Add capture group -> non capture group mutation

- Mutates regexps like (foo) to (?:foo).

config/flay.yml

@@ -1,3 +1,3 @@
 ---
 threshold: 16
-total_score: 1306
+total_score: 1300

lib/mutant.rb

@@ -92,6 +92,7 @@ require 'mutant/mutator/node'
 require 'mutant/mutator/node/generic'
 require 'mutant/mutator/node/regexp'
 require 'mutant/mutator/node/regexp/alternation_meta'
+require 'mutant/mutator/node/regexp/capture_group'
 require 'mutant/mutator/node/regexp/character_type'
 require 'mutant/mutator/node/regexp/end_of_line_anchor'
 require 'mutant/mutator/node/regexp/end_of_string_or_before_end_of_line_anchor'

lib/mutant/mutator/node/generic.rb

@@ -39,6 +39,7 @@ module Mutant
         unsupported_regexp_nodes = AST::Types::REGEXP.to_a - %i[
           regexp_alternation_meta
           regexp_bol_anchor
+          regexp_capture_group
           regexp_digit_type
           regexp_eol_anchor
           regexp_eos_ob_eol_anchor

lib/mutant/mutator/node/regexp/capture_group.rb

@@ -0,0 +1,26 @@
+module Mutant
+  class Mutator
+    class Node
+      module Regexp
+        # Mutator for regexp capture groups, such as `/(foo)/`
+        class CaptureGroup < Node
+          handle(:regexp_capture_group)
+
+          children :group
+
+          # Emit mutations
+          #
+          # Replace `(captured_group)` with `(?:non_captured_group)`
+          #
+          # @return [undefined]
+          def dispatch
+            return unless group
+
+            emit(s(:regexp_passive_group, group))
+            emit_group_mutations
+          end
+        end # EndOfLineAnchor
+      end # Regexp
+    end # Node
+  end # Mutator
+end # Mutant

meta/regexp/regexp_alternation_meta.rb

@@ -7,4 +7,5 @@ Mutant::Meta::Example.add :regexp_alternation_meta do
   mutation '/\A(foo|bar)\z/'
   mutation '/\A(foo|baz)\z/'
   mutation '/\A(bar|baz)\z/'
+  mutation '/\A(?:foo|bar|baz)\z/'
 end

meta/regexp/regexp_capture_group.rb

@@ -0,0 +1,17 @@
+Mutant::Meta::Example.add :regexp_capture_group do
+  source '/()/'
+
+  singleton_mutations
+  regexp_mutations
+end
+
+Mutant::Meta::Example.add :regexp_capture_group do
+  source '/(foo|bar)/'
+
+  singleton_mutations
+  regexp_mutations
+
+  mutation '/(?:foo|bar)/'
+  mutation '/(foo)/'
+  mutation '/(bar)/'
+end