fix RSTRUCT_LEN macro in public C API

rb_struct_size returns an Integer VALUE, so it must be converted
to a `long` for compatibility with previous Ruby C API versions.

* ext/-test-/struct/len.c: new
* test/-ext-/struct/test_len.rb: new
* include/ruby/ruby.h (RSTRUCT_LEN): use NUM2LONG
  [ruby-core:80692] [Bug #13439]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58359 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/-test-/struct/len.c

@@ -0,0 +1,13 @@
+#include "ruby.h"
+
+static VALUE
+bug_struct_len(VALUE obj)
+{
+    return LONG2NUM(RSTRUCT_LEN(obj));
+}
+
+void
+Init_len(VALUE klass)
+{
+    rb_define_method(klass, "rstruct_len", bug_struct_len, 0);
+}

include/ruby/ruby.h

@@ -1183,7 +1183,7 @@ void *rb_check_typeddata(VALUE, const rb_data_type_t *);
 #define TypedData_Get_Struct(obj,type,data_type,sval) \
     ((sval) = (type*)rb_check_typeddata((obj), (data_type)))
 
-#define RSTRUCT_LEN(st)         rb_struct_size(st)
+#define RSTRUCT_LEN(st)         NUM2LONG(rb_struct_size(st))
 #define RSTRUCT_PTR(st)         rb_struct_ptr(st)
 #define RSTRUCT_SET(st, idx, v) rb_struct_aset(st, INT2NUM(idx), (v))
 #define RSTRUCT_GET(st, idx)    rb_struct_aref(st, INT2NUM(idx))

test/-ext-/struct/test_len.rb

@@ -0,0 +1,10 @@
+# frozen_string_literal: false
+require 'test/unit'
+require "-test-/struct"
+
+class Bug::Struct::Test_Len < Test::Unit::TestCase
+  def test_rstruct_len
+    klass = Bug::Struct.new(:a, :b, :c)
+    assert_equal 3, klass.new.rstruct_len
+  end
+end