From 99ab9ff1713b161509a0b28b68dbffc933e82aca Mon Sep 17 00:00:00 2001
From: Markus Schirp <mbj@schirp-dso.com>
Date: Wed, 12 Sep 2018 14:21:24 +0000
Subject: [PATCH] Change required Ruby version to >=2.3

* This commit does every 2.3 change required to get the build pass
* None of the changes can be extracted, without changing the build setup
---
 .rubocop.yml                                     |  2 +-
 Changelog.md                                     |  5 +++++
 Gemfile                                          |  1 +
 Gemfile.lock                                     | 16 ++++++++--------
 Rakefile                                         |  1 +
 bin/mutant                                       |  1 +
 config/flay.yml                                  |  2 +-
 lib/mutant.rb                                    |  1 +
 lib/mutant/actor.rb                              |  1 +
 lib/mutant/actor/env.rb                          |  1 +
 lib/mutant/actor/mailbox.rb                      |  1 +
 lib/mutant/actor/receiver.rb                     |  1 +
 lib/mutant/actor/sender.rb                       |  1 +
 lib/mutant/ast.rb                                |  1 +
 lib/mutant/ast/meta.rb                           |  1 +
 lib/mutant/ast/meta/const.rb                     |  1 +
 lib/mutant/ast/meta/optarg.rb                    |  1 +
 lib/mutant/ast/meta/resbody.rb                   |  1 +
 lib/mutant/ast/meta/restarg.rb                   |  1 +
 lib/mutant/ast/meta/send.rb                      |  1 +
 lib/mutant/ast/meta/symbol.rb                    |  1 +
 lib/mutant/ast/named_children.rb                 |  1 +
 lib/mutant/ast/node_predicates.rb                |  1 +
 lib/mutant/ast/nodes.rb                          |  1 +
 lib/mutant/ast/regexp.rb                         |  1 +
 lib/mutant/ast/regexp/transformer.rb             |  1 +
 lib/mutant/ast/regexp/transformer/alternative.rb |  1 +
 .../ast/regexp/transformer/character_set.rb      |  1 +
 lib/mutant/ast/regexp/transformer/direct.rb      |  1 +
 .../ast/regexp/transformer/options_group.rb      |  1 +
 lib/mutant/ast/regexp/transformer/quantifier.rb  |  3 ++-
 lib/mutant/ast/regexp/transformer/recursive.rb   |  1 +
 lib/mutant/ast/regexp/transformer/root.rb        |  1 +
 lib/mutant/ast/regexp/transformer/text.rb        |  1 +
 lib/mutant/ast/sexp.rb                           |  1 +
 lib/mutant/ast/types.rb                          |  1 +
 lib/mutant/cli.rb                                |  1 +
 lib/mutant/color.rb                              |  1 +
 lib/mutant/config.rb                             |  1 +
 lib/mutant/context.rb                            |  1 +
 lib/mutant/diff.rb                               |  5 +++--
 lib/mutant/env.rb                                |  1 +
 lib/mutant/env/bootstrap.rb                      |  1 +
 lib/mutant/expression.rb                         |  1 +
 lib/mutant/expression/method.rb                  |  1 +
 lib/mutant/expression/methods.rb                 |  1 +
 lib/mutant/expression/namespace.rb               |  1 +
 lib/mutant/expression/parser.rb                  |  1 +
 lib/mutant/integration.rb                        |  1 +
 lib/mutant/integration/rspec.rb                  |  1 +
 lib/mutant/isolation.rb                          |  1 +
 lib/mutant/isolation/fork.rb                     |  1 +
 lib/mutant/isolation/none.rb                     |  1 +
 lib/mutant/loader.rb                             |  1 +
 lib/mutant/matcher.rb                            |  1 +
 lib/mutant/matcher/chain.rb                      |  1 +
 lib/mutant/matcher/compiler.rb                   |  1 +
 lib/mutant/matcher/config.rb                     |  1 +
 lib/mutant/matcher/filter.rb                     |  1 +
 lib/mutant/matcher/method.rb                     |  1 +
 lib/mutant/matcher/method/instance.rb            |  1 +
 lib/mutant/matcher/method/singleton.rb           |  1 +
 lib/mutant/matcher/methods.rb                    |  1 +
 lib/mutant/matcher/namespace.rb                  |  1 +
 lib/mutant/matcher/null.rb                       |  1 +
 lib/mutant/matcher/scope.rb                      |  1 +
 lib/mutant/matcher/static.rb                     |  1 +
 lib/mutant/meta.rb                               |  1 +
 lib/mutant/meta/example.rb                       |  1 +
 lib/mutant/meta/example/dsl.rb                   |  1 +
 lib/mutant/meta/example/verification.rb          |  1 +
 lib/mutant/mutation.rb                           |  1 +
 lib/mutant/mutator.rb                            |  1 +
 lib/mutant/mutator/node.rb                       |  5 +++--
 lib/mutant/mutator/node/and_asgn.rb              |  1 +
 lib/mutant/mutator/node/argument.rb              |  1 +
 lib/mutant/mutator/node/arguments.rb             |  1 +
 lib/mutant/mutator/node/begin.rb                 |  1 +
 lib/mutant/mutator/node/binary.rb                |  1 +
 lib/mutant/mutator/node/block.rb                 |  1 +
 lib/mutant/mutator/node/break.rb                 |  1 +
 lib/mutant/mutator/node/case.rb                  |  1 +
 lib/mutant/mutator/node/class.rb                 |  1 +
 lib/mutant/mutator/node/conditional_loop.rb      |  1 +
 lib/mutant/mutator/node/const.rb                 |  1 +
 lib/mutant/mutator/node/define.rb                |  1 +
 lib/mutant/mutator/node/defined.rb               |  1 +
 lib/mutant/mutator/node/dstr.rb                  |  1 +
 lib/mutant/mutator/node/dsym.rb                  |  1 +
 lib/mutant/mutator/node/generic.rb               |  1 +
 lib/mutant/mutator/node/if.rb                    |  1 +
 lib/mutant/mutator/node/kwbegin.rb               |  1 +
 lib/mutant/mutator/node/literal.rb               |  1 +
 lib/mutant/mutator/node/literal/array.rb         |  1 +
 lib/mutant/mutator/node/literal/boolean.rb       |  1 +
 lib/mutant/mutator/node/literal/float.rb         |  1 +
 lib/mutant/mutator/node/literal/hash.rb          |  1 +
 lib/mutant/mutator/node/literal/integer.rb       |  1 +
 lib/mutant/mutator/node/literal/nil.rb           |  1 +
 lib/mutant/mutator/node/literal/range.rb         |  1 +
 lib/mutant/mutator/node/literal/regex.rb         |  1 +
 lib/mutant/mutator/node/literal/string.rb        |  1 +
 lib/mutant/mutator/node/literal/symbol.rb        |  1 +
 lib/mutant/mutator/node/masgn.rb                 |  1 +
 lib/mutant/mutator/node/match_current_line.rb    |  1 +
 lib/mutant/mutator/node/mlhs.rb                  |  1 +
 lib/mutant/mutator/node/named_value/access.rb    |  1 +
 .../node/named_value/constant_assignment.rb      |  1 +
 .../node/named_value/variable_assignment.rb      |  1 +
 lib/mutant/mutator/node/next.rb                  |  1 +
 lib/mutant/mutator/node/noop.rb                  |  1 +
 lib/mutant/mutator/node/nthref.rb                |  1 +
 lib/mutant/mutator/node/op_asgn.rb               |  1 +
 lib/mutant/mutator/node/or_asgn.rb               |  1 +
 lib/mutant/mutator/node/regexp.rb                |  1 +
 .../mutator/node/regexp/alternation_meta.rb      |  1 +
 lib/mutant/mutator/node/regexp/capture_group.rb  |  1 +
 lib/mutant/mutator/node/regexp/character_type.rb |  1 +
 .../mutator/node/regexp/end_of_line_anchor.rb    |  1 +
 ...end_of_string_or_before_end_of_line_anchor.rb |  1 +
 .../mutator/node/regexp/greedy_zero_or_more.rb   |  1 +
 lib/mutant/mutator/node/regopt.rb                |  1 +
 lib/mutant/mutator/node/resbody.rb               |  1 +
 lib/mutant/mutator/node/rescue.rb                |  1 +
 lib/mutant/mutator/node/return.rb                |  1 +
 lib/mutant/mutator/node/send.rb                  |  1 +
 .../mutator/node/send/attribute_assignment.rb    |  1 +
 lib/mutant/mutator/node/send/binary.rb           |  1 +
 lib/mutant/mutator/node/send/conditional.rb      |  1 +
 lib/mutant/mutator/node/send/index.rb            |  1 +
 lib/mutant/mutator/node/splat.rb                 |  1 +
 lib/mutant/mutator/node/super.rb                 |  1 +
 lib/mutant/mutator/node/when.rb                  |  1 +
 lib/mutant/mutator/node/yield.rb                 |  1 +
 lib/mutant/mutator/node/zsuper.rb                |  1 +
 lib/mutant/mutator/util.rb                       |  1 +
 lib/mutant/mutator/util/array.rb                 |  1 +
 lib/mutant/mutator/util/symbol.rb                |  1 +
 lib/mutant/parallel.rb                           |  1 +
 lib/mutant/parallel/master.rb                    |  1 +
 lib/mutant/parallel/source.rb                    |  1 +
 lib/mutant/parallel/worker.rb                    |  1 +
 lib/mutant/parser.rb                             |  1 +
 lib/mutant/registry.rb                           |  1 +
 lib/mutant/reporter.rb                           |  1 +
 lib/mutant/reporter/cli.rb                       |  1 +
 lib/mutant/reporter/cli/format.rb                |  1 +
 lib/mutant/reporter/cli/printer.rb               |  1 +
 lib/mutant/reporter/cli/printer/config.rb        |  1 +
 lib/mutant/reporter/cli/printer/env_progress.rb  |  1 +
 lib/mutant/reporter/cli/printer/env_result.rb    |  1 +
 .../cli/printer/mutation_progress_result.rb      |  1 +
 .../reporter/cli/printer/mutation_result.rb      |  1 +
 lib/mutant/reporter/cli/printer/status.rb        |  1 +
 .../reporter/cli/printer/status_progressive.rb   |  1 +
 .../reporter/cli/printer/subject_progress.rb     |  1 +
 .../reporter/cli/printer/subject_result.rb       |  1 +
 lib/mutant/reporter/cli/printer/test_result.rb   |  1 +
 lib/mutant/reporter/cli/tput.rb                  |  1 +
 lib/mutant/reporter/null.rb                      |  1 +
 lib/mutant/reporter/sequence.rb                  |  1 +
 lib/mutant/repository.rb                         |  1 +
 lib/mutant/require_highjack.rb                   |  1 +
 lib/mutant/result.rb                             |  1 +
 lib/mutant/runner.rb                             |  1 +
 lib/mutant/runner/sink.rb                        |  1 +
 lib/mutant/scope.rb                              |  1 +
 lib/mutant/selector.rb                           |  1 +
 lib/mutant/selector/expression.rb                |  1 +
 lib/mutant/subject.rb                            |  1 +
 lib/mutant/subject/method.rb                     |  1 +
 lib/mutant/subject/method/instance.rb            |  1 +
 lib/mutant/subject/method/singleton.rb           |  1 +
 lib/mutant/test.rb                               |  1 +
 lib/mutant/util.rb                               |  1 +
 lib/mutant/version.rb                            |  1 +
 lib/mutant/warning_filter.rb                     |  1 +
 lib/mutant/zombifier.rb                          |  1 +
 meta/and.rb                                      |  1 +
