SIGILL when compiling `cc` with `-C instrument-coverage` `-Z coverage-options=mcdc`

[esarver]

As part of my project, I tried to compile cc (1.1.16) with -C instrument-coverage (enabled using cargo-llvm-cov). This was run on GitHub Actions Runners. These tools seem to work without issue on Desktop Windows (have not tried macOS or Linux).

I expected to see this happen: cc successfully compiled with -C instrument-coverage enabled

Instead, this happened: I get a SIGILL error under Linux, [SIGTRAP](signal: 5, SIGTRAP: trace/breakpoint trap) under macOS, and STATUS_ILLEGAL_INSTRUCTION on Windows.

Meta

rustc --version --verbose:

Linux

rustc 1.88.0-nightly (13e879094 2025-05-04)
binary: rustc
commit-hash: 13e87909494f2f31bce51bc106e2b629420d1235
commit-date: 2025-05-04
host: x86_64-unknown-linux-gnu
release: 1.88.0-nightly
LLVM version: 20.1.4

Windows

rustc 1.88.0-nightly (13e879094 2025-05-04)
binary: rustc
commit-hash: 13e87909494f2f31bce51bc106e2b629420d1235
commit-date: 2025-05-04
host: x86_64-pc-windows-msvc
release: 1.88.0-nightly
LLVM version: 20.1.4

macOS

rustc 1.88.0-nightly (13e879094 2025-05-04)
binary: rustc
commit-hash: 13e87909494f2f31bce51bc106e2b629420d1235
commit-date: 2025-05-04
host: aarch64-apple-darwin
release: 1.88.0-nightly
LLVM version: 20.1.4

Backtrace Linux

cargo llvm-cov nextest --cobertura --mcdc > "test-results/REDACTED.cobertura.xml"
warning: --mcdc option is unstable
info: cargo-llvm-cov currently setting cfg(coverage) and cfg(coverage_nightly); you can opt-out it by passing --no-cfg-coverage and --no-cfg-coverage-nightly

[...]

   Compiling cc v1.1.16
     Running `/usr/local/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc --crate-name cc --edition=2018 /__w/REDACTED/REDACTED/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/cc-1.1.16/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no --cfg 'feature="parallel"' --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values("jobserver", "parallel"))' -C metadata=8996d7acb1b451e7 -C extra-filename=-f8adae57810bd71c --out-dir /__w/REDACTED/REDACTED/target/llvm-cov-target/debug/deps -L dependency=/__w/REDACTED/REDACTED/target/llvm-cov-target/debug/deps --extern jobserver=/__w/REDACTED/REDACTED/target/llvm-cov-target/debug/deps/libjobserver-88b92c2e8f8204fb.rmeta --extern libc=/__w/REDACTED/REDACTED/target/llvm-cov-target/debug/deps/liblibc-03b59914ade46f2d.rmeta --extern shlex=/__w/REDACTED/REDACTED/target/llvm-cov-target/debug/deps/libshlex-be5013b75a6c003a.rmeta --cap-lints allow -Cinstrument-coverage -L ./ -C instrument-coverage -Z coverage-options=mcdc --cfg=coverage --cfg=coverage_nightly --cfg=trybuild_no_target`
error: rustc interrupted by SIGILL, printing backtrace

