release/build/blake3-4b710a6cade225a4/out" "-L" "<sysroot>-rustc/x86_64-unknown-linux-musl/release/build/blake3-4b710a6cade225a4/out" "-L" "<sysroot>-rustc/x86_64-unknown-linux-musl/release/build/rustc_llvm-53d79c3d485dbcab/out" "-L" "/usr/lib/llvm/20/lib" "-L" "<sysroot>/lib/rustlib/x86_64-unknown-linux-musl/lib" "-o" "<sysroot>-rustc/x86_64-unknown-linux-musl/release/deps/rustc_main-bbfd9d6158d1a34d" "-Wl,--gc-sections" "-pie" "-Wl,-z,relro,-z,now" "-Wl,-O1" "-nodefaultlibs" "-Wl,-z,origin" "-Wl,-rpath,$ORIGIN/../lib" "-Wl,--icf=all"
  = note: some arguments are omitted. use `--verbose` to show all linker arguments
  = note: ld.lld: error: undefined symbol: rustc_driver_impl::main
          >>> referenced by rustc_main.e7d35cd2037fc768-cgu.0
          >>>               /var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/x86_64-unknown-linux-musl/release/deps/rustc_main-bbfd9d6158d1a34d.lto.rustc_main-bbfd9d6158d1a34d.rustc_main.e7d35cd2037fc768-cgu.0.rcgu.o:(rustc_main::main)
          clang-21: error: linker command failed with exit code 1 (use -v to see invocation)



Did not run successfully: exit status: 1
LD_LIBRARY_PATH="/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1/lib:/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/release/deps" "/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1/bin/rustc" "--crate-name" "rustc_main" "--edition=2024" "compiler/rustc/src/main.rs" "--error-format=json" "--json=diagnostic-rendered-ansi,artifacts,future-incompat" "--crate-type" "bin" "--emit=dep-info,link" "-C" "opt-level=3" "-C" "embed-bitcode=no" "--cfg" "feature=\"llvm\"" "--cfg" "feature=\"max_level_info\"" "--check-cfg" "cfg(docsrs,test)" "--check-cfg" "cfg(feature, values(\"check_only\", \"jemalloc\", \"llvm\", \"max_level_info\", \"rustc_randomized_layouts\"))" "-C" "metadata=7a6468985b0b531f" "-C" "extra-filename=-bbfd9d6158d1a34d" "--out-dir" "/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/x86_64-unknown-linux-musl/release/deps" "--target" "x86_64-unknown-linux-musl" "-C" "linker=clang-21" "-L" "dependency=/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/x86_64-unknown-linux-musl/release/deps" "-L" "dependency=/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/release/deps" "--extern" "rustc_codegen_ssa=/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/x86_64-unknown-linux-musl/release/deps/librustc_codegen_ssa-13d457041e1f2354.rlib" "--extern" "rustc_driver=/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/x86_64-unknown-linux-musl/release/deps/librustc_driver-d2380b03692a4f0a.so" "--extern" "rustc_driver_impl=/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/x86_64-unknown-linux-musl/release/deps/librustc_driver_impl-254b9ca3ac44ae12.rlib" "--extern" "rustc_public=/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/x86_64-unknown-linux-musl/release/deps/librustc_public-57fa484b4e3c29ff.rlib" "--extern" "rustc_public_bridge=/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/x86_64-unknown-linux-musl/release/deps/librustc_public_bridge-7d273d7c58e4934f.rlib" "-C" "target-cpu=native" "-C" "linker-plugin-lto" "-Lnative=/usr/lib/llvm/20/lib" "-C" "target-feature=-crt-static" "--cfg=windows_raw_dylib" "-Csymbol-mangling-version=v0" "-Zunstable-options" "--check-cfg=cfg(bootstrap)" "--check-cfg=cfg(llvm_enzyme)" "-Zmacro-backtrace" "-Csplit-debuginfo=off" "-Ctarget-feature=-crt-static" "-Clink-args=-Wl,-z,origin" "-Clink-args=-Wl,-rpath,$ORIGIN/../lib" "-Clinker-features=+lld" "-Zunstable-options" "-Alinker-messages" "-Zon-broken-pipe=kill" "-Zdefault-visibility=protected" "-Zdylib-lto" "-Clto=thin" "-Cembed-bitcode=yes" "-Clink-args=-Wl,--icf=all" "-Z" "binary-dep-depinfo" "-L" "native=/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/x86_64-unknown-linux-musl/release/build/psm-4c52a2a752ad411a/out" "-L" "native=/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/x86_64-unknown-linux-musl/release/build/blake3-4b710a6cade225a4/out" "-L" "native=/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/x86_64-unknown-linux-musl/release/build/blake3-4b710a6cade225a4/out" "-L" "native=/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1-rustc/x86_64-unknown-linux-musl/release/build/rustc_llvm-53d79c3d485dbcab/out" "-L" "native=/usr/lib/llvm/20/lib" "-Wrust_2018_idioms" "-Wunused_lifetimes" "-Dwarnings" "-Wrustc::internal" "-Drustc::symbol_intern_string_literal" "-Wkeyword_idents_2024" "-Wunreachable_pub" "-Wunsafe_op_in_unsafe_fn" "-Wunused_crate_dependencies" "--sysroot" "/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/build/x86_64-unknown-linux-musl/stage1" "-Ztls-model=initial-exec" "--remap-path-prefix" "/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src=/rustc-dev/1159e78c4747b02ef996e55082b704c09b970588" "--remap-path-prefix" "/var/tmp/portage/dev-lang/rust-1.90.0-r1/work/rustc-1.90.0-src/vendor=/rust/deps" "-Z" "force-unstable-if-unmarked"

Full log attached at the end.