 meta/and_asgn.rb                                 |  1 +
 meta/array.rb                                    |  1 +
 meta/begin.rb                                    |  1 +
 meta/block.rb                                    |  1 +
 meta/block_pass.rb                               |  1 +
 meta/blockarg.rb                                 |  1 +
 meta/break.rb                                    |  1 +
 meta/case.rb                                     |  1 +
 meta/casgn.rb                                    |  1 +
 meta/cbase.rb                                    |  1 +
 meta/class.rb                                    |  1 +
 meta/const.rb                                    |  1 +
 meta/csend.rb                                    |  1 +
 meta/cvar.rb                                     |  1 +
 meta/cvasgn.rb                                   |  1 +
 meta/date.rb                                     |  1 +
 meta/def.rb                                      |  1 +
 meta/defined.rb                                  |  1 +
 meta/dstr.rb                                     |  1 +
 meta/dsym.rb                                     |  1 +
 meta/ensure.rb                                   |  1 +
 meta/false.rb                                    |  1 +
 meta/float.rb                                    |  1 +
 meta/gvar.rb                                     |  1 +
 meta/gvasgn.rb                                   |  1 +
 meta/hash.rb                                     |  1 +
 meta/if.rb                                       |  1 +
 meta/int.rb                                      |  1 +
 meta/ivar.rb                                     |  1 +
 meta/ivasgn.rb                                   |  1 +
 meta/kwarg.rb                                    |  1 +
 meta/kwbegin.rb                                  |  1 +
 meta/kwoptarg.rb                                 |  1 +
 meta/lvar.rb                                     |  1 +
 meta/lvasgn.rb                                   |  1 +
 meta/masgn.rb                                    |  1 +
 meta/match_current_line.rb                       |  1 +
 meta/next.rb                                     |  1 +
 meta/nil.rb                                      |  1 +
 meta/nthref.rb                                   |  1 +
 meta/op_assgn.rb                                 |  1 +
 meta/or.rb                                       |  1 +
 meta/or_asgn.rb                                  |  1 +
 meta/range.rb                                    |  1 +
 meta/redo.rb                                     |  1 +
 meta/regexp.rb                                   |  1 +
 meta/regexp/character_types.rb                   |  1 +
 meta/regexp/regexp_alternation_meta.rb           |  1 +
 meta/regexp/regexp_bol_anchor.rb                 |  1 +
 meta/regexp/regexp_bos_anchor.rb                 |  1 +
 meta/regexp/regexp_capture_group.rb              |  1 +
 meta/regexp/regexp_eol_anchor.rb                 |  1 +
 meta/regexp/regexp_eos_anchor.rb                 |  1 +
 meta/regexp/regexp_eos_ob_eol_anchor.rb          |  1 +
 meta/regexp/regexp_greedy_zero_or_more.rb        |  1 +
 meta/regexp/regexp_root_expression.rb            |  1 +
 meta/regopt.rb                                   |  1 +
 meta/rescue.rb                                   |  1 +
 meta/restarg.rb                                  |  1 +
 meta/return.rb                                   |  1 +
 meta/self.rb                                     |  1 +
 meta/send.rb                                     |  1 +
 meta/str.rb                                      |  1 +
 meta/super.rb                                    |  1 +
 meta/sym.rb                                      |  1 +
 meta/true.rb                                     |  1 +
 meta/until.rb                                    |  1 +
 meta/while.rb                                    |  1 +
 meta/yield.rb                                    |  1 +
 mutant-rspec.gemspec                             |  1 +
 mutant.gemspec                                   |  1 +
 spec/integration/mutant/corpus_spec.rb           |  1 +
 spec/integration/mutant/isolation/fork_spec.rb   |  1 +
 spec/integration/mutant/null_spec.rb             |  1 +
 spec/integration/mutant/rspec_spec.rb            |  1 +
 .../mutant/test_mutator_handles_types_spec.rb    |  1 +
 spec/shared/framework_integration_behavior.rb    |  1 +
 spec/shared/method_matcher_behavior.rb           |  1 +
 spec/spec_helper.rb                              |  1 +
 spec/support/compress_helper.rb                  |  1 +
 spec/support/corpus.rb                           |  1 +
 spec/support/fake_actor.rb                       |  3 ++-
 spec/support/file_system.rb                      |  1 +
 spec/support/ice_nine_config.rb                  |  1 +
 spec/support/rb_bug.rb                           |  1 +
 spec/support/ruby_vm.rb                          |  1 +
 spec/support/shared_context.rb                   |  1 +
 spec/support/test_app.rb                         |  1 +
 spec/support/warning.rb                          |  1 +
 spec/support/xspec.rb                            |  1 +
 spec/unit/mutant/actor/binding_spec.rb           |  1 +
 spec/unit/mutant/actor/env_spec.rb               |  1 +
 spec/unit/mutant/actor/mailbox_spec.rb           |  1 +
 spec/unit/mutant/actor/message_spec.rb           |  1 +
 spec/unit/mutant/actor/receiver_spec.rb          |  1 +
 spec/unit/mutant/actor/sender_spec.rb            |  1 +
 spec/unit/mutant/ast/meta/optarg_spec.rb         |  1 +
 .../mutant/ast/meta/send/proc_predicate_spec.rb  |  1 +
 ...er_possible_top_level_const_predicate_spec.rb |  1 +
 spec/unit/mutant/ast/meta/send_spec.rb           |  1 +
 spec/unit/mutant/ast/named_children_spec.rb      |  1 +
 spec/unit/mutant/ast/regexp/parse_spec.rb        |  1 +
 .../ast/regexp/supported_predicate_spec.rb       |  1 +
 .../transformer/lookup_table/table_spec.rb       |  1 +
 .../ast/regexp/transformer/lookup_table_spec.rb  |  1 +
 spec/unit/mutant/ast/regexp/transformer_spec.rb  |  1 +
 spec/unit/mutant/ast/regexp_spec.rb              |  1 +
 spec/unit/mutant/ast/sexp_spec.rb                |  1 +
 spec/unit/mutant/ast_spec.rb                     |  1 +
 spec/unit/mutant/cli_spec.rb                     |  1 +
 spec/unit/mutant/context_spec.rb                 |  1 +
 spec/unit/mutant/diff_spec.rb                    |  1 +
 spec/unit/mutant/env/bootstrap_spec.rb           |  1 +
 spec/unit/mutant/env_spec.rb                     |  1 +
 spec/unit/mutant/expression/method_spec.rb       |  1 +
 spec/unit/mutant/expression/methods_spec.rb      |  1 +
 .../mutant/expression/namespace/exact_spec.rb    |  1 +
 .../expression/namespace/recursive_spec.rb       |  1 +
 spec/unit/mutant/expression/parser_spec.rb       |  1 +
 spec/unit/mutant/expression_spec.rb              |  1 +
 spec/unit/mutant/integration/rspec_spec.rb       |  1 +
 spec/unit/mutant/integration_spec.rb             |  1 +
 spec/unit/mutant/isolation/fork_spec.rb          |  1 +
 spec/unit/mutant/isolation/none_spec.rb          |  1 +
 spec/unit/mutant/loader_spec.rb                  |  1 +
 spec/unit/mutant/matcher/chain_spec.rb           |  1 +
 .../matcher/compiler/subject_prefix_spec.rb      |  1 +
 spec/unit/mutant/matcher/compiler_spec.rb        |  1 +
 spec/unit/mutant/matcher/config_spec.rb          |  1 +
 spec/unit/mutant/matcher/filter_spec.rb          |  1 +
 spec/unit/mutant/matcher/method/instance_spec.rb |  1 +
 .../unit/mutant/matcher/method/singleton_spec.rb |  1 +
 .../unit/mutant/matcher/methods/instance_spec.rb |  1 +
 .../mutant/matcher/methods/singleton_spec.rb     |  1 +
 spec/unit/mutant/matcher/namespace_spec.rb       |  1 +
 spec/unit/mutant/matcher/null_spec.rb            |  1 +
 spec/unit/mutant/matcher/scope_spec.rb           |  1 +
 spec/unit/mutant/matcher/static_spec.rb          |  1 +
 spec/unit/mutant/meta/example/dsl_spec.rb        |  1 +
 .../mutant/meta/example/verification_spec.rb     |  1 +
 spec/unit/mutant/meta/example_spec.rb            |  1 +
 spec/unit/mutant/mutation_spec.rb                |  1 +
 spec/unit/mutant/mutator/node_spec.rb            |  1 +
 spec/unit/mutant/mutator_spec.rb                 |  1 +
 spec/unit/mutant/parallel/driver_spec.rb         |  1 +
 spec/unit/mutant/parallel/master_spec.rb         |  1 +
 spec/unit/mutant/parallel/source/array_spec.rb   |  1 +
 spec/unit/mutant/parallel/worker_spec.rb         |  1 +
 spec/unit/mutant/parallel_spec.rb                |  1 +
 spec/unit/mutant/parser_spec.rb                  |  1 +
 spec/unit/mutant/registry_spec.rb                |  1 +
 .../mutant/reporter/cli/printer/config_spec.rb   |  1 +
 .../reporter/cli/printer/env_progress_spec.rb    |  1 +
 .../reporter/cli/printer/env_result_spec.rb      |  1 +
 .../cli/printer/mutation_progress_result_spec.rb |  1 +
 .../reporter/cli/printer/mutation_result_spec.rb |  1 +
 .../cli/printer/status_progressive_spec.rb       |  1 +
 .../mutant/reporter/cli/printer/status_spec.rb   |  1 +
 .../cli/printer/subject_progress_spec.rb         |  1 +
 .../reporter/cli/printer/subject_result_spec.rb  |  1 +
 .../reporter/cli/printer/test_result_spec.rb     |  1 +
 spec/unit/mutant/reporter/cli/printer_spec.rb    |  7 ++++---
 spec/unit/mutant/reporter/cli/tput_spec.rb       |  1 +
 spec/unit/mutant/reporter/cli_spec.rb            |  3 ++-
 spec/unit/mutant/reporter/null_spec.rb           |  1 +
 spec/unit/mutant/reporter/sequence_spec.rb       |  1 +
 spec/unit/mutant/repository/diff_spec.rb         |  1 +
 .../mutant/repository/subject_filter_spec.rb     |  1 +
 spec/unit/mutant/require_highjack_spec.rb        |  1 +
 spec/unit/mutant/result/class_methods_spec.rb    |  1 +
 spec/unit/mutant/result/env_spec.rb              |  1 +
 spec/unit/mutant/result/mutation_spec.rb         |  1 +
 spec/unit/mutant/result/subject_spec.rb          |  1 +
 spec/unit/mutant/result_spec.rb                  |  1 +
 spec/unit/mutant/runner/sink_spec.rb             |  1 +
 spec/unit/mutant/runner_spec.rb                  |  1 +
 spec/unit/mutant/selector/expression_spec.rb     |  1 +
 spec/unit/mutant/subject/method/instance_spec.rb |  1 +
 .../unit/mutant/subject/method/singleton_spec.rb |  1 +
 spec/unit/mutant/subject_spec.rb                 |  1 +
 spec/unit/mutant/util/one_spec.rb                |  1 +
 spec/unit/mutant/warning_filter_spec.rb          |  1 +
 spec/unit/mutant/zombifier_spec.rb               |  1 +
 spec/unit/mutant_spec.rb                         |  1 +
 363 files changed, 384 insertions(+), 20 deletions(-)