/usr/local/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-0f4a9aaf6ae6cf24.so(+0x398101f)[0x7fc5ae58101f]
/lib/x86_64-linux-gnu/libc.so.6(+0x3c050)[0x7fc5aaa5b050]
/usr/local/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/libLLVM.so.20.1-rust-1.88.0-nightly(+0x8fa70bd)[0x7fc5a9da70bd]
/usr/local/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/libLLVM.so.20.1-rust-1.88.0-nightly(_ZN4llvm19SelectionDAGBuilder5visitERKNS_11InstructionE+0x1eb)[0x7fc5a8d8e92b]
/usr/local/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/libLLVM.so.20.1-rust-1.88.0-nightly(_ZN4llvm16SelectionDAGISel20SelectAllBasicBlocksERKNS_8FunctionE+0x13b7)[0x7fc5a8d909b9]
/usr/local/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/libLLVM.so.20.1-rust-1.88.0-nightly(_ZN4llvm16SelectionDAGISel20runOnMachineFunctionERNS_15MachineFunctionE+0xc1)[0x7fc5a8dad55f]
/usr/local/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/libLLVM.so.20.1-rust-1.88.0-nightly(_ZN4llvm22SelectionDAGISelLegacy20runOnMachineFunctionERNS_15MachineFunctionE+0x99)[0x7fc5a8dad3d9]
/usr/local/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/libLLVM.so.20.1-rust-1.88.0-nightly(_ZN4llvm13FPPassManager13runOnFunctionERNS_8FunctionE+0xb05)[0x7fc5a8c28291]
/usr/local/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/libLLVM.so.20.1-rust-1.88.0-nightly(_ZN4llvm13FPPassManager11runOnModuleERNS_6ModuleE+0x2f)[0x7fc5a8c276ef]
/usr/local/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/libLLVM.so.20.1-rust-1.88.0-nightly(_ZN4llvm6legacy15PassManagerImpl3runERNS_6ModuleE+0x26f)[0x7fc5a9267383]
/usr/local/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-0f4a9aaf6ae6cf24.so(+0x6008ac2)[0x7fc5b0c08ac2]
/usr/local/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-0f4a9aaf6ae6cf24.so(+0x6008621)[0x7fc5b0c08621]
/usr/local/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-0f4a9aaf6ae6cf24.so(+0x6005be6)[0x7fc5b0c05be6]
/usr/local/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-0f4a9aaf6ae6cf24.so(+0x60058b4)[0x7fc5b0c058b4]
/usr/local/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-0f4a9aaf6ae6cf24.so(_RINvNtNtCs1FpB9HpxKJ1_3std3sys9backtrace28___rust_begin_short_backtraceNCINvXs0_CshDqY5VxCEGQ_18rustc_codegen_llvmNtB1g_18LlvmCodegenBackendNtNtNtCs34iPl0uoQEf_17rustc_codegen_ssa6traits7backend19ExtraBackendMethods18spawn_named_threadNCINvNtNtB2k_4back5write10spawn_workB1O_E0uE0uEB1g_+0x251)[0x7fc5b0c04011]
/usr/local/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-0f4a9aaf6ae6cf24.so(+0x609669c)[0x7fc5b0c9669c]
/usr/local/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-0f4a9aaf6ae6cf24.so(+0x6096a2b)[0x7fc5b0c96a2b]
/lib/x86_64-linux-gnu/libc.so.6(+0x891f5)[0x7fc5aaaa81f5]
/lib/x86_64-linux-gnu/libc.so.6(__clone+0x40)[0x7fc5aab27b00]

note: we would appreciate a report at https://github.com/rust-lang/rust
error: could not compile `cc` (lib)

Caused by:
  process didn't exit successfully: `/usr/local/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc --crate-name cc --edition=2018 /__w/REDACTED/REDACTED/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/cc-1.1.16/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no --cfg 'feature="parallel"' --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values("jobserver", "parallel"))' -C metadata=8996d7acb1b451e7 -C extra-filename=-f8adae57810bd71c --out-dir /__w/REDACTED/REDACTED/target/llvm-cov-target/debug/deps -L dependency=/__w/REDACTED/REDACTED/target/llvm-cov-target/debug/deps --extern jobserver=/__w/REDACTED/REDACTED/target/llvm-cov-target/debug/deps/libjobserver-88b92c2e8f8204fb.rmeta --extern libc=/__w/REDACTED/REDACTED/target/llvm-cov-target/debug/deps/liblibc-03b59914ade46f2d.rmeta --extern shlex=/__w/REDACTED/REDACTED/target/llvm-cov-target/debug/deps/libshlex-be5013b75a6c003a.rmeta --cap-lints allow -Cinstrument-coverage -L ./ -C instrument-coverage -Z coverage-options=mcdc --cfg=coverage --cfg=coverage_nightly --cfg=trybuild_no_target` (signal: 4, SIGILL: illegal instruction)
warning: build failed, waiting for other jobs to finish...
error: command `/usr/local/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/cargo test --no-run --message-format json-render-diagnostics --target-dir /__w/REDACTED/REDACTED/target/llvm-cov-target` exited with code 101
error: process didn't exit successfully: `/usr/local/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/cargo nextest run --manifest-path /__w/REDACTED/REDACTED/Cargo.toml --target-dir /__w/REDACTED/REDACTED/target/llvm-cov-target` (exit status: 101)

Backtrace Windows

cargo llvm-cov nextest --cobertura --mcdc > "test-results/REDACTED.cobertura.xml"
warning: --mcdc option is unstable
info: cargo-llvm-cov currently setting cfg(coverage) and cfg(coverage_nightly); you can opt-out it by passing --no-cfg-coverage and --no-cfg-coverage-nightly

[...]

