Add pushtoarray insn to fix segfault with forwarding + splat

Author: rwstauner

bootstraptest/test_method.rb

@@ -1427,3 +1427,10 @@ def test(*, kw: false)
 
   test
 RUBY
+
+assert_equal '[1, 2, 3]', %q{
+  def target(*args) = args
+  def x = [1]
+  def forwarder(...) = target(*x, 2, ...)
+  forwarder(3).inspect
+}, '[Bug #21192] post-splat args before forwarding'

prism_compile.c

@@ -1855,6 +1855,11 @@ pm_setup_args_core(const pm_arguments_node_t *arguments_node, const pm_node_t *b
                             PUSH_INSN(ret, location, concatarray);
                             break;
                           }
+                          case PM_FORWARDING_ARGUMENTS_NODE: {
+                            // foo(*a, b, ...) - push into splat for correct stack layout
+                            PUSH_INSN1(ret, location, pushtoarray, INT2FIX(post_splat_counter));
+                            break;
+                          }
                           default:
                             break;
                         }