diff --git a/.rubocop.yml b/.rubocop.yml
index 17439fba..2e0afbaa 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -4,4 +4,4 @@ AllCops:
     - 'test_app/**/*'
     - 'tmp/**/*'
     - 'vendor/**/*'
-  TargetRubyVersion: 2.2
+  TargetRubyVersion: 2.3
diff --git a/Changelog.md b/Changelog.md
index d525cb16..e5226176 100644
--- a/Changelog.md
+++ b/Changelog.md
@@ -1,3 +1,8 @@
+# v0.8.17 2018-09-12
+
+* Drop support for Ruby < 2.3
+* Use frozen string literals globally
+
 # v0.8.16 2018-08-03
 
 * Support for rspec-3.x
diff --git a/Gemfile b/Gemfile
index 85ca0406..d8ae1079 100644
--- a/Gemfile
+++ b/Gemfile
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 source 'https://rubygems.org'
 
 gemspec name: 'mutant'
diff --git a/Gemfile.lock b/Gemfile.lock
index ef3d933f..10b53e18 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -1,7 +1,7 @@
 PATH
   remote: .
   specs:
-    mutant (0.8.14)
+    mutant (0.8.16)
       abstract_type (~> 0.0.7)
       adamantium (~> 0.2.0)
       anima (~> 0.3.0)
@@ -17,9 +17,9 @@ PATH
       procto (~> 0.0.2)
       regexp_parser (~> 0.4.9)
       unparser (~> 0.2.5)
-    mutant-rspec (0.8.14)
-      mutant (~> 0.8.14)
-      rspec-core (>= 3.4.0, < 3.8.0)
+    mutant-rspec (0.8.16)
+      mutant (~> 0.8.16)
+      rspec-core (>= 3.4.0, < 4.0.0)
 
 GEM
   remote: https://rubygems.org/
@@ -105,7 +105,7 @@ GEM
       codeclimate-engine-rb (~> 0.4.0)
       parser (>= 2.4.0.0, < 2.5)
       rainbow (~> 2.0)
-    regexp_parser (0.4.9)
+    regexp_parser (0.4.13)
     rspec (3.5.0)
       rspec-core (~> 3.5.0)
       rspec-expectations (~> 3.5.0)
@@ -139,13 +139,13 @@ GEM
     simplecov-html (0.10.1)
     thread_safe (0.3.6)
     unicode-display_width (1.3.0)
-    unparser (0.2.6)
+    unparser (0.2.8)
       abstract_type (~> 0.0.7)
       adamantium (~> 0.2.0)
       concord (~> 0.1.5)
       diff-lcs (~> 1.3)
       equalizer (~> 0.0.9)
-      parser (>= 2.3.1.2, < 2.5)
+      parser (>= 2.3.1.2, < 2.6)
       procto (~> 0.0.2)
     virtus (1.0.5)
       axiom-types (~> 0.1)
@@ -166,4 +166,4 @@ DEPENDENCIES
   mutant!
 
 BUNDLED WITH
-   1.15.4
+   1.16.4
diff --git a/Rakefile b/Rakefile
index aebe5ca7..dc3686d2 100644
--- a/Rakefile
+++ b/Rakefile
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require 'devtools'
 