   Compiling cc v1.1.16
     Running `C:\Users\runneradmin\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\bin\rustc.exe --crate-name cc --edition=2018 D:\a\tsp-toolkit-script-gen\tsp-toolkit-script-gen\.cargo\registry\src\index.crates.io-1949cf8c6b5b557f\cc-1.1.16\src\lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no --cfg "feature=\"parallel\"" --check-cfg cfg(docsrs,test) --check-cfg "cfg(feature, values(\"jobserver\", \"parallel\"))" -C metadata=2dce8f0fbf27253e -C extra-filename=-991b9d71cf2ca524 --out-dir D:\a\tsp-toolkit-script-gen\tsp-toolkit-script-gen\target\llvm-cov-target\debug\deps -L dependency=D:\a\tsp-toolkit-script-gen\tsp-toolkit-script-gen\target\llvm-cov-target\debug\deps --extern jobserver=D:\a\tsp-toolkit-script-gen\tsp-toolkit-script-gen\target\llvm-cov-target\debug\deps\libjobserver-771c36a572dff3c3.rmeta --extern shlex=D:\a\tsp-toolkit-script-gen\tsp-toolkit-script-gen\target\llvm-cov-target\debug\deps\libshlex-34bee54341b4da13.rmeta --cap-lints allow -C instrument-coverage -Z coverage-options=mcdc --cfg=coverage --cfg=coverage_nightly --cfg=trybuild_no_target`
error: could not compile `cc` (lib)

Caused by:
  process didn't exit successfully: `C:\Users\runneradmin\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\bin\rustc.exe --crate-name cc --edition=2018 D:\a\tsp-toolkit-script-gen\tsp-toolkit-script-gen\.cargo\registry\src\index.crates.io-1949cf8c6b5b557f\cc-1.1.16\src\lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no --cfg "feature=\"parallel\"" --check-cfg cfg(docsrs,test) --check-cfg "cfg(feature, values(\"jobserver\", \"parallel\"))" -C metadata=2dce8f0fbf27253e -C extra-filename=-991b9d71cf2ca524 --out-dir D:\a\tsp-toolkit-script-gen\tsp-toolkit-script-gen\target\llvm-cov-target\debug\deps -L dependency=D:\a\tsp-toolkit-script-gen\tsp-toolkit-script-gen\target\llvm-cov-target\debug\deps --extern jobserver=D:\a\tsp-toolkit-script-gen\tsp-toolkit-script-gen\target\llvm-cov-target\debug\deps\libjobserver-771c36a572dff3c3.rmeta --extern shlex=D:\a\tsp-toolkit-script-gen\tsp-toolkit-script-gen\target\llvm-cov-target\debug\deps\libshlex-34bee54341b4da13.rmeta --cap-lints allow -C instrument-coverage -Z coverage-options=mcdc --cfg=coverage --cfg=coverage_nightly --cfg=trybuild_no_target` (exit code: 0xc000001d, STATUS_ILLEGAL_INSTRUCTION)
warning: build failed, waiting for other jobs to finish...
error: command `'C:\Users\runneradmin\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\bin\cargo.exe' test --no-run --message-format json-render-diagnostics --target-dir 'D:\a\tsp-toolkit-script-gen\tsp-toolkit-script-gen\target\llvm-cov-target'` exited with code 101
error: process didn't exit successfully: `C:\Users\runneradmin\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\bin\cargo.exe nextest run --manifest-path D:\a\tsp-toolkit-script-gen\tsp-toolkit-script-gen\Cargo.toml --target-dir D:\a\tsp-toolkit-script-gen\tsp-toolkit-script-gen\target\llvm-cov-target` (exit code: 101)

Backtrace macOS

cargo llvm-cov nextest --cobertura --mcdc > "test-results/REDACTED.cobertura.xml"
warning: --mcdc option is unstable
info: cargo-llvm-cov currently setting cfg(coverage) and cfg(coverage_nightly); you can opt-out it by passing --no-cfg-coverage and --no-cfg-coverage-nightly

[...]

