To the best of my knowledge, there's no Rust-based compiler that comes anywhere close to 99% on the GCC torture test suite, or able to compile Doom. So even if it saw the internals of GCC and a lot of other compilers, the ability to recreate this step-by-step in Rust is extremely impressive to me.

The impressiveness of converting C to Rust by any means is kind of contingent on how much unnecessary unsafe there is in the end result though.

None - all references to 'unsafe' are in comments about the codegen: https://github.com/search?q=repo%3Aanthropics%2Fclaudes-c-co...

I think the careful response to this is:

(1) There are compilers written in C in the training set

(2) LLMs demonstrably can near-perfectly memorize training-set inputs (see other comments here)

(3) LLMs are very good at translation tasks (natural language or code, e.g.: C to Rust)

I don't think this necessarily completely deflates the impressiveness of this accomplishment, but it does qualify it to some degree.