Running Servo’s unit tests started failing on CI a couple nightlies ago. A customized CI task to add -j1 shows:

https://tools.taskcluster.net/groups/Pl5QK7YnS3eldtQ6EwR09A/tasks/Pl5QK7YnS3eldtQ6EwR09A/runs/0/logs/public%2Flogs%2Flive.log

   Compiling servo_remutex v0.0.1 (/repo/components/remutex)
     Running `sccache rustc --edition=2018 --crate-name smoke components/remutex/tests/smoke.rs --error-format=json --json=diagnostic-rendered-ansi --emit=dep-info,link -C debuginfo=2 --test -C metadata=5076d3288909fc2b -C extra-filename=-5076d3288909fc2b --out-dir /repo/target/debug/deps -L dependency=/repo/target/debug/deps --extern lazy_static=/repo/target/debug/deps/liblazy_static-30d72531331b6d3e.rlib --extern log=/repo/target/debug/deps/liblog-147e2d153a2e1822.rlib --extern servo_remutex=/repo/target/debug/deps/libservo_remutex-8b018814b17ada98.rlib -C link-args=-fuse-ld=gold -W unused-extern-crates`
error: cannot specify the `--color` option with `--json`

error: could not compile `servo_remutex`.

The error message appears to come from here:

Reading through the call sites of this function I quickly reach env::args_os() in rustc_driver::main, so I don’t understand how this message could be emitted while --color is not actually present in the command-line arguments given to rustc by Cargo.

I have also not managed to reproduce this outside of the CI environment.

I don’t have an exact regression range, but this started happening a couple days ago which is close to when #65445 landed. This PR updated Cargo, including rust-lang/cargo#7450 whose description mentions using rustc’s JSON output more. I suspect this is what caused the issue to be uncovered, but it looks like the root cause is in rustc’s argument parsing.