I mean... rust modules aren't typically in your CWD, no? they're not in some node_modules that you can grep, but in a cargo folder with /all of the libraries you ever used/, not just the ones you have for this one project.
not sure how the CSS subsystem is included (I remember it is shared with Servo?), but in general all of the Rust dependencies in mozilla-central is vendored, so nothing is in your ~/.cargo directory.
