Forgot Expr and got "cannot borrow as mutable more than once" is lifetime elision really the cause?

Yesterday I decided to switch my project's Strings over to &str. In the process I forgot to make my parser functions return Expr<'a> instead of plain Expr, and started getting "cannot borrow *self as mutable more than once at a time". I dug into it and also asked an AI about it the explanation I got was that this comes down to lifetime elision: when I don't write Expr<'a> explicitly, the compiler treats it as Expr<'s> instead, tied to the &mut self borrow rather than the struct's own lifetime,...