I went down a rabbithole and wow.

Found a comment from the author of https://github.com/stclib/STC apparently and then came across this example:

https://stackoverflow.com/a/76887723

  int coro_a(struct a* g)
  {
   cco_routine (g) {
    printf("entering a\n");
    for (g->i = 0; g->i < 3; g->i++) {
     printf("A step %d\n", g->i);
     cco_yield();
    }
    cco_final:
    printf("returning from a\n");
   }
   return 0; // done
  }

gcc -E -ISTC/include co.c

After running it through a preprocessor, it gives me this.

  int coro_a(struct a* g)
   {
    for (int* _state = &(g)->cco_state; *_state != CCO_STATE_DONE; *_state = CCO_STATE_DONE) _resume: switch (*_state) case 0: {
     printf("entering a\n");
     for (g->i = 0; g->i < 3; g->i++) {
      printf("A step %d\n", g->i);
      do { *_state = 14; return CCO_YIELD; goto _resume; case 14:; } while (0);
     }
     *_state = CCO_STATE_FINAL; case CCO_STATE_FINAL:
     printf("returning from a\n");
    }
    return 0;
   }