Add Nokogiri::XML::Node#line=

Add support for setting the line number on elements from Ruby. This only
works when Nokogiri uses `libxml2`.

Line numbers less than 65535 may be set. Larger line numbers require
`libxml2` to be compiled with a particular option and even then, it is
buggy. See https://bugzilla.gnome.org/show_bug.cgi?id=767138.

ext/nokogiri/xml_node.c

@@ -1332,6 +1332,25 @@ static VALUE line(VALUE self)
   return INT2NUM(xmlGetLineNo(node));
 }
 
+/*
+ * call-seq:
+ *  line=(num)
+ *
+ * Sets the line for this Node. num must be less than 65535.
+ */
+static VALUE set_line(VALUE self, VALUE num)
+{
+  xmlNodePtr node;
+  Data_Get_Struct(self, xmlNode, node);
+
+  int value = NUM2INT(num);
+  if (value < 65535) {
+    node->line = value;
+  }
+
+  return num;
+}
+
 /*
  * call-seq:
  *  add_namespace_definition(prefix, href)
@@ -1728,6 +1747,7 @@ void init_xml_node()
   rb_define_method(klass, "create_external_subset", create_external_subset, 3);
   rb_define_method(klass, "pointer_id", pointer_id, 0);
   rb_define_method(klass, "line", line, 0);
+  rb_define_method(klass, "line=", set_line, 1);
   rb_define_method(klass, "content", get_native_content, 0);
   rb_define_method(klass, "native_content=", set_native_content, 1);
   rb_define_method(klass, "lang", get_lang, 0);

test/xml/test_node.rb

@@ -1222,6 +1222,14 @@ def test_line
         assert_equal 2, node.line
       end
 
+      def test_set_line
+        skip "Only supported with libxml2" unless Nokogiri.uses_libxml?
+        document = Nokogiri::XML::Document.new
+        node = document.create_element('a')
+        node.line = 54321
+        assert_equal 54321, node.line
+      end
+
       def test_xpath_results_have_document_and_are_decorated
         x = Module.new do
           def awesome! ; end