✨ Add Data polyfill for ruby 3.1

For new data structs, I don't want to commit to supporting the entire
Struct API and I'd prefer frozen by default.  `Data` is exactly what I
want 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 can be a drop-in replacement for Data.  Net::IMAP::Data is an
alias for Net::IMAP::DataLite, so when we remove this implementation,
the constant will resolve to ruby's ::Data.  The most noticable
incompatibility is that member names must be valid local variable names.

Ideally, we wouldn't define this on newer ruby versions at all, but that
breaks the YAML serialization for our test fixtures.  So, on newer
versions of ruby, this class inherits from `Data` and only reimplements
the two methods that are needed for YAML serialization.  This serves the
additional purpose of ensuring that the same tests pass for both the
core `Data` class and our reimplementation.

Most of the test code and some of the implementation code has 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 <[email protected]>

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 +1,225 @@
+# 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.
+
+
+module Net
+  class IMAP
+    data_or_object = RUBY_VERSION >= "3.2.0" ? ::Data : Object
+    class DataLite < data_or_object
+      def encode_with(coder) coder.map = attributes.transform_keys(&:to_s) end
+      def init_with(coder) initialize(**coder.map.transform_keys(&:to_sym)) end
+    end
+
+    Data = DataLite
+  end
+end
+
+# :nocov:
+# Need to skip test coverage for the rest, because it isn't loaded by ruby 3.2+.
+return if RUBY_VERSION >= "3.2.0"
+
+module Net
+  class IMAP
+    # DataLite is a temporary substitute for ruby 3.2's +Data+ class.  DataLite
+    # is aliased as Net::IMAP::Data, so that code using it won't need to be
+    # updated when it is removed.
+    #
+    # See {ruby 3.2's documentation for Data}[https://docs.ruby-lang.org/en/3.2/Data.html].
+    #
+    # [When running ruby 3.1]
+    #    This class reimplements the API for ruby 3.2's +Data+, and should be
+    #    compatible for nearly all use-cases.  This reimplementation <em>will be
+    #    removed</em> in +net-imap+ 0.6, when support for ruby 3.1 is dropped.
+    #
+    #    _NOTE:_ +net-imap+ no longer supports ruby versions prior to 3.1.
+    # [When running ruby >= 3.2]
+    #    This class inherits from +Data+ and _only_ defines the methods needed
+    #    for YAML serialization.  This will be dropped when +psych+ adds support
+    #    for +Data+.
+    #
+    # 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
+
+      # Defines a new Data class.
+      #
+      # _NOTE:_ Unlike ruby 3.2's +Data.define+, DataLite.define only supports
+      # member names which are valid local variable names.  Member names can't
+      # be keywords (e.g: +next+ or +class+) or start with capital letters, "@",
+      # etc.
+      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
+
+      ##
+      # singleton-method: new
+      # call-seq:
+      #   new(*args) -> instance
+      #   new(**kwargs) -> instance
+      #
+      # Constuctor for classes defined with ::define.
+      #
+      # Aliased as ::[].
+
+      ##
+      # singleton-method: []
+      # call-seq:
+      #   ::[](*args) -> instance
+      #   ::[](**kwargs) -> instance
+      #
+      # Constuctor for classes defined with ::define.
+      #
+      # Alias for ::new
+
+      ##
+      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;        [self.class, attributes].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
+
+      def inspect
+        __inspect_guard__(self) do |seen|
+          return "#<data #{self.class}:...>" if seen
+          attrs = attributes.map {|kv| "%s=%p" % kv }.join(", ")
+          display = ["data", self.class.name, attrs].compact.join(" ")
+          "#<#{display}>"
+        end
+      end
+      alias_method :to_s, :inspect
+
+      private
+
+      def initialize_copy(source) super.freeze end
+      def marshal_dump; attributes end
+
+      # Yields +true+ if +obj+ has been seen already, +false+ if it hasn't.
+      # Marks +obj+ as seen inside the block, so circuler references don't
+      # recursively trigger a SystemStackError (stack level too deep).
+      #
+      # Making circular references inside a Data object _should_ be very
+      # uncommon, but we'll support them for the sake of completeness.
+      def __inspect_guard__(obj)
+        preexisting = Thread.current[:__net_imap_data__inspect__]
+        Thread.current[:__net_imap_data__inspect__] ||= {}.compare_by_identity
+        inspect_guard = Thread.current[:__net_imap_data__inspect__]
+        if inspect_guard.include?(obj)
+          yield true
+        else
+          begin
+            inspect_guard[obj] = true
+            yield false
+          ensure
+            inspect_guard.delete(obj)
+          end
+        end
+      ensure
+        unless preexisting.equal?(inspect_guard)
+          Thread.current[:__net_imap_data__inspect__] = preexisting
+        end
+      end
+
+    end
+
+  end
+end
+# :nocov: