I don’t understand the confusion. It’s all right there, on page 404 of the rust spec:
> Parameters with a recursive lifetime can only be used in a tail-call position within a function. If a function with only recursive params compiles successfully, it is guaranteed not to grow the stack.
For the self-recursive case, people are likely better off just writing the imperative version. But using these with mutual recursive calls lets you build safely build state machines that compile down to the same performant code a classic goto-based implementation would in C.