 Devtools.init_rake_tasks
diff --git a/bin/mutant b/bin/mutant
index 7472a46c..e4606d3e 100755
--- a/bin/mutant
+++ b/bin/mutant
@@ -1,4 +1,5 @@
 #!/usr/bin/env ruby
+# frozen_string_literal: true
 
 trap('INT') do |status|
   effective_status = status ? status + 128 : 128
diff --git a/config/flay.yml b/config/flay.yml
index 09a2c4f7..7c6ca935 100644
--- a/config/flay.yml
+++ b/config/flay.yml
@@ -1,3 +1,3 @@
 ---
 threshold: 16
-total_score: 1316
+total_score: 1318
diff --git a/lib/mutant.rb b/lib/mutant.rb
index 9e56253b..63341890 100644
--- a/lib/mutant.rb
+++ b/lib/mutant.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require 'abstract_type'
 require 'adamantium'
 require 'anima'
diff --git a/lib/mutant/actor.rb b/lib/mutant/actor.rb
index 1250aaa8..cba5fa62 100644
--- a/lib/mutant/actor.rb
+++ b/lib/mutant/actor.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # A minimal actor implementation
   module Actor
diff --git a/lib/mutant/actor/env.rb b/lib/mutant/actor/env.rb
index 5ffc1cfd..7cee88f4 100644
--- a/lib/mutant/actor/env.rb
+++ b/lib/mutant/actor/env.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module Actor
     # Actor root environment
diff --git a/lib/mutant/actor/mailbox.rb b/lib/mutant/actor/mailbox.rb
index f9958e0a..ae0e637a 100644
--- a/lib/mutant/actor/mailbox.rb
+++ b/lib/mutant/actor/mailbox.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module Actor
     # Unbound mailbox
diff --git a/lib/mutant/actor/receiver.rb b/lib/mutant/actor/receiver.rb
index a4766ddb..239f532a 100644
--- a/lib/mutant/actor/receiver.rb
+++ b/lib/mutant/actor/receiver.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module Actor
     # Receiver side of an actor
diff --git a/lib/mutant/actor/sender.rb b/lib/mutant/actor/sender.rb
index 9c903ae2..3d9c1d34 100644
--- a/lib/mutant/actor/sender.rb
+++ b/lib/mutant/actor/sender.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module Actor
 
diff --git a/lib/mutant/ast.rb b/lib/mutant/ast.rb
index 7c161bec..3f4bcd38 100644
--- a/lib/mutant/ast.rb
+++ b/lib/mutant/ast.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # AST helpers
   module AST
diff --git a/lib/mutant/ast/meta.rb b/lib/mutant/ast/meta.rb
index ed0ab95c..2bf949b5 100644
--- a/lib/mutant/ast/meta.rb
+++ b/lib/mutant/ast/meta.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module AST
     # Node meta information mixin
diff --git a/lib/mutant/ast/meta/const.rb b/lib/mutant/ast/meta/const.rb
index 2f3b01d6..5878b181 100644
--- a/lib/mutant/ast/meta/const.rb
+++ b/lib/mutant/ast/meta/const.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module AST
     # Node meta information mixin
diff --git a/lib/mutant/ast/meta/optarg.rb b/lib/mutant/ast/meta/optarg.rb
index a2fcfe9f..85cbf9c8 100644
--- a/lib/mutant/ast/meta/optarg.rb
+++ b/lib/mutant/ast/meta/optarg.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module AST
     # Node meta information mixin
diff --git a/lib/mutant/ast/meta/resbody.rb b/lib/mutant/ast/meta/resbody.rb
index fe34790d..f297f77c 100644
--- a/lib/mutant/ast/meta/resbody.rb
+++ b/lib/mutant/ast/meta/resbody.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module AST
     # Node meta information mixin
diff --git a/lib/mutant/ast/meta/restarg.rb b/lib/mutant/ast/meta/restarg.rb
index 43705868..48757ad9 100644
--- a/lib/mutant/ast/meta/restarg.rb
+++ b/lib/mutant/ast/meta/restarg.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module AST
     # Node meta information mixin
diff --git a/lib/mutant/ast/meta/send.rb b/lib/mutant/ast/meta/send.rb
index e529cc52..4c6a0b39 100644
--- a/lib/mutant/ast/meta/send.rb
+++ b/lib/mutant/ast/meta/send.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module AST
     # Node meta information mixin
diff --git a/lib/mutant/ast/meta/symbol.rb b/lib/mutant/ast/meta/symbol.rb
index 941738ed..3668efc9 100644
--- a/lib/mutant/ast/meta/symbol.rb
+++ b/lib/mutant/ast/meta/symbol.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module AST
     # Node meta information mixin
diff --git a/lib/mutant/ast/named_children.rb b/lib/mutant/ast/named_children.rb
index 1f499cc6..9f7cc7ce 100644
--- a/lib/mutant/ast/named_children.rb
+++ b/lib/mutant/ast/named_children.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module AST
 
diff --git a/lib/mutant/ast/node_predicates.rb b/lib/mutant/ast/node_predicates.rb
index b3143d8d..01181b53 100644
--- a/lib/mutant/ast/node_predicates.rb
+++ b/lib/mutant/ast/node_predicates.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module AST
     # Module for node predicates
diff --git a/lib/mutant/ast/nodes.rb b/lib/mutant/ast/nodes.rb
index bb5eef74..f63bcee1 100644
--- a/lib/mutant/ast/nodes.rb
+++ b/lib/mutant/ast/nodes.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module AST
     # Singleton nodes
diff --git a/lib/mutant/ast/regexp.rb b/lib/mutant/ast/regexp.rb
index 545e8081..02186732 100644
--- a/lib/mutant/ast/regexp.rb
+++ b/lib/mutant/ast/regexp.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module AST
     # Regexp source mapper
diff --git a/lib/mutant/ast/regexp/transformer.rb b/lib/mutant/ast/regexp/transformer.rb
index 482431c2..bd40ec7b 100644
--- a/lib/mutant/ast/regexp/transformer.rb
+++ b/lib/mutant/ast/regexp/transformer.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module AST
     module Regexp
diff --git a/lib/mutant/ast/regexp/transformer/alternative.rb b/lib/mutant/ast/regexp/transformer/alternative.rb
index 5e4db606..2fc029b8 100644
--- a/lib/mutant/ast/regexp/transformer/alternative.rb
+++ b/lib/mutant/ast/regexp/transformer/alternative.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module AST
     module Regexp
diff --git a/lib/mutant/ast/regexp/transformer/character_set.rb b/lib/mutant/ast/regexp/transformer/character_set.rb
index 4030aa03..b83ff399 100644
--- a/lib/mutant/ast/regexp/transformer/character_set.rb
+++ b/lib/mutant/ast/regexp/transformer/character_set.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module AST
     module Regexp
diff --git a/lib/mutant/ast/regexp/transformer/direct.rb b/lib/mutant/ast/regexp/transformer/direct.rb
index 70994cd0..1d0e8cea 100644
--- a/lib/mutant/ast/regexp/transformer/direct.rb
+++ b/lib/mutant/ast/regexp/transformer/direct.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module AST
     module Regexp
diff --git a/lib/mutant/ast/regexp/transformer/options_group.rb b/lib/mutant/ast/regexp/transformer/options_group.rb
index 5918b910..345ade30 100644
--- a/lib/mutant/ast/regexp/transformer/options_group.rb
+++ b/lib/mutant/ast/regexp/transformer/options_group.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module AST
     module Regexp
diff --git a/lib/mutant/ast/regexp/transformer/quantifier.rb b/lib/mutant/ast/regexp/transformer/quantifier.rb
index 90d3b2e6..b1184396 100644
--- a/lib/mutant/ast/regexp/transformer/quantifier.rb
+++ b/lib/mutant/ast/regexp/transformer/quantifier.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module AST
     module Regexp
@@ -99,7 +100,7 @@ module Mutant
             #
             # @return [String]
             def interval_text
-              interval = [min, max].map { |num| num if num > 0 }.uniq
+              interval = [min, max].map { |num| num if num.positive? }.uniq
               "{#{interval.join(',')}}"
             end
           end # ASTToExpression
diff --git a/lib/mutant/ast/regexp/transformer/recursive.rb b/lib/mutant/ast/regexp/transformer/recursive.rb
index 914750ad..bf5a1dad 100644
--- a/lib/mutant/ast/regexp/transformer/recursive.rb
+++ b/lib/mutant/ast/regexp/transformer/recursive.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module AST
     module Regexp
diff --git a/lib/mutant/ast/regexp/transformer/root.rb b/lib/mutant/ast/regexp/transformer/root.rb
index b7c196b2..b1b73533 100644
--- a/lib/mutant/ast/regexp/transformer/root.rb
+++ b/lib/mutant/ast/regexp/transformer/root.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module AST
     module Regexp
diff --git a/lib/mutant/ast/regexp/transformer/text.rb b/lib/mutant/ast/regexp/transformer/text.rb
index f2c2cc9b..bec52e9c 100644
--- a/lib/mutant/ast/regexp/transformer/text.rb
+++ b/lib/mutant/ast/regexp/transformer/text.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module AST
     module Regexp
diff --git a/lib/mutant/ast/sexp.rb b/lib/mutant/ast/sexp.rb
index 1bb2ed9d..f06a9ecc 100644
--- a/lib/mutant/ast/sexp.rb
+++ b/lib/mutant/ast/sexp.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module AST
     # Mixin for node sexp syntax
diff --git a/lib/mutant/ast/types.rb b/lib/mutant/ast/types.rb
index 48818017..18f68344 100644
--- a/lib/mutant/ast/types.rb
+++ b/lib/mutant/ast/types.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module AST
     # Groups of node types
diff --git a/lib/mutant/cli.rb b/lib/mutant/cli.rb
index f70580da..b5fd7994 100644
--- a/lib/mutant/cli.rb
+++ b/lib/mutant/cli.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # Commandline parser / runner
   class CLI
diff --git a/lib/mutant/color.rb b/lib/mutant/color.rb
index 703eeeed..75680b43 100644
--- a/lib/mutant/color.rb
+++ b/lib/mutant/color.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # Class to colorize strings
   class Color
diff --git a/lib/mutant/config.rb b/lib/mutant/config.rb
index 3bfee82a..d2ea5cb5 100644
--- a/lib/mutant/config.rb
+++ b/lib/mutant/config.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # Standalone configuration of a mutant execution.
   #
diff --git a/lib/mutant/context.rb b/lib/mutant/context.rb
index 52697ac8..c725a83f 100644
--- a/lib/mutant/context.rb
+++ b/lib/mutant/context.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # An abstract context where mutations can be applied to.
   class Context
diff --git a/lib/mutant/diff.rb b/lib/mutant/diff.rb
index 8bdaea59..16e0fda2 100644
--- a/lib/mutant/diff.rb
+++ b/lib/mutant/diff.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # Class to create diffs from source code
   class Diff
@@ -17,7 +18,7 @@ module Mutant
     def diff
       return if diffs.empty?
 
