When GPT4's training data was only up to Sep 2021, I fed it a Rust solution to one of the 2022 AoC problems I had lying around on my PC. I asked it to annotate it with with "formal doc comments".
Now keep in mind that it had never seen this problem, and it was written in a very terse "competition solving" style with zero comments, one-letter identifiers, and generic function names like "day1" and "parse".
It figured it all out. It worked out that it was for solving a maze -- even though the word "maze" was not used anywhere in the code!
It worked out that a constant "(-1,0)" in one isolated bit of code was "orienting the character west", even though to figure that out it had to trace the logic through 4 or 5 layers of indirection. It connected a single 'w' character in the parser to a vector somewhere else!
Etc...
PS: It wrote a more useful, more accurate, more coherent, and more grammatically correct comment than I had seen in any codebase I had worked on in something like two years.