   Compiling cc v1.1.16
     Running `/Users/runner/.rustup/toolchains/nightly-aarch64-apple-darwin/bin/rustc --crate-name cc --edition=2018 /Users/runner/work/REDACTED/REDACTED/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/cc-1.1.16/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no --cfg 'feature="parallel"' --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values("jobserver", "parallel"))' -C metadata=c3b8ebb0918e49dc -C extra-filename=-969d5a180171e87e --out-dir /Users/runner/work/REDACTED/REDACTED/target/llvm-cov-target/debug/deps -L dependency=/Users/runner/work/REDACTED/REDACTED/target/llvm-cov-target/debug/deps --extern jobserver=/Users/runner/work/REDACTED/REDACTED/target/llvm-cov-target/debug/deps/libjobserver-dfe5801ba8ebe052.rmeta --extern libc=/Users/runner/work/REDACTED/REDACTED/target/llvm-cov-target/debug/deps/liblibc-12708f473cc4487d.rmeta --extern shlex=/Users/runner/work/REDACTED/REDACTED/target/llvm-cov-target/debug/deps/libshlex-282d31caf026f140.rmeta --cap-lints allow -C instrument-coverage -Z coverage-options=mcdc --cfg=coverage --cfg=coverage_nightly --cfg=trybuild_no_target`

error: could not compile `cc` (lib)

Caused by:
  process didn't exit successfully: `/Users/runner/.rustup/toolchains/nightly-aarch64-apple-darwin/bin/rustc --crate-name cc --edition=2018 /Users/runner/work/REDACTED/REDACTED/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/cc-1.1.16/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no --cfg 'feature="parallel"' --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values("jobserver", "parallel"))' -C metadata=c3b8ebb0918e49dc -C extra-filename=-969d5a180171e87e --out-dir /Users/runner/work/REDACTED/REDACTED/target/llvm-cov-target/debug/deps -L dependency=/Users/runner/work/REDACTED/REDACTED/target/llvm-cov-target/debug/deps --extern jobserver=/Users/runner/work/REDACTED/REDACTED/target/llvm-cov-target/debug/deps/libjobserver-dfe5801ba8ebe052.rmeta --extern libc=/Users/runner/work/REDACTED/REDACTED/target/llvm-cov-target/debug/deps/liblibc-12708f473cc4487d.rmeta --extern shlex=/Users/runner/work/REDACTED/REDACTED/target/llvm-cov-target/debug/deps/libshlex-282d31caf026f140.rmeta --cap-lints allow -C instrument-coverage -Z coverage-options=mcdc --cfg=coverage --cfg=coverage_nightly --cfg=trybuild_no_target` (signal: 5, SIGTRAP: trace/breakpoint trap)
warning: build failed, waiting for other jobs to finish...
error: command `/Users/runner/.rustup/toolchains/nightly-aarch64-apple-darwin/bin/cargo test --no-run --message-format json-render-diagnostics --target-dir /Users/runner/work/REDACTED/REDACTED/target/llvm-cov-target` exited with code 101
error: process didn't exit successfully: `/Users/runner/.rustup/toolchains/nightly-aarch64-apple-darwin/bin/cargo nextest run --manifest-path /Users/runner/work/REDACTED/REDACTED/Cargo.toml --target-dir /Users/runner/work/REDACTED/REDACTED/target/llvm-cov-target` (exit status: 101)

[jieyouxu]

Triage notes: -Z coverage-options=mcdc

[esarver]

@jieyouxu Thanks for pointing that out. I changed to using -Z coverage-options=branch and that seems to resolve the problem in my build.

[jieyouxu]

I changed to using -Z coverage-options=branch and that seems to resolve the problem in my build.

Keeping this open, because if someone is able to, a reproducer for the -Z coverage-options=mcdc case would still be very helpful (most likely a genuine bug).

[jieyouxu]

AFAIK the MCDC impl is still quite immature, backlinking to tracking issue #124144

[Lambdaris]

It might be same as #129989. I also encountered such errors with cc. Change rustc version might escape it.