✨ Add Data polyfill for ruby 3.1

For new data structs, I'd prefer frozen by default and I don't want to
support the entire Struct API.  Data is perfect, but it's not available
until ruby 3.2.

So this adds a DataLite class that closely matches ruby 3.2's Data
class, and it can be a drop-in replacement for Data.  Net::IMAP::Data
is an alias for Net::IMAP::DataLite, so when we remove our
implementation, the constant will resolve to ruby's ::Data.

Ideally, we wouldn't define this on newer ruby versions at all, but that
breaks the YAML serialization for our test fixtures.

The tests (and some of the implementation) have been copied from the
polyfill-data gem and updated so that they use "Data" as it is resolved
inside the "Net::IMAP" namespace.  Copyright notices have been added to
the appropriate files to satisfy the MIT license terms.

Co-authored-by: Jim Gay <jim@saturnflyer.com>

Author: nevans

lib/net/imap.rb

@@ -3276,6 +3276,7 @@ def self.saslprep(string, **opts)
 require_relative "imap/config"
 require_relative "imap/command_data"
 require_relative "imap/data_encoding"
+require_relative "imap/data_lite"
 require_relative "imap/flags"
 require_relative "imap/response_data"
 require_relative "imap/response_parser"

lib/net/imap/data_lite.rb

@@ -0,0 +1,165 @@
+# frozen_string_literal: true
+
+# Some of the code in this file was copied from the polyfill-data gem.
+#
+# MIT License
+#
+# Copyright (c) 2023 Jim Gay, Joel Drapper, Nicholas Evans
+#
+# Permission is hereby granted, free of charge, to any person obtaining a copy
+# of this software and associated documentation files (the "Software"), to deal
+# in the Software without restriction, including without limitation the rights
+# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+# copies of the Software, and to permit persons to whom the Software is
+# furnished to do so, subject to the following conditions:
+#
+# The above copyright notice and this permission notice shall be included in all
+# copies or substantial portions of the Software.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+# SOFTWARE.
+
+# TODO: fix tests to allow same yaml for both Data and DataLite
+# NOTE: psych 5.1.2 doesn't encode ::Data correctly!
+# if RUBY_VERSION >= "3.2.0"
+#   return
+# end
+
+module Net
+  class IMAP
+
+    # See {ruby's documentation for Data}[https://docs.ruby-lang.org/en/3.3/Data.html].
+    #
+    # DataLite is a temporary polyfill for ruby 3.2's
+    # Data[https://docs.ruby-lang.org/en/3.3/Data.html].  <em>This class is
+    # not defined for ruby versions >= 3.2.</em>  It will only be defined when
+    # using ruby 3.1 (+net-imap+ no longer supports ruby versions < 3.1).  It
+    # <em>will be removed</em> in +net-imap+ 0.6, when support for ruby 3.1 is
+    # dropped.
+    #
+    # It is aliased as Net::IMAP::Data so that, in ruby 3.1, any reference to
+    # "Data" that is namespaced inside Net::IMAP will use it.  This way,
+    # Net::IMAP's code shouldn't need to change to work with both
+    # Net::IMAP::DataLite and
+    # {::Data}[https://docs.ruby-lang.org/en/3.3/Data.html].
+    #
+    # Some of the code in this class was copied or adapted from the
+    # {polyfill-data gem}[https://rubygems.org/gems/polyfill-data], by Jim Gay
+    # and Joel Drapper, under the MIT license terms.
+    class DataLite
+      singleton_class.undef_method :new
+
+      TYPE_ERROR    = "%p is not a symbol nor a string"
+      ATTRSET_ERROR = "invalid data member: %p"
+      DUP_ERROR     = "duplicate member: %p"
+      ARITY_ERROR   = "wrong number of arguments (given %d, expected %s)"
+      private_constant :TYPE_ERROR, :ATTRSET_ERROR, :DUP_ERROR, :ARITY_ERROR
+
+      # *NOTE:+ DataLite.define does not support member names which are not
+      # valid local variable names.
+      def self.define(*args, &block)
