This assumes that the compiler cannot optimize the calculation of your hashed value, which is not necessarily the case if you are not careful.

Printing the hash of the result is a nice trick I've used a couple of times myself, but jumping through the hoops of making the inputs parametrizeable (a la env vars) is only going to make your benchmarks less reliable, not more. It could be that the request for the env var gets moved past you let start = now() call, and therefore you're now partly benchmarking std::env::get().

Instead, stick with std::hint::black_box for both inputs and outputs. Your benchmark will have no overhead from them, and you're benchmarking exactly what you intend. For low-level benchmarks, use repeats (again with std::hint::black_box on independent loops, or a proper microbenchmarking framework