See https://github.com/xml4r/libxml-ruby/issues/198

test_libxml_warnings.rb

# Results in comments are from Ruby 3.2.2, libxml-ruby 4.1.1 on macOS

require 'libxml-ruby'

# new() is undefined for Writer
# https://github.com/xml4r/libxml-ruby/blob/master/ext/libxml/ruby_xml_writer.c#L1134
# LibXML::XML::Writer.new()  # crash: undefined method `new' for LibXML::XML::Writer:Class (NoMethodError)
LibXML::XML::Writer.io(nil)  # warning: undefining the allocator of T_DATA class LibXML::XML::Writer

# new() is neither undefined nor redefined for Reader
# https://github.com/xml4r/libxml-ruby/blob/master/ext/libxml/ruby_xml_reader.c#L1140-L1241
LibXML::XML::Reader.new()  # no warning
LibXML::XML::Reader.string("_")  # warning: undefining the allocator of T_DATA class LibXML::XML::Reader
# LibXML::XML::Reader.new()  # crash: allocator undefined for LibXML::XML::Reader (TypeError)

LibXML::XML::Error.new()  # no warning

LibXML::XML::XInclude.new()  # no warning

context = LibXML::XML::Parser::Context.string("_")  # no warning

LibXML::XML::Parser.string("_")  # no warning
LibXML::XML::Parser.new(context)  # no warning

LibXML::XML::SaxParser.string("_")  # no warning
LibXML::XML::SaxParser.new(context)  # no warning

htmlContext = LibXML::XML::HTMLParser::Context.string("_")  # no warning
LibXML::XML::HTMLParser.string("_")  # no warning
LibXML::XML::HTMLParser.new(htmlContext)  # no warning

LibXML::XML::InputCallbacks.new()  # no warning


doc = LibXML::XML::Document.new()  # no warning
LibXML::XML::Document.string("<x/>")  # no warning

node = LibXML::XML::Node.new(nil)  # no warning
LibXML::XML::Attr.new(node, "", "")  # no warning
LibXML::XML::Namespaces.new(node)  # no warning
LibXML::XML::Namespace.new(node, "", "")  # no warning

# new() is redefined for Schema
# https://github.com/xml4r/libxml-ruby/blob/master/ext/libxml/ruby_xml_schema.c#L397
LibXML::XML::Schema.new("http://www.w3.org/2001/XMLSchema.xsd")  # warning: undefining the allocator of T_DATA class LibXML::XML::Schema
LibXML::XML::Schema.from_string("<schema xmlns='http://www.w3.org/2001/XMLSchema'/>")  # warning: undefining the allocator of T_DATA class LibXML::XML::Schema
LibXML::XML::Schema.new("http://www.w3.org/2001/XMLSchema.xsd")  # warning: undefining the allocator of T_DATA class LibXML::XML::Schema

LibXML::XML::Schema::Attribute.new()  # no warning
LibXML::XML::Schema::Element.new()  # no warning
LibXML::XML::Schema::Facet.new()  # no warning
LibXML::XML::Schema::Type.new()  # no warning

LibXML::XML::Attributes.new()  # no warning
LibXML::XML::AttrDecl.new()  # no warning

LibXML::XML::Dtd.new("")  # no warning

LibXML::XML::RelaxNG.from_string("<element name='_' xmlns='http://relaxng.org/ns/structure/1.0'><text/></element>")  # warning: undefining the allocator of T_DATA class LibXML::XML::RelaxNG

LibXML::XML::XPath::Context.new(doc)  # no warning
LibXML::XML::XPath::Expression.new('/')  # no warning
LibXML::XML::XPath::Object.new()  # no warning