-      minimized_hunk.diff(:unified) << NEWLINE
+      minimized_hunk.diff(:unified) + NEWLINE
     end
     memoize :diff
 
@@ -68,7 +69,7 @@ module Mutant
     # @return [Array<Diff::LCS::Hunk>]
     def hunks
       diffs.map do |diff|
-        ::Diff::LCS::Hunk.new(old, new, diff, max_length, 0)
+        ::Diff::LCS::Hunk.new(old.map(&:dup), new, diff, max_length, 0)
       end
     end
 
diff --git a/lib/mutant/env.rb b/lib/mutant/env.rb
index 06a0215a..7b62ef7b 100644
--- a/lib/mutant/env.rb
+++ b/lib/mutant/env.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # Abstract base class for mutant environments
   class Env
diff --git a/lib/mutant/env/bootstrap.rb b/lib/mutant/env/bootstrap.rb
index 6675d056..03b62114 100644
--- a/lib/mutant/env/bootstrap.rb
+++ b/lib/mutant/env/bootstrap.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Env
     # Bootstrap environment
diff --git a/lib/mutant/expression.rb b/lib/mutant/expression.rb
index 6661bb66..33ecd64a 100644
--- a/lib/mutant/expression.rb
+++ b/lib/mutant/expression.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
 
   # Abstract base class for match expression
diff --git a/lib/mutant/expression/method.rb b/lib/mutant/expression/method.rb
index 4910c490..ae743206 100644
--- a/lib/mutant/expression/method.rb
+++ b/lib/mutant/expression/method.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Expression
 
diff --git a/lib/mutant/expression/methods.rb b/lib/mutant/expression/methods.rb
index 2d390985..e0395e9b 100644
--- a/lib/mutant/expression/methods.rb
+++ b/lib/mutant/expression/methods.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Expression
 
diff --git a/lib/mutant/expression/namespace.rb b/lib/mutant/expression/namespace.rb
index e787a187..11e9d991 100644
--- a/lib/mutant/expression/namespace.rb
+++ b/lib/mutant/expression/namespace.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Expression
     # Abstract base class for expressions matching namespaces
diff --git a/lib/mutant/expression/parser.rb b/lib/mutant/expression/parser.rb
index e886276c..0b39b468 100644
--- a/lib/mutant/expression/parser.rb
+++ b/lib/mutant/expression/parser.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Expression
     class Parser
diff --git a/lib/mutant/integration.rb b/lib/mutant/integration.rb
index 994a4a66..7acffe31 100644
--- a/lib/mutant/integration.rb
+++ b/lib/mutant/integration.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
 
   # Abstract base class mutant test framework integrations
diff --git a/lib/mutant/integration/rspec.rb b/lib/mutant/integration/rspec.rb
index 29ab09fc..e313b0c1 100644
--- a/lib/mutant/integration/rspec.rb
+++ b/lib/mutant/integration/rspec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require 'rspec/core'
 
 module Mutant
diff --git a/lib/mutant/isolation.rb b/lib/mutant/isolation.rb
index 0216f753..fbc68421 100644
--- a/lib/mutant/isolation.rb
+++ b/lib/mutant/isolation.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Isolation
     include AbstractType
diff --git a/lib/mutant/isolation/fork.rb b/lib/mutant/isolation/fork.rb
index e2476478..6a5a18be 100644
--- a/lib/mutant/isolation/fork.rb
+++ b/lib/mutant/isolation/fork.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Isolation
     # Isolation via the fork(2) systemcall.
diff --git a/lib/mutant/isolation/none.rb b/lib/mutant/isolation/none.rb
index 157ef979..d91afbaa 100644
--- a/lib/mutant/isolation/none.rb
+++ b/lib/mutant/isolation/none.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # Module providing isolation
   class Isolation
diff --git a/lib/mutant/loader.rb b/lib/mutant/loader.rb
index a83b75f2..a159f885 100644
--- a/lib/mutant/loader.rb
+++ b/lib/mutant/loader.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # Base class for code loaders
   class Loader
diff --git a/lib/mutant/matcher.rb b/lib/mutant/matcher.rb
index 7f8336b1..46303686 100644
--- a/lib/mutant/matcher.rb
+++ b/lib/mutant/matcher.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # Abstract matcher to find subjects to mutate
   class Matcher
diff --git a/lib/mutant/matcher/chain.rb b/lib/mutant/matcher/chain.rb
index 7f1d2e29..62473ac9 100644
--- a/lib/mutant/matcher/chain.rb
+++ b/lib/mutant/matcher/chain.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Matcher
     # Matcher chaining results of other matchers together
diff --git a/lib/mutant/matcher/compiler.rb b/lib/mutant/matcher/compiler.rb
index 5841664e..0f39075d 100644
--- a/lib/mutant/matcher/compiler.rb
+++ b/lib/mutant/matcher/compiler.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Matcher
 
diff --git a/lib/mutant/matcher/config.rb b/lib/mutant/matcher/config.rb
index 19586230..df6ae34a 100644
--- a/lib/mutant/matcher/config.rb
+++ b/lib/mutant/matcher/config.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Matcher
     # Subject matcher configuration
diff --git a/lib/mutant/matcher/filter.rb b/lib/mutant/matcher/filter.rb
index 11ad2662..02bf5c8a 100644
--- a/lib/mutant/matcher/filter.rb
+++ b/lib/mutant/matcher/filter.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Matcher
     # Matcher filter
diff --git a/lib/mutant/matcher/method.rb b/lib/mutant/matcher/method.rb
index 6e175999..3bdeb43b 100644
--- a/lib/mutant/matcher/method.rb
+++ b/lib/mutant/matcher/method.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Matcher
     # Abstract base class for method matchers
diff --git a/lib/mutant/matcher/method/instance.rb b/lib/mutant/matcher/method/instance.rb
index 2ac92d74..a96a8bf5 100644
--- a/lib/mutant/matcher/method/instance.rb
+++ b/lib/mutant/matcher/method/instance.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Matcher
     class Method
diff --git a/lib/mutant/matcher/method/singleton.rb b/lib/mutant/matcher/method/singleton.rb
index 4d5d2d59..579fcd02 100644
--- a/lib/mutant/matcher/method/singleton.rb
+++ b/lib/mutant/matcher/method/singleton.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Matcher
     class Method
diff --git a/lib/mutant/matcher/methods.rb b/lib/mutant/matcher/methods.rb
index 77b11962..f23e6fd5 100644
--- a/lib/mutant/matcher/methods.rb
+++ b/lib/mutant/matcher/methods.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Matcher
     # Abstract base class for matcher that returns method subjects from scope
diff --git a/lib/mutant/matcher/namespace.rb b/lib/mutant/matcher/namespace.rb
index 71069a5e..7c8a69ab 100644
--- a/lib/mutant/matcher/namespace.rb
+++ b/lib/mutant/matcher/namespace.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Matcher
     # Matcher for specific namespace
diff --git a/lib/mutant/matcher/null.rb b/lib/mutant/matcher/null.rb
index 131cda5e..133906c0 100644
--- a/lib/mutant/matcher/null.rb
+++ b/lib/mutant/matcher/null.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Matcher
     # A null matcher, that does not match any subjects
diff --git a/lib/mutant/matcher/scope.rb b/lib/mutant/matcher/scope.rb
index 025fe299..b56edc9b 100644
--- a/lib/mutant/matcher/scope.rb
+++ b/lib/mutant/matcher/scope.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Matcher
     # Matcher expanding Mutant::Scope objects into method matches
diff --git a/lib/mutant/matcher/static.rb b/lib/mutant/matcher/static.rb
index f84d0b3a..fbced4ef 100644
--- a/lib/mutant/matcher/static.rb
+++ b/lib/mutant/matcher/static.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Matcher
     # Matcher returning subjects already known at its creation time
diff --git a/lib/mutant/meta.rb b/lib/mutant/meta.rb
index 951976bf..8d29bd33 100644
--- a/lib/mutant/meta.rb
+++ b/lib/mutant/meta.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # Namespace for mutant metadata
   module Meta
diff --git a/lib/mutant/meta/example.rb b/lib/mutant/meta/example.rb
index 80dfcef1..7b2ac6a9 100644
--- a/lib/mutant/meta/example.rb
+++ b/lib/mutant/meta/example.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module Meta
     class Example
diff --git a/lib/mutant/meta/example/dsl.rb b/lib/mutant/meta/example/dsl.rb
index 091f38a0..7b408298 100644
--- a/lib/mutant/meta/example/dsl.rb
+++ b/lib/mutant/meta/example/dsl.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module Meta
     class Example
diff --git a/lib/mutant/meta/example/verification.rb b/lib/mutant/meta/example/verification.rb
index 2f1992c9..79fedd98 100644
--- a/lib/mutant/meta/example/verification.rb
+++ b/lib/mutant/meta/example/verification.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module Meta
     class Example
diff --git a/lib/mutant/mutation.rb b/lib/mutant/mutation.rb
index 77e26fff..573e0daf 100644
--- a/lib/mutant/mutation.rb
+++ b/lib/mutant/mutation.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # Represent a mutated node with its subject
   class Mutation
diff --git a/lib/mutant/mutator.rb b/lib/mutant/mutator.rb
index 29f24ec8..43a115d9 100644
--- a/lib/mutant/mutator.rb
+++ b/lib/mutant/mutator.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # Generator for mutations
   class Mutator
diff --git a/lib/mutant/mutator/node.rb b/lib/mutant/mutator/node.rb
index 9824a0f9..e3602289 100644
--- a/lib/mutant/mutator/node.rb
+++ b/lib/mutant/mutator/node.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
 
