Add mutation foo[n..-1] -> foo.drop(n)

Fixes #521

config/flay.yml

@@ -1,3 +1,3 @@
 ---
 threshold: 16
-total_score: 1166
+total_score: 1171

lib/mutant/mutator/node/send.rb

@@ -105,6 +105,7 @@ module Mutant
           emit_const_get_mutation
           emit_integer_mutation
           emit_dig_mutation
+          emit_drop_mutation
         end
 
         # Emit selector mutations specific to top level constants
@@ -137,6 +138,19 @@ module Mutant
           emit(s(:send, fetch_mutation, :dig, *tail))
         end
 
+        # Emit mutation `foo[n..-1]` -> `foo.drop(n)`
+        #
+        # @return [undefined]
+        def emit_drop_mutation
+          return if !selector.equal?(:[]) || !arguments.one? || !n_irange?(arguments.first)
+
+          start, ending = *arguments.first
+
+          return unless ending.eql?(s(:int, -1))
+
+          emit(s(:send, receiver, :drop, start))
+        end
+
         # Emit mutation from `to_i` to `Integer(...)`
         #
         # @return [undefined]

meta/send.rb

@@ -597,6 +597,7 @@ Mutant::Meta::Example.add :send do
   mutation 'foo[n..(1.0 / 0.0)]'
   mutation 'foo[n..(0.0 / 0.0)]'
   mutation 'foo[(0.0 / 0.0)..-1]'
+  mutation 'foo.drop(n)'
 end
 
 Mutant::Meta::Example.add :send do