c2rust exists today, and pretty much satisfies this. I've used it to convert a few legacy math libraries to unsafe rust, and then been able to do the unsafe->safe refactor in the relative comfort of the full rust toolset (analyser + IDE + tests)

There is real utility in slowly fleshing out the number of transforms in a tool like c2rust that can recognise high-level constructs in C code and produce idiomatic safe equivalents in rust