Passing --@rules_rust//:error_format=json triggers a full rebuild

[rdelfin]

Adding and/or removing the flag --@rules_rust//:error_format=json seems to trigger a rebuild of all rust dependencies. You can see an example of it here: https://github.com/rdelfin/min_repro_rules_rust

Simply run:

bazel build //:main

Followed by

bazel build --@rules_rust//:error_format=json //:main

And you'll see the second command rebuild all of tokio anew.

This seems like a bug as the flag should not affect cached artifacts, given it just provides a distinct error output format. This is an issue as this flag is a common way of integrating displaying errors in some IDEs manually or otherwise for scripting error checks.

[dzbarsky]

If you run the build with the --subcommands flag you should see that the flag is passed to the rustc process-wrapper, which is why every compilation is invalidated. I think to make it work the way you'd hope, you would need to collect the compilation output and have a separate build action format it.