This is still a meaningless question, because different languages are good for different things.
Are you as productive in rust when doing web front-end development as in javascript? Are you as productive when doing one-of scripts as in perl? Are you as productive when doing exploratory data analysis as in python? Are you as productive when doing database queries as in SQL?
It's not meaningless; it's just not very meaningful. You can doctor the phrasing to filter out the specific issues you raised (hence, "or similar"). More importantly though, it's never going to be a perfect metric because there is some inherent subjectivity; however, it's still far more meaningful than the current question. There is a difference between "meaningless" and "not 100% meaningful".
This is obviously a subjective question.
You will feel productive once you have a sufficient grasp on the language and the parts of the ecosystem you need.
Basically no-one will have Rust as a first language so there will be a point of comparison to other languages too.
Are you as productive in rust when doing web front-end development as in javascript? Are you as productive when doing one-of scripts as in perl? Are you as productive when doing exploratory data analysis as in python? Are you as productive when doing database queries as in SQL?