   # Generator for mutations
@@ -124,7 +125,7 @@ module Mutant
       # @return [nil]
       #   otherwise
       def parent_node
-        parent.node if parent
+        parent&.node
       end
 
       # Parent type
@@ -135,7 +136,7 @@ module Mutant
       # @return [nil]
       #   otherwise
       def parent_type
-        parent_node.type if parent_node
+        parent_node&.type
       end
 
       # Test if the node is the left of an or_asgn or op_asgn
diff --git a/lib/mutant/mutator/node/and_asgn.rb b/lib/mutant/mutator/node/and_asgn.rb
index 7fa5b475..1cfdccde 100644
--- a/lib/mutant/mutator/node/and_asgn.rb
+++ b/lib/mutant/mutator/node/and_asgn.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/argument.rb b/lib/mutant/mutator/node/argument.rb
index fbb2266c..48089788 100644
--- a/lib/mutant/mutator/node/argument.rb
+++ b/lib/mutant/mutator/node/argument.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/arguments.rb b/lib/mutant/mutator/node/arguments.rb
index 1114a05a..93fa9015 100644
--- a/lib/mutant/mutator/node/arguments.rb
+++ b/lib/mutant/mutator/node/arguments.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/begin.rb b/lib/mutant/mutator/node/begin.rb
index 8c4b6cb4..2f2184b9 100644
--- a/lib/mutant/mutator/node/begin.rb
+++ b/lib/mutant/mutator/node/begin.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/binary.rb b/lib/mutant/mutator/node/binary.rb
index a951408b..9e5cbc11 100644
--- a/lib/mutant/mutator/node/binary.rb
+++ b/lib/mutant/mutator/node/binary.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/block.rb b/lib/mutant/mutator/node/block.rb
index ba67179b..cd681a96 100644
--- a/lib/mutant/mutator/node/block.rb
+++ b/lib/mutant/mutator/node/block.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/break.rb b/lib/mutant/mutator/node/break.rb
index 4ad86df0..d52da168 100644
--- a/lib/mutant/mutator/node/break.rb
+++ b/lib/mutant/mutator/node/break.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/case.rb b/lib/mutant/mutator/node/case.rb
index d3ca016b..eff072bb 100644
--- a/lib/mutant/mutator/node/case.rb
+++ b/lib/mutant/mutator/node/case.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/class.rb b/lib/mutant/mutator/node/class.rb
index 2553b428..b5e16fc9 100644
--- a/lib/mutant/mutator/node/class.rb
+++ b/lib/mutant/mutator/node/class.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/conditional_loop.rb b/lib/mutant/mutator/node/conditional_loop.rb
index 0afda4ee..91b6589c 100644
--- a/lib/mutant/mutator/node/conditional_loop.rb
+++ b/lib/mutant/mutator/node/conditional_loop.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/const.rb b/lib/mutant/mutator/node/const.rb
index b63819f6..4470a912 100644
--- a/lib/mutant/mutator/node/const.rb
+++ b/lib/mutant/mutator/node/const.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/define.rb b/lib/mutant/mutator/node/define.rb
index 5e18d214..b475903f 100644
--- a/lib/mutant/mutator/node/define.rb
+++ b/lib/mutant/mutator/node/define.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/defined.rb b/lib/mutant/mutator/node/defined.rb
index a98426bb..48628e42 100644
--- a/lib/mutant/mutator/node/defined.rb
+++ b/lib/mutant/mutator/node/defined.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/dstr.rb b/lib/mutant/mutator/node/dstr.rb
index 0a6335e5..5f318bb2 100644
--- a/lib/mutant/mutator/node/dstr.rb
+++ b/lib/mutant/mutator/node/dstr.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/dsym.rb b/lib/mutant/mutator/node/dsym.rb
index 973d8878..12646d37 100644
--- a/lib/mutant/mutator/node/dsym.rb
+++ b/lib/mutant/mutator/node/dsym.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/generic.rb b/lib/mutant/mutator/node/generic.rb
index 90f86fb3..ec8c3b05 100644
--- a/lib/mutant/mutator/node/generic.rb
+++ b/lib/mutant/mutator/node/generic.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/if.rb b/lib/mutant/mutator/node/if.rb
index 91c9b2ba..85b7a5f5 100644
--- a/lib/mutant/mutator/node/if.rb
+++ b/lib/mutant/mutator/node/if.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/kwbegin.rb b/lib/mutant/mutator/node/kwbegin.rb
index 1ad6185e..e5134a3e 100644
--- a/lib/mutant/mutator/node/kwbegin.rb
+++ b/lib/mutant/mutator/node/kwbegin.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/literal.rb b/lib/mutant/mutator/node/literal.rb
index 87831284..084c735f 100644
--- a/lib/mutant/mutator/node/literal.rb
+++ b/lib/mutant/mutator/node/literal.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/literal/array.rb b/lib/mutant/mutator/node/literal/array.rb
index bde60a52..ed81ca59 100644
--- a/lib/mutant/mutator/node/literal/array.rb
+++ b/lib/mutant/mutator/node/literal/array.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/literal/boolean.rb b/lib/mutant/mutator/node/literal/boolean.rb
index 7f879fd9..9f70cf74 100644
--- a/lib/mutant/mutator/node/literal/boolean.rb
+++ b/lib/mutant/mutator/node/literal/boolean.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/literal/float.rb b/lib/mutant/mutator/node/literal/float.rb
index 52c90b2e..ac3104e9 100644
--- a/lib/mutant/mutator/node/literal/float.rb
+++ b/lib/mutant/mutator/node/literal/float.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/literal/hash.rb b/lib/mutant/mutator/node/literal/hash.rb
index 60a016be..f1ba62e8 100644
--- a/lib/mutant/mutator/node/literal/hash.rb
+++ b/lib/mutant/mutator/node/literal/hash.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/literal/integer.rb b/lib/mutant/mutator/node/literal/integer.rb
index 62d291e6..0f76da84 100644
--- a/lib/mutant/mutator/node/literal/integer.rb
+++ b/lib/mutant/mutator/node/literal/integer.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/literal/nil.rb b/lib/mutant/mutator/node/literal/nil.rb
index f6d6ad8c..f6f796a0 100644
--- a/lib/mutant/mutator/node/literal/nil.rb
+++ b/lib/mutant/mutator/node/literal/nil.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/literal/range.rb b/lib/mutant/mutator/node/literal/range.rb
index db2ee90a..53db12d4 100644
--- a/lib/mutant/mutator/node/literal/range.rb
+++ b/lib/mutant/mutator/node/literal/range.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/literal/regex.rb b/lib/mutant/mutator/node/literal/regex.rb
index 8085f7ac..b5892e57 100644
--- a/lib/mutant/mutator/node/literal/regex.rb
+++ b/lib/mutant/mutator/node/literal/regex.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/literal/string.rb b/lib/mutant/mutator/node/literal/string.rb
index 2645cd0e..ed3bc87e 100644
--- a/lib/mutant/mutator/node/literal/string.rb
+++ b/lib/mutant/mutator/node/literal/string.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/literal/symbol.rb b/lib/mutant/mutator/node/literal/symbol.rb
index ae52222d..4b05fc74 100644
--- a/lib/mutant/mutator/node/literal/symbol.rb
+++ b/lib/mutant/mutator/node/literal/symbol.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/masgn.rb b/lib/mutant/mutator/node/masgn.rb
index ec2feeb0..c5ffb4bb 100644
--- a/lib/mutant/mutator/node/masgn.rb
+++ b/lib/mutant/mutator/node/masgn.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/match_current_line.rb b/lib/mutant/mutator/node/match_current_line.rb
index 57420a06..6849a4d2 100644
--- a/lib/mutant/mutator/node/match_current_line.rb
+++ b/lib/mutant/mutator/node/match_current_line.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/mlhs.rb b/lib/mutant/mutator/node/mlhs.rb
index 1ef1b899..601e55a2 100644
--- a/lib/mutant/mutator/node/mlhs.rb
+++ b/lib/mutant/mutator/node/mlhs.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/named_value/access.rb b/lib/mutant/mutator/node/named_value/access.rb
index 85855ae0..95090030 100644
--- a/lib/mutant/mutator/node/named_value/access.rb
+++ b/lib/mutant/mutator/node/named_value/access.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/named_value/constant_assignment.rb b/lib/mutant/mutator/node/named_value/constant_assignment.rb
index f67adfe1..fc4cc0d9 100644
--- a/lib/mutant/mutator/node/named_value/constant_assignment.rb
+++ b/lib/mutant/mutator/node/named_value/constant_assignment.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/named_value/variable_assignment.rb b/lib/mutant/mutator/node/named_value/variable_assignment.rb
index 493cf68f..bdcbd6db 100644
--- a/lib/mutant/mutator/node/named_value/variable_assignment.rb
+++ b/lib/mutant/mutator/node/named_value/variable_assignment.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/next.rb b/lib/mutant/mutator/node/next.rb
index 7791b717..8463d711 100644
--- a/lib/mutant/mutator/node/next.rb
+++ b/lib/mutant/mutator/node/next.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/noop.rb b/lib/mutant/mutator/node/noop.rb
index 81021ade..8ba82cdb 100644
--- a/lib/mutant/mutator/node/noop.rb
+++ b/lib/mutant/mutator/node/noop.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/nthref.rb b/lib/mutant/mutator/node/nthref.rb
index 68c13448..d09e6426 100644
--- a/lib/mutant/mutator/node/nthref.rb
+++ b/lib/mutant/mutator/node/nthref.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/op_asgn.rb b/lib/mutant/mutator/node/op_asgn.rb
index 7d03ab95..ab1e78bf 100644
--- a/lib/mutant/mutator/node/op_asgn.rb
+++ b/lib/mutant/mutator/node/op_asgn.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/or_asgn.rb b/lib/mutant/mutator/node/or_asgn.rb
index 2a108888..5ef04f34 100644
--- a/lib/mutant/mutator/node/or_asgn.rb
+++ b/lib/mutant/mutator/node/or_asgn.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/regexp.rb b/lib/mutant/mutator/node/regexp.rb
index 5c6e44d3..0fbaa4c0 100644
--- a/lib/mutant/mutator/node/regexp.rb
+++ b/lib/mutant/mutator/node/regexp.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/regexp/alternation_meta.rb b/lib/mutant/mutator/node/regexp/alternation_meta.rb
index 53a3f387..038c27df 100644
--- a/lib/mutant/mutator/node/regexp/alternation_meta.rb
+++ b/lib/mutant/mutator/node/regexp/alternation_meta.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/regexp/capture_group.rb b/lib/mutant/mutator/node/regexp/capture_group.rb
index c41981b9..a1d6d569 100644
--- a/lib/mutant/mutator/node/regexp/capture_group.rb
+++ b/lib/mutant/mutator/node/regexp/capture_group.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/regexp/character_type.rb b/lib/mutant/mutator/node/regexp/character_type.rb
index e4c75af5..b9ca5ebf 100644
--- a/lib/mutant/mutator/node/regexp/character_type.rb
+++ b/lib/mutant/mutator/node/regexp/character_type.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/regexp/end_of_line_anchor.rb b/lib/mutant/mutator/node/regexp/end_of_line_anchor.rb
index ac91ef8e..e26fdd1b 100644
--- a/lib/mutant/mutator/node/regexp/end_of_line_anchor.rb
+++ b/lib/mutant/mutator/node/regexp/end_of_line_anchor.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/regexp/end_of_string_or_before_end_of_line_anchor.rb b/lib/mutant/mutator/node/regexp/end_of_string_or_before_end_of_line_anchor.rb
