I have to think the approach will be something like "AI summarizes the features of the program into some kind of technical language, then the AI synthesizes Rust code that covers the same feature set".
It would be most interesting if the approach was not to feed the program the original program but rather the manual for the program. That said it's rare that a manual captures all of the nuances of the program so a view into the source code is probably necessary, at least for getting the ground truth.
"AI more or less sort of summarizes the features of the program into some approximate kind of technical language, then the AI synthesizes something not too far from Rust code that hopefully covers aspirationally the same feature set".
Ghidra, which is decompilation software, already manages to produce almost-valid C from assembly, and it does so without AI. I know nothing about how it works, but just from that, I'm guessing that producing almost-valid Rust from C code would be a simpler problem to solve.
In theory, a codebase is a language precisely describing a program. The same program can be described in other languages. So that’s what you’re asking the LLM to do, in the same way you can describe a flower in either English or Spanish.
It would be most interesting if the approach was not to feed the program the original program but rather the manual for the program. That said it's rare that a manual captures all of the nuances of the program so a view into the source code is probably necessary, at least for getting the ground truth.