I'm not sure how this would be useful in Rust, but macros and tail calls are what allows one to (for example) write iterative loops in Scheme, which doesn't have a native loop syntax.

Maybe the same idea can be used in Rust where some constructs are easier to write in recursive form instead of a loop?

In any case, here's a silly example of a `for-loop` macro in Scheme using a tail call:

    (define-syntax for-loop
      (syntax-rules ()
        ((for-loop var start end body ...)
         (letrec ((loop (lambda (var)
                          (unless (>= var end)
                            body ...
                            (loop (+ var 1))))))  ; <-- tail call
           (loop start)))))

And here's how you'd use it to print the numbers 0 to 9:

    (for-loop i 0 10
              (display i)
              (newline))

This macro expands to a function that calls itself to loop. Since Scheme is guaranteed to have proper tail calls, the calls are guaranteed to not blow the stack.

(Note that you'll probably never see a `letrec` used like this: people would use a named `let`, which is syntax sugar for that exact usage of `letrec`. I wrote it the `letrec` way to make the function explicit).