Fix invalid ASTs on mutating attribute assignments

* Also move index reference / assignment mutators to dedicated file
* Move attribute assignment cases to subclass
* Mutations emitted never had been invalid. Since invalid AST was parsed
  to valid source as unparser trusts the AST.

lib/mutant.rb

@@ -127,6 +127,8 @@ require 'mutant/mutator/node/zsuper'
 require 'mutant/mutator/node/restarg'
 require 'mutant/mutator/node/send'
 require 'mutant/mutator/node/send/binary'
+require 'mutant/mutator/node/send/attribute_assignment'
+require 'mutant/mutator/node/send/index'
 require 'mutant/mutator/node/when'
 require 'mutant/mutator/node/define'
 require 'mutant/mutator/node/mlhs'

lib/mutant/mutator/node.rb

@@ -46,6 +46,10 @@ module Mutant
           children.each_with_index.drop(names.length)
         end
 
+        define_method(:remaining_children_indices) do
+          children.each_index.drop(names.length)
+        end
+
         define_method(:remaining_children) do
           children.drop(names.length)
         end

lib/mutant/mutator/node/send.rb

@@ -22,40 +22,6 @@ module Mutant
         INDEX_ASSIGN    = :[]=
         ASSIGN_SUFFIX   = '='.freeze
 
-        # Base mutator for index operations
-        class Index < self
-
-          # Mutator for index references
-          class Reference < self
-
-            # Perform dispatch
-            #
-            # @return [undefined]
-            #
-            # @api private
-            #
-            def dispatch
-              emit(receiver)
-            end
-
-          end # Reference
-
-          # Mutator for index assignments
-          class Assign < self
-
-            # Perform dispatch
-            #
-            # @return [undefined]
-            #
-            # @api private
-            #
-            def dispatch
-              emit(receiver)
-            end
-
-          end # Assign
-        end # Index
-
       private
 
         # Perform dispatch
@@ -65,6 +31,7 @@ module Mutant
         # @api private
         #
         def dispatch
+          emit_nil
           case selector
           when INDEX_REFERENCE
             run(Index::Reference)
@@ -73,7 +40,6 @@ module Mutant
           else
             non_index_dispatch
           end
-          emit_nil
         end
 
         # Perform non index dispatch
@@ -86,6 +52,8 @@ module Mutant
           case
           when binary_operator?
             run(Binary)
+          when attribute_assignment?
+            run(AttributeAssignment)
           else
             normal_dispatch
           end
@@ -173,7 +141,6 @@ module Mutant
         # @api private
         #
         def mutate_arguments
-          return if arguments.empty?
           emit_self(receiver, selector)
           remaining_children_with_index.each do |node, index|
             mutate_child(index)

lib/mutant/mutator/node/send/attribute_assignment.rb

@@ -0,0 +1,52 @@
+# encoding: utf-8
+
+module Mutant
+  class Mutator
+    class Node
+      class Send
+
+        # Mutator for sends that correspond to an attribute assignment
+        class AttributeAssignment < self
+
+        private
+
+          # Emit mutations
+          #
+          # @return [undefined]
+          #
+          # @api private
+          #
+          def dispatch
+            normal_dispatch
+            emit_attribute_read
+          end
+
+          # Mutate arguments
+          #
+          # @return [undefined]
+          #
+          # @api private
+          #
+          def mutate_arguments
+            remaining_children_indices.each do |index|
+              mutate_child(index)
+            end
+          end
+
+          # Emit attribute read
+          #
+          # @return [undefined]
+          #
+          # @api private
+          #
+          def emit_attribute_read
+            emit_self(receiver, selector.to_s[0..-2].to_sym)
+          end
+
+        end # AttributeAssignment
+
+      end # Send
+    end # Node
+  end # Mutator
+end # Mutant
+

lib/mutant/mutator/node/send/index.rb

@@ -0,0 +1,43 @@
+# encoding: UTF-8
+
+module Mutant
+  class Mutator
+    class Node
+      class Send
+        # Base mutator for index operations
+        class Index < self
+
+          # Mutator for index references
+          class Reference < self
+
+            # Perform dispatch
+            #
+            # @return [undefined]
+            #
+            # @api private
+            #
+            def dispatch
+              emit(receiver)
+            end
+
+          end # Reference
+
+          # Mutator for index assignments
+          class Assign < self
+
+            # Perform dispatch
+            #
+            # @return [undefined]
+            #
+            # @api private
+            #
+            def dispatch
+              emit(receiver)
+            end
+
+          end # Assign
+        end # Index
+      end # Send
+    end # Node
+  end # Mutator
+end # Mutant