I think it's due to the lack of quality instructions on what is good Rust code; AI often literally doesn't know what idiomatic Rust is. It can be good to have a reference where you write the basic rules that you want it to follow (ideal to assume it has no idea why spamming clone is bad and you're speaking to someone who has just watched one of those youtube videos with a dude in black t-shirt speaking very slowly and going over basic programming concepts as if they're breaking you out of the matrix).