There are two compilers that can handle the Linux kernel. GCC and LLVM. Both are written in C, not Rust. It's "in distribution" only if you really stretch the meaning of the term. A generic C compiler isn't going to be anywhere near the level of rigour of this one.
There are several C compilers written in Rust from scratch of comparable quality.
We do not know whether Anthropic has a closed source C compiler written in Rust in their training data. We also do not know whether Anthropic validated their models on their ability to implement C compiler from scratch before releasing this experiment.
That language J I proposed does not have any C compiler implemented in it at all. Idiomatic J expertise is scarce and expensive so that it would be a significant expense for Anthropic to have C compiler in J for their training data. Being Turing-complete, J can express all typical compiler tips and tricks from compiler books, albeit in an unusual way.
