`bazel coverage` fails for Rust targets: `-u` linker flag incorrectly parsed as filename

[electronjoe]

When using hermetic_cc_toolchain with rules_rust, running bazel coverage on Rust targets fails with:

ld.lld: error: cannot open __llvm_profile_runtime: No such file or directory

This is caused by Zig's clang driver misinterpreting the -u SYMBOL linker flag format.

Environment

• hermetic_cc_toolchain: 4.1.0
• rules_rust: 0.68.1
• Zig: 0.12.0 (bundled with hermetic_cc_toolchain)
• Bazel: 8.x
• OS: Linux x86_64

Steps to Reproduce

Open GitHub codespace from my minimal reproduction repo: https://github.com/anthropics/bazel-cpp-rust-codecov

bazel coverage //rust:greeter_test
<succeeds as patch has been applied>
git rebase to remove single git commit patch
bazel coverage //rust:greeter_test
<fails>: ld.lld: error: cannot open __llvm_profile_runtime: No such file or directory

Expected Behavior

Coverage builds and tests run successfully, generating LCOV coverage data.

Actual Behavior

Build fails during linking with:

error: linking with `external/hermetic_cc_toolchain~~toolchains~zig_config/tools/x86_64-linux-gnu.2.17/c++` failed: exit status: 1
  |
  = note: ld.lld: error: cannot open __llvm_profile_runtime: No such file or directory

Root Cause

When bazel coverage runs, rustc passes coverage instrumentation flags to the linker:

-u __llvm_profile_runtime -fprofile-instr-generate

The -u SYMBOL flag tells the linker to force a symbol to be undefined (ensuring it gets linked in). However, Zig's clang driver incorrectly parses this:

1. It matches -u against the Windows /u CLFlag (undefine macro) instead of the Unix linker -u flag
2. This consumes -u as a standalone flag
3. __llvm_profile_runtime is then treated as a positional argument (input filename)
4. Zig tries to open it as a file → FileNotFound / No such file or directory

This can be verified by testing directly with Zig:

# Fails - Zig misparses -u
echo 'int main(){}' | zig c++ -x c - -u __test_symbol -target x86_64-linux-gnu -o /dev/null
# __test_symbol:1:1: error: FileNotFound

# Works - using -Wl to bypass Zig's argument parsing
echo 'int main(){}' | zig c++ -x c - -Wl,-u,__test_symbol -target x86_64-linux-gnu -o /dev/null
# Success

Upstream Zig Bug

This is a known bug in Zig, tracked at:

• Issue: https://codeberg.org/ziglang/zig/issues/30613
• Fix PR: https://codeberg.org/ziglang/zig/pulls/30749

The fix modifies src/clang_options_data.zig to prevent the Windows /u CLFlag from matching when using a dash prefix (-u).

Workaround

Until the Zig fix is released, the issue can be worked around by patching zig-wrapper.zig to transform -u SYMBOL into -Wl,-u,SYMBOL.

I have posted #242 for consideration with this workaround.

Suggested Resolution

1. Short-term: Consider including this patch in hermetic_cc_toolchain until Zig releases a fix
2. Long-term: Remove the patch once a fixed Zig version is bundled

Related Issues

• Zig issue: https://codeberg.org/ziglang/zig/issues/30613
• Zig fix PR: https://codeberg.org/ziglang/zig/pulls/30749