Add mutation a.dig(b, c) -> a.fetch(b).dig(c)

Closes #480

config/flay.yml

@@ -1,3 +1,3 @@
 ---
 threshold: 18
-total_score: 1174
+total_score: 1181

lib/mutant/mutator/node/send.rb

@@ -103,6 +103,7 @@ module Mutant
         def emit_selector_specific_mutations
           emit_const_get_mutation
           emit_integer_mutation
+          emit_dig_mutation
         end
 
         # Emit selector mutations specific to top level constants
@@ -117,6 +118,24 @@ module Mutant
             .each(&method(:emit_selector))
         end
 
+        # Emit mutation for `#dig`
+        #
+        # - Mutates `foo.dig(a, b)` to `foo.fetch(a).dig(b)`
+        # - Mutates `foo.dig(a)` to `foo.fetch(a)`
+        #
+        # @return [undefined]
+        def emit_dig_mutation
+          return if !selector.equal?(:dig) || arguments.none?
+
+          head, *tail = arguments
+
+          fetch_mutation = s(:send, receiver, :fetch, head)
+
+          return emit(fetch_mutation) if tail.empty?
+
+          emit(s(:send, fetch_mutation, :dig, *tail))
+        end
+
         # Emit mutation from `to_i` to `Integer(...)`
         #
         # @return [undefined]

meta/send.rb

@@ -255,6 +255,7 @@ Mutant::Meta::Example.add do
   source 'foo.dig(a, b)'
 
   singleton_mutations
+  mutation 'foo.fetch(a).dig(b)'
   mutation 'foo'
   mutation 'self.dig(a, b)'
   mutation 'foo.dig(a)'
@@ -270,6 +271,7 @@ Mutant::Meta::Example.add do
   source 'foo.dig(a)'
 
   singleton_mutations
+  mutation 'foo.fetch(a)'
   mutation 'foo'
   mutation 'self.dig(a)'
   mutation 'foo.dig(nil)'