index db5224d5..bb9139fa 100644
--- a/lib/mutant/mutator/node/regexp/end_of_string_or_before_end_of_line_anchor.rb
+++ b/lib/mutant/mutator/node/regexp/end_of_string_or_before_end_of_line_anchor.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/regexp/greedy_zero_or_more.rb b/lib/mutant/mutator/node/regexp/greedy_zero_or_more.rb
index 2071717b..368ba921 100644
--- a/lib/mutant/mutator/node/regexp/greedy_zero_or_more.rb
+++ b/lib/mutant/mutator/node/regexp/greedy_zero_or_more.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/regopt.rb b/lib/mutant/mutator/node/regopt.rb
index 5b344d80..8a7343ec 100644
--- a/lib/mutant/mutator/node/regopt.rb
+++ b/lib/mutant/mutator/node/regopt.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/resbody.rb b/lib/mutant/mutator/node/resbody.rb
index 03025f05..0b2cbd9c 100644
--- a/lib/mutant/mutator/node/resbody.rb
+++ b/lib/mutant/mutator/node/resbody.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/rescue.rb b/lib/mutant/mutator/node/rescue.rb
index 33c4adf3..49cebfec 100644
--- a/lib/mutant/mutator/node/rescue.rb
+++ b/lib/mutant/mutator/node/rescue.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/return.rb b/lib/mutant/mutator/node/return.rb
index 3c0f5897..052e98d2 100644
--- a/lib/mutant/mutator/node/return.rb
+++ b/lib/mutant/mutator/node/return.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/send.rb b/lib/mutant/mutator/node/send.rb
index 3dd6a03d..a34721af 100644
--- a/lib/mutant/mutator/node/send.rb
+++ b/lib/mutant/mutator/node/send.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/send/attribute_assignment.rb b/lib/mutant/mutator/node/send/attribute_assignment.rb
index 037f66d5..77798a2b 100644
--- a/lib/mutant/mutator/node/send/attribute_assignment.rb
+++ b/lib/mutant/mutator/node/send/attribute_assignment.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/send/binary.rb b/lib/mutant/mutator/node/send/binary.rb
index 8411f89d..f306f21f 100644
--- a/lib/mutant/mutator/node/send/binary.rb
+++ b/lib/mutant/mutator/node/send/binary.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/send/conditional.rb b/lib/mutant/mutator/node/send/conditional.rb
index e8ea5233..bb2992a0 100644
--- a/lib/mutant/mutator/node/send/conditional.rb
+++ b/lib/mutant/mutator/node/send/conditional.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/send/index.rb b/lib/mutant/mutator/node/send/index.rb
index c0ef2f96..38462f7e 100644
--- a/lib/mutant/mutator/node/send/index.rb
+++ b/lib/mutant/mutator/node/send/index.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/splat.rb b/lib/mutant/mutator/node/splat.rb
index 6e20ddb7..0a7c8889 100644
--- a/lib/mutant/mutator/node/splat.rb
+++ b/lib/mutant/mutator/node/splat.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/super.rb b/lib/mutant/mutator/node/super.rb
index 2d8006f5..19238035 100644
--- a/lib/mutant/mutator/node/super.rb
+++ b/lib/mutant/mutator/node/super.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/when.rb b/lib/mutant/mutator/node/when.rb
index 5f25e205..459888da 100644
--- a/lib/mutant/mutator/node/when.rb
+++ b/lib/mutant/mutator/node/when.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/yield.rb b/lib/mutant/mutator/node/yield.rb
index 2683ead4..7bdc2f54 100644
--- a/lib/mutant/mutator/node/yield.rb
+++ b/lib/mutant/mutator/node/yield.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/node/zsuper.rb b/lib/mutant/mutator/node/zsuper.rb
index 06a495a1..978a1cf8 100644
--- a/lib/mutant/mutator/node/zsuper.rb
+++ b/lib/mutant/mutator/node/zsuper.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Node
diff --git a/lib/mutant/mutator/util.rb b/lib/mutant/mutator/util.rb
index 9d219b79..add7a4c4 100644
--- a/lib/mutant/mutator/util.rb
+++ b/lib/mutant/mutator/util.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     # Namespace for utility mutators
diff --git a/lib/mutant/mutator/util/array.rb b/lib/mutant/mutator/util/array.rb
index 32843e02..a5fd1491 100644
--- a/lib/mutant/mutator/util/array.rb
+++ b/lib/mutant/mutator/util/array.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Util
diff --git a/lib/mutant/mutator/util/symbol.rb b/lib/mutant/mutator/util/symbol.rb
index cf699db1..fdcea9e7 100644
--- a/lib/mutant/mutator/util/symbol.rb
+++ b/lib/mutant/mutator/util/symbol.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Mutator
     class Util
diff --git a/lib/mutant/parallel.rb b/lib/mutant/parallel.rb
index 64c57d9f..50423890 100644
--- a/lib/mutant/parallel.rb
+++ b/lib/mutant/parallel.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # Parallel execution engine of arbitrary payloads
   module Parallel
diff --git a/lib/mutant/parallel/master.rb b/lib/mutant/parallel/master.rb
index e8a95a84..f09eb639 100644
--- a/lib/mutant/parallel/master.rb
+++ b/lib/mutant/parallel/master.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module Parallel
     # Master parallel worker
diff --git a/lib/mutant/parallel/source.rb b/lib/mutant/parallel/source.rb
index 471645e8..edc22470 100644
--- a/lib/mutant/parallel/source.rb
+++ b/lib/mutant/parallel/source.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module Parallel
     # Job source for parallel execution
diff --git a/lib/mutant/parallel/worker.rb b/lib/mutant/parallel/worker.rb
index 0cb9610b..e1a3f2a4 100644
--- a/lib/mutant/parallel/worker.rb
+++ b/lib/mutant/parallel/worker.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module Parallel
     # Parallel execution worker
diff --git a/lib/mutant/parser.rb b/lib/mutant/parser.rb
index 12cd6a6e..22377576 100644
--- a/lib/mutant/parser.rb
+++ b/lib/mutant/parser.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # An AST Parser
   class Parser
diff --git a/lib/mutant/registry.rb b/lib/mutant/registry.rb
index d109cfc3..3bf82143 100644
--- a/lib/mutant/registry.rb
+++ b/lib/mutant/registry.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # Registry for mapping AST types to classes
   class Registry
diff --git a/lib/mutant/reporter.rb b/lib/mutant/reporter.rb
index cccda784..2b0f250c 100644
--- a/lib/mutant/reporter.rb
+++ b/lib/mutant/reporter.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # Abstract base class for reporters
   class Reporter
diff --git a/lib/mutant/reporter/cli.rb b/lib/mutant/reporter/cli.rb
index a9fbd1f4..6a57b395 100644
--- a/lib/mutant/reporter/cli.rb
+++ b/lib/mutant/reporter/cli.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Reporter
     # Reporter that reports in human readable format
diff --git a/lib/mutant/reporter/cli/format.rb b/lib/mutant/reporter/cli/format.rb
index 533ec8bf..3524a5c4 100644
--- a/lib/mutant/reporter/cli/format.rb
+++ b/lib/mutant/reporter/cli/format.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Reporter
     class CLI
diff --git a/lib/mutant/reporter/cli/printer.rb b/lib/mutant/reporter/cli/printer.rb
index 04716829..783f6ca4 100644
--- a/lib/mutant/reporter/cli/printer.rb
+++ b/lib/mutant/reporter/cli/printer.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Reporter
     class CLI
diff --git a/lib/mutant/reporter/cli/printer/config.rb b/lib/mutant/reporter/cli/printer/config.rb
index e0d00720..09ed2729 100644
--- a/lib/mutant/reporter/cli/printer/config.rb
+++ b/lib/mutant/reporter/cli/printer/config.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Reporter
     class CLI
diff --git a/lib/mutant/reporter/cli/printer/env_progress.rb b/lib/mutant/reporter/cli/printer/env_progress.rb
index 1fffdb07..014ead40 100644
--- a/lib/mutant/reporter/cli/printer/env_progress.rb
+++ b/lib/mutant/reporter/cli/printer/env_progress.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Reporter
     class CLI
diff --git a/lib/mutant/reporter/cli/printer/env_result.rb b/lib/mutant/reporter/cli/printer/env_result.rb
index c6ec2bc5..20ca5f08 100644
--- a/lib/mutant/reporter/cli/printer/env_result.rb
+++ b/lib/mutant/reporter/cli/printer/env_result.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Reporter
     class CLI
diff --git a/lib/mutant/reporter/cli/printer/mutation_progress_result.rb b/lib/mutant/reporter/cli/printer/mutation_progress_result.rb
index 4f421cde..baea78d1 100644
--- a/lib/mutant/reporter/cli/printer/mutation_progress_result.rb
+++ b/lib/mutant/reporter/cli/printer/mutation_progress_result.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Reporter
     class CLI
diff --git a/lib/mutant/reporter/cli/printer/mutation_result.rb b/lib/mutant/reporter/cli/printer/mutation_result.rb
index bb9c9092..ed7d149c 100644
--- a/lib/mutant/reporter/cli/printer/mutation_result.rb
+++ b/lib/mutant/reporter/cli/printer/mutation_result.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Reporter
     class CLI
diff --git a/lib/mutant/reporter/cli/printer/status.rb b/lib/mutant/reporter/cli/printer/status.rb
index c3b22633..32044ef9 100644
--- a/lib/mutant/reporter/cli/printer/status.rb
+++ b/lib/mutant/reporter/cli/printer/status.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Reporter
     class CLI
diff --git a/lib/mutant/reporter/cli/printer/status_progressive.rb b/lib/mutant/reporter/cli/printer/status_progressive.rb
index e373b955..ecca5fb9 100644
--- a/lib/mutant/reporter/cli/printer/status_progressive.rb
+++ b/lib/mutant/reporter/cli/printer/status_progressive.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Reporter
     class CLI
diff --git a/lib/mutant/reporter/cli/printer/subject_progress.rb b/lib/mutant/reporter/cli/printer/subject_progress.rb
index e4c22969..b42f67a8 100644
--- a/lib/mutant/reporter/cli/printer/subject_progress.rb
+++ b/lib/mutant/reporter/cli/printer/subject_progress.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Reporter
     class CLI
diff --git a/lib/mutant/reporter/cli/printer/subject_result.rb b/lib/mutant/reporter/cli/printer/subject_result.rb
index 8db29857..52d27b69 100644
--- a/lib/mutant/reporter/cli/printer/subject_result.rb
+++ b/lib/mutant/reporter/cli/printer/subject_result.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Reporter
     class CLI
diff --git a/lib/mutant/reporter/cli/printer/test_result.rb b/lib/mutant/reporter/cli/printer/test_result.rb
index 3533adf4..29937390 100644
--- a/lib/mutant/reporter/cli/printer/test_result.rb
+++ b/lib/mutant/reporter/cli/printer/test_result.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Reporter
     class CLI
diff --git a/lib/mutant/reporter/cli/tput.rb b/lib/mutant/reporter/cli/tput.rb
index 5b9d3cf3..550d2bf9 100644
--- a/lib/mutant/reporter/cli/tput.rb
+++ b/lib/mutant/reporter/cli/tput.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Reporter
     class CLI
diff --git a/lib/mutant/reporter/null.rb b/lib/mutant/reporter/null.rb
index d2e108a8..b1702de4 100644
--- a/lib/mutant/reporter/null.rb
+++ b/lib/mutant/reporter/null.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Reporter
 
