Everything about lifetimes
Learning Rust is... an experience. An emotional journey. I've rarely been more frustrated than in my first few months of trying to learn Rust.
As I've said before, I'm working on a book about lifetimes. Or maybe it's just a long series - I haven't decided the specifics yet. Like every one of my series/book things, it's long, and it starts you off way in the periphery of the subject, and takes a lot of detours to get there.