Tail Modulo Cons [pdf] (arxiv.org)

Though this is about OCaml, LLVM also does this optimization. From this Reddit post[1], you can see how this gets optimized into an iterative function:

  size_t my_strlen(const char *s) {
      return (*s == '\0') ? 0 : 1 + my_strlen(s + 1);
  }

[1]: https://www.reddit.com/r/Compilers/comments/187ng5s/how_does...