Make attr* methods define public methods if self in caller is not same as receiver

Previously, attr* methods could be private even if not in the
private section of a class/module block.

This uses the same approach that ruby started using for define_method
in 1fc3319973.

Fixes [Bug #4537]

test/ruby/test_module.rb

@@ -705,6 +705,32 @@ class TestModule < Test::Unit::TestCase
     assert_equal(false, o.respond_to?(:bar=))
   end
 
+  def test_attr_public_at_toplevel
+    s = Object.new
+    TOPLEVEL_BINDING.eval(<<-END).call(s.singleton_class)
+      proc do |c|
+        c.send(:attr_accessor, :x)
+        c.send(:attr, :y)
+        c.send(:attr_reader, :z)
+        c.send(:attr_writer, :w)
+      end
+    END
+    assert_nil s.x
+    s.x = 1
+    assert_equal 1, s.x
+
+    assert_nil s.y
+    s.instance_variable_set(:@y, 2)
+    assert_equal 2, s.y
+
+    assert_nil s.z
+    s.instance_variable_set(:@z, 3)
+    assert_equal 3, s.z
+
+    s.w = 4
+    assert_equal 4, s.instance_variable_get(:@w)
+  end
+
   def test_const_get_evaled
     c1 = Class.new
     c2 = Class.new(c1)

vm.c

@@ -1383,7 +1383,7 @@ rb_vm_cref_in_context(VALUE self, VALUE cbase)
     const rb_execution_context_t *ec = GET_EC();
     const rb_control_frame_t *cfp = rb_vm_get_ruby_level_next_cfp(ec, ec->cfp);
     const rb_cref_t *cref;
-    if (cfp->self != self) return NULL;
+    if (!cfp || cfp->self != self) return NULL;
     if (!vm_env_cref_by_cref(cfp->ep)) return NULL;
     cref = vm_get_cref(cfp->ep);
     if (CREF_CLASS(cref) != cbase) return NULL;

vm_method.c

@@ -1135,14 +1135,16 @@ rb_scope_module_func_set(void)
     vm_cref_set_visibility(METHOD_VISI_PRIVATE, TRUE);
 }
 
+const rb_cref_t *rb_vm_cref_in_context(VALUE self, VALUE cbase);
 void
 rb_attr(VALUE klass, ID id, int read, int write, int ex)
 {
     ID attriv;
     rb_method_visibility_t visi;
     const rb_execution_context_t *ec = GET_EC();
+    const rb_cref_t *cref = rb_vm_cref_in_context(klass, klass);
 
-    if (!ex) {
+    if (!ex || !cref) {
 	visi = METHOD_VISI_PUBLIC;
     }
     else {