Add ImmutableStruct implementation

Author: pitr-ch

lib/concurrent/synchronization.rb

@@ -27,3 +27,5 @@ class Object < Implementation
     end
   end
 end
+
+require 'concurrent/synchronization/immutable_struct'

lib/concurrent/synchronization/immutable_struct.rb

@@ -0,0 +1,23 @@
+module Concurrent
+  module Synchronization
+    class ImmutableStruct < Synchronization::Object
+      def self.with_fields(*names, &block)
+        Class.new(self) do
+          attr_reader(*names)
+          instance_eval &block if block
+
+          class_eval <<-RUBY, __FILE__, __LINE__ + 1
+            def initialize(#{names.join(', ')})
+              #{names.map { |n| '@' + n.to_s }.join(', ')} = #{names.join(', ')}
+              ensure_ivar_visibility!
+            end
+          RUBY
+        end
+      end
+
+      def self.[](*args)
+        new *args
+      end
+    end
+  end
+end

spec/concurrent/synchronized_object_spec.rb

@@ -105,5 +105,16 @@ def ns_initialize
       end
     end
 
+    describe Synchronization::ImmutableStruct do
+      let(:klass) { described_class.with_fields(:a, :b) }
+      subject { klass[1, 'a'] }
+
+      specify do
+        expect(klass.superclass).to eq described_class
+        expect(subject.a).to eq 1
+        expect(subject.b).to eq 'a'
+      end
+    end
+
   end
 end