some types of comments are redundant.

These days I write my code as comments first and then write the code.

But for example this one

        // If either token bucket capacity or refill time is 0, disable limiting.
        if size == 0 || complete_refill_time_ms == 0 {
            return None;
        }

The comment is the same as the code. It is pointless. Instead I might say under what circumstances I expect them to be zero or why Im disabling limiting when they are zero.

This one is great, though at first glance I cant relate the actual code to the comment, at least I understand why the code is there.

            // We still have burst budget for *some* of the tokens requests.
            // The tokens left unfulfilled will be consumed from current `self.budget`.
            tokens -= self.one_time_burst;
            self.one_time_burst = 0;