Bootstrap configuration (bootstrap.toml)

change-id = "ignore"
profile = "dist"
[llvm]
download-ci-llvm = false
optimize = true
release-debuginfo = false
assertions = false
ninja = true
targets = "ARM;BPF;RISCV;WebAssembly;X86"
experimental-targets = ""
link-shared = true
use-libcxx = true
static-libstdcpp = false

enable-warnings = false
[llvm.build-config]
CMAKE_VERBOSE_MAKEFILE = "ON"
CMAKE_C_FLAGS_RELEASE = "-O2 -pipe -march=native -Werror=odr -Werror=strict-aliasing"
CMAKE_CXX_FLAGS_RELEASE = "-O2 -pipe -march=native -Werror=odr -Werror=strict-aliasing"
CMAKE_EXE_LINKER_FLAGS_RELEASE = "-Wl,-O1 -Wl,--as-needed -Wl,-z,pack-relative-relocs -Wl,--as-needed"
CMAKE_MODULE_LINKER_FLAGS_RELEASE = "-Wl,-O1 -Wl,--as-needed -Wl,-z,pack-relative-relocs -Wl,--as-needed"
CMAKE_SHARED_LINKER_FLAGS_RELEASE = "-Wl,-O1 -Wl,--as-needed -Wl,-z,pack-relative-relocs -Wl,--as-needed"
CMAKE_STATIC_LINKER_FLAGS_RELEASE = ""
[build]
build-stage = 2
test-stage = 2
build = "x86_64-unknown-linux-musl"
host = ["x86_64-unknown-linux-musl"]
target = ["x86_64-unknown-linux-musl","bpfeb-unknown-none","bpfel-unknown-none","wasm32-unknown-unknown"]
cargo = "/usr/lib/rust/1.90.0/bin/cargo"
rustc = "/usr/lib/rust/1.90.0/bin/rustc"
rustfmt = "/usr/lib/rust/1.90.0/bin/rustfmt"
description = "gentoo"
docs = false
compiler-docs = false
submodules = false
python = "python3.14"
locked-deps = true
vendor = true
extended = true
tools = ["cargo","rustdoc","clippy","rustfmt","src"]
verbose = 2
sanitizers = false
profiler = true
cargo-native-static = false
[install]
prefix = "/usr/lib/rust/1.90.0"
sysconfdir = "etc"
docdir = "share/doc/rust"
bindir = "bin"
libdir = "lib"
mandir = "share/man"
[rust]
# https://github.com/rust-lang/rust/issues/54872
codegen-units-std = 1
optimize = true
debug = false
debug-assertions = false
debug-assertions-std = false
debuginfo-level = 0
debuginfo-level-rustc = 0
debuginfo-level-std = 0
debuginfo-level-tools = 0
debuginfo-level-tests = 0
backtrace = true
incremental = false
default-linker = "x86_64-pc-linux-musl-cc"
channel = "stable"
rpath = true
verbose-tests = true
optimize-tests = true
codegen-tests = true
omit-git-hash = false
dist-src = false
remap-debuginfo = true
lld = false
use-lld = true
# only deny warnings if doc+wasm are NOT requested, documenting stage0 wasm std fails without it
# https://github.com/rust-lang/rust/issues/74976
# https://github.com/rust-lang/rust/issues/76526
deny-warnings = true
backtrace-on-ice = true
jemalloc = false
# See https://github.com/rust-lang/rust/issues/121124
lto = "thin"
[dist]
src-tarball = false
compression-formats = ["xz"]
compression-profile = "balanced"
[target.x86_64-unknown-linux-musl]
ar = "/usr/lib/llvm/21/bin/llvm-ar"
cc = "clang-21"
cxx = "clang++-21"
linker = "clang-21"
ranlib = "/usr/lib/llvm/21/bin/llvm-ranlib"
llvm-libunwind = "system"
llvm-config = "/usr/lib/llvm/20/bin/llvm-config"
crt-static = false
musl-root = "/usr"
[target.wasm32-unknown-unknown]
linker = "lld"
# wasm target does not have profiler_builtins https://bugs.gentoo.org/848483
profiler = false

Operating system

Gentoo Linux, musl-llvm profile

HEAD

starting from 1.90.0. 1159e78c4747b02ef996e55082b704c09b970588

Additional context

I applied https://github.com/rust-lang/rust/pull/144997/commits/351e4bd1066a9d89b4d9f06c628ba0733c1850b5 as a patch to be able to bootstrap 1.90.0 with 1.90.0

Build Log

rust-lto.log

Start by reproducing the failure with x.py and bootstrap.toml using clang/lld 21 and global -C linker-plugin-lto. Inspect compiler/rustc/src/main.rs and the rustc_driver_impl build path to determine where the undefined rustc_driver_impl::main symbol is introduced. Done means the documented configuration builds successfully, or the affected build step conditionally handles the flag with coverage for the chosen behavior.