Line by line is infeasible, which is precisely why you need to use AI to make larger semantic inferences.

You also don't have to one-shot translate everything. One of the valuable things about the Rust compiler is it gives lots of specific information that you can feed back into an LLM to iterate.

I've been working on similar problems for my startup (grit.io) and think C -> Rust is definitely tractable in the near term. Definitely not easy but certainly solvable.