Add empty collection return value mutations

ruby/lib/mutant/mutator/node/literal/string.rb

@@ -13,6 +13,7 @@ class String < self
 
           def dispatch
             emit_singletons
+            emit(s(:str, ''))
           end
 
         end # String

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

@@ -65,6 +65,7 @@ def normal_dispatch
           mutate_arguments
         end
 
+        # rubocop:disable Metrics/MethodLength
         def emit_selector_specific_mutations
           emit_reduce_to_sum_mutation
           emit_start_end_with_mutations
@@ -76,7 +77,9 @@ def emit_selector_specific_mutations
           emit_dig_mutation
           emit_double_negation_mutation
           emit_lambda_mutation
+          emit_empty_collection_mutation
         end
+        # rubocop:enable Metrics/MethodLength
 
         def emit_reduce_to_sum_mutation
           return unless selector.equal?(:reduce)
@@ -171,6 +174,17 @@ def emit_lambda_mutation
           emit(s(:send, nil, :lambda)) if meta.proc?
         end
 
+        def emit_empty_collection_mutation
+          case selector
+          when :to_a, :to_ary
+            emit(s(:array))
+          when :to_h, :to_hash
+            emit(s(:hash))
+          when :to_s, :to_str
+            emit(s(:str, ''))
+          end
+        end
+
         def emit_dig_mutation
           return if !selector.equal?(:dig) || arguments.none?
 

ruby/meta/send.rb

@@ -149,6 +149,7 @@
   mutation 'foo'
   mutation 'self.to_s'
   mutation 'foo.to_str'
+  mutation '""'
 end
 
 Mutant::Meta::Example.add :send do
@@ -158,6 +159,7 @@
   mutation 'foo'
   mutation 'self.to_a'
   mutation 'foo.to_ary'
+  mutation '[]'
 end
 
 Mutant::Meta::Example.add :send do
@@ -177,6 +179,34 @@
   mutation 'foo'
   mutation 'self.to_h'
   mutation 'foo.to_hash'
+  mutation '{}'
+end
+
+Mutant::Meta::Example.add :send do
+  source 'foo.to_ary'
+
+  singleton_mutations
+  mutation 'foo'
+  mutation 'self.to_ary'
+  mutation '[]'
+end
+
+Mutant::Meta::Example.add :send do
+  source 'foo.to_hash'
+
+  singleton_mutations
+  mutation 'foo'
+  mutation 'self.to_hash'
+  mutation '{}'
+end
+
+Mutant::Meta::Example.add :send do
+  source 'foo.to_str'
+
+  singleton_mutations
+  mutation 'foo'
+  mutation 'self.to_str'
+  mutation '""'
 end
 
 Mutant::Meta::Example.add :send, operators: :full do

ruby/meta/str.rb

@@ -3,5 +3,12 @@
 Mutant::Meta::Example.add :str do
   source '"foo"'
 
+  singleton_mutations
+  mutation '""'
+end
+
+Mutant::Meta::Example.add :str do
+  source '""'
+
   singleton_mutations
 end