This topic is really tricky, and there's a lot of different ways to slice it. In the most general sense, if you take a C expert, and a Rust expert, and ask them to do something, they should be able to get the exact same results.

But that leaves out other interesting questions that inform this comparison. For example:

* What about the average Rust programmer vs the average C programmer? Maybe two experts can produce identical results, but does one language vs the other make it so that a non-expert tends to produce faster code?

* What about "no unsafe Rust other than what's in the implementation of the standard library"? Most Rust code doesn't use unsafe, so is it fair to let the Rust use it? How much?

* What about "no code other than the standard libraries." I may not be an expert, but Rust's package ecosystem makes it easier to use code that's developed by them. This is sort of a slightly different take on the two previous questions at the same time.

* Should there be a time limit on development? If so, should the C have the same requirements of demonstrating safety as the Rust, that is, sketching out an informal proof of correctness?

* What about some sort of "how maintainable is this" metric. Mozilla tried to implement parallel CSS in C++ twice, and couldn't do it, because the concurrency bugs were just too many. Their Rust attempt succeeded, thanks to the safety guidelines. Is this comparison valid? Or could some sort of person who was better at C++ have implemented it in theory with no bugs? (this is sort of question 1, but slightly different.)

Lots and lots of ways to do this.