+        members = args.each_with_object({}) do |arg, members|
+          arg = arg.to_str unless arg in Symbol | String if arg.respond_to?(:to_str)
+          arg = arg.to_sym if     arg in String
+          arg in Symbol     or  raise TypeError,     TYPE_ERROR    % [arg]
+          arg in %r{=}      and raise ArgumentError, ATTRSET_ERROR % [arg]
+          members.key?(arg) and raise ArgumentError, DUP_ERROR     % [arg]
+          members[arg] = true
+        end
+        members = members.keys.freeze
+
+        klass = ::Class.new(self)
+
+        klass.singleton_class.undef_method :define
+        klass.define_singleton_method(:members) { members }
+
+        def klass.new(*args, **kwargs, &block)
+          if kwargs.size.positive?
+            if args.size.positive?
+              raise ArgumentError, ARITY_ERROR % [args.size, 0]
+            end
+          elsif members.size < args.size
+            expected = members.size.zero? ? 0 : 0..members.size
+            raise ArgumentError, ARITY_ERROR % [args.size, expected]
+          else
+            kwargs = Hash[members.take(args.size).zip(args)]
+          end
+          allocate.tap do |instance|
+            instance.__send__(:initialize, **kwargs, &block)
+          end.freeze
+        end
+
+        klass.singleton_class.alias_method :[], :new
+        klass.attr_reader(*members)
+
+        # Dynamically defined initializer methods are in an included module,
+        # rather than directly on DataLite (like in ruby 3.2+):
+        # * simpler to handle required kwarg ArgumentErrors
+        # * easier to ensure consistent ivar assignment order (object shape)
+        # * faster than instance_variable_set
+        klass.include(Module.new do
+          if members.any?
+            kwargs = members.map{"#{_1.name}:"}.join(", ")
+            params = members.map(&:name).join(", ")
+            ivars  = members.map{"@#{_1.name}"}.join(", ")
+            attrs  = members.map{"attrs[:#{_1.name}]"}.join(", ")
+            module_eval <<~RUBY, __FILE__, __LINE__ + 1
+              protected
+              def initialize(#{kwargs}) #{ivars} = #{params}; freeze end
+              def marshal_load(attrs)   #{ivars} = #{attrs};  freeze end
+            RUBY
+          end
+        end)
+
+        klass.module_eval do _1.module_eval(&block) end if block_given?
+
+        klass
+      end
+
+      def members;     self.class.members                              end
+      def attributes;  Hash[members.map {|m| [m, send(m)] }]           end
+      def to_h(&block) attributes.to_h(&block)                         end
+      def hash;        to_h.hash                                       end
+      def ==(other)    self.class == other.class && to_h == other.to_h end
+      def eql?(other)  self.class == other.class && hash == other.hash end
+      def deconstruct; attributes.values                               end
+
+      def deconstruct_keys(keys)
+        raise TypeError unless keys.is_a?(Array) || keys.nil?
+        return attributes if keys&.first.nil?
+        attributes.slice(*keys)
+      end
+
+      def with(**kwargs)
+        return self if kwargs.empty?
+        self.class.new(**attributes.merge(kwargs))
+      end
+
+      # +NOTE:+ Unlike ruby 3.2's <tt>Data#inspect</tt>, this has no guard
+      # against infinite recursion.
+      def inspect
+        attrs   = attributes.map {|kv| "%s=%p" % kv }.join(", ")
+        display = ["data", self.class.name, attrs].compact.join(" ")
+        "#<#{display}>"
+      end
+      alias_method :to_s, :inspect
+
+      def encode_with(coder) coder.map = attributes.transform_keys(&:to_s) end
+      def init_with(coder) marshal_load(coder.map.transform_keys(&:to_sym)) end
+
+      private
+
+      def initialize_copy(source) super.freeze end
+      def marshal_dump; attributes end
+
+    end
+
+    Data = DataLite
+
+  end
+end