seriously.c

/*
 * Evaluate the expression used in a ":for var in expr" command.
 * "arg" points to "var".
 * Set "*errp" to TRUE for an error, FALSE otherwise;
 * Return a pointer that holds the info.  Null when there is an error.
 */
void *eval_for_line(char_u *arg, int *errp, char_u **nextcmdp, int skip)
{
  forinfo_T   *fi;
  char_u      *expr;
  typval_T tv;
  list_T      *l;

  *errp = TRUE;         /* default: there is an error */

  fi = (forinfo_T *)alloc_clear(sizeof(forinfo_T));
  if (fi == NULL)
    return NULL;

  expr = skip_var_list(arg, &fi->fi_varcount, &fi->fi_semicolon);
  if (expr == NULL)
    return fi;

  expr = skipwhite(expr);
  if (expr[0] != 'i' || expr[1] != 'n' || !vim_iswhite(expr[2])) {
    EMSG(_("E690: Missing \"in\" after :for"));
    return fi;
  }

  if (skip)
    ++emsg_skip;
  if (eval0(skipwhite(expr + 2), &tv, nextcmdp, !skip) == OK) {
    *errp = FALSE;
    if (!skip) {
      l = tv.vval.v_list;
      if (tv.v_type != VAR_LIST || l == NULL) {
        EMSG(_(e_listreq));
        clear_tv(&tv);
      } else {
        /* No need to increment the refcount, it's already set for the
         * list being used in "tv". */
        fi->fi_list = l;
        list_add_watch(l, &fi->fi_lw);
        fi->fi_lw.lw_item = l->lv_first;
      }
    }
  }
  if (skip)
    --emsg_skip;

  return fi;
}

yesreally.c

  if (expr[0] != 'i' || expr[1] != 'n' || !vim_iswhite(expr[2])) {
    EMSG(_("E690: Missing \"in\" after :for"));
    return fi;
  }