Avoid unused unwindlib flag in rustc links

[pdeva]

Summary

• stop forwarding Clang's -unwindlib=none / --unwindlib=none arguments through rustc when rustc disables the linker's default libraries
• preserve both spellings when the effective rustc flags enable default linker libraries
• preserve them conservatively when opaque Args or @response-file contents leave the effective setting unknown
• preserve every other C++ linker-driver argument

Root cause

rules_rust asks Bazel's C++ toolchain for its executable-link command line and forwards those arguments to rustc as -C link-arg. Hermetic Clang toolchains can include an unwind-library selection in that command line.

By default, rustc invokes a compiler driver with -nodefaultlibs and supplies the Rust runtime and unwind library explicitly. Clang therefore cannot act on -unwindlib=none or --unwindlib=none and correctly reports an unused-command-line warning.

Rust 1.97's linker_messages lint surfaces successful-link stderr as a Rust warning, so the redundant toolchain argument became visible across every Rust binary and test link. See rust-lang/rust#136096 for the linker-message behavior.

Conditional behavior

Targets can opt back into compiler-driver defaults with bare -Cdefault-linker-libraries or an affirmative value such as -Cdefault-linker-libraries=yes. In those modes Clang can use the unwind-library selection, so this change preserves it.

Rustc uses the last occurrence of the option. The implementation therefore evaluates statically available flag sources in their real command-line order, including later disabling values such as =no. Opaque Args and @response-file contents are treated as unknown; the unwind selection is preserved unless a later static option establishes that default linker libraries are disabled.

Why remove the arguments conditionally

Adding -Wno-unused-command-line-argument would hide an accurate diagnostic and could conceal other genuinely stale linker-driver options. This change removes only the two accepted spellings of the known-inapplicable unwind option when rustc's effective behavior makes them unusable. All other toolchain arguments remain intact.

Verbose-link inspection confirmed that removing these arguments in the default mode does not change the final LLD inputs: rustc still links the explicit unwind runtime.

Validation

• bazelisk test //test/unit/... — 404 tests passed, 24 skipped
• analysis tests verify:
  • normal rustc links remove both unwind-option spellings while retaining an unrelated linker argument
  • bare and explicit-yes default-linker-library modes retain both spellings
  • a later =no overrides an earlier =yes and removes both spellings
  • response-file flags conservatively retain both spellings
• reproduced rustc 1.97.1 linker invocations directly: the default mode passes -nodefaultlibs, while both the bare and =yes forms omit it
• built //services/oriel/pid-gen:pid-gen-test in the downstream monorepo using this checkout via --override_repository, with unused-command-line warnings explicitly re-enabled; the target built successfully without the warning
• formatting and typo hooks pass; repository-wide buildifier lint reports only warnings already present on main

AI assistance

OpenAI Codex assisted with diagnosis, implementation, and test construction. I reviewed the resulting code and validated it against both the upstream unit suite and the downstream reproducer.

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