diff --git a/lib/mutant/reporter/sequence.rb b/lib/mutant/reporter/sequence.rb
index 6fe51653..6703eaa0 100644
--- a/lib/mutant/reporter/sequence.rb
+++ b/lib/mutant/reporter/sequence.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Reporter
     class Sequence < self
diff --git a/lib/mutant/repository.rb b/lib/mutant/repository.rb
index 9acb718b..cb6797d2 100644
--- a/lib/mutant/repository.rb
+++ b/lib/mutant/repository.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   module Repository
     # Error raised on repository interaction problems
diff --git a/lib/mutant/require_highjack.rb b/lib/mutant/require_highjack.rb
index 507acf76..3e58c0d0 100644
--- a/lib/mutant/require_highjack.rb
+++ b/lib/mutant/require_highjack.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # Require highjack
   module RequireHighjack
diff --git a/lib/mutant/result.rb b/lib/mutant/result.rb
index a13e4cea..2c54c78a 100644
--- a/lib/mutant/result.rb
+++ b/lib/mutant/result.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # Namespace and mixin module for results
   module Result
diff --git a/lib/mutant/runner.rb b/lib/mutant/runner.rb
index 29f9188a..e28c990a 100644
--- a/lib/mutant/runner.rb
+++ b/lib/mutant/runner.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # Runner baseclass
   class Runner
diff --git a/lib/mutant/runner/sink.rb b/lib/mutant/runner/sink.rb
index 426e2ebc..c7a3844d 100644
--- a/lib/mutant/runner/sink.rb
+++ b/lib/mutant/runner/sink.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Runner
     class Sink
diff --git a/lib/mutant/scope.rb b/lib/mutant/scope.rb
index f4334c1f..dc636b99 100644
--- a/lib/mutant/scope.rb
+++ b/lib/mutant/scope.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # Class or Module bound to an exact expression
   class Scope
diff --git a/lib/mutant/selector.rb b/lib/mutant/selector.rb
index 9f09ce13..d2fef7be 100644
--- a/lib/mutant/selector.rb
+++ b/lib/mutant/selector.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # Abstract base class for test selectors
   class Selector
diff --git a/lib/mutant/selector/expression.rb b/lib/mutant/selector/expression.rb
index f8b8f8e3..0e0a989a 100644
--- a/lib/mutant/selector/expression.rb
+++ b/lib/mutant/selector/expression.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Selector
     # Expression based test selector
diff --git a/lib/mutant/subject.rb b/lib/mutant/subject.rb
index 902daae6..b4ab876d 100644
--- a/lib/mutant/subject.rb
+++ b/lib/mutant/subject.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # Subject of a mutation
   class Subject
diff --git a/lib/mutant/subject/method.rb b/lib/mutant/subject/method.rb
index 12416b89..3fa3b493 100644
--- a/lib/mutant/subject/method.rb
+++ b/lib/mutant/subject/method.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Subject
     # Abstract base class for method subjects
diff --git a/lib/mutant/subject/method/instance.rb b/lib/mutant/subject/method/instance.rb
index 9a991579..27fd8dd8 100644
--- a/lib/mutant/subject/method/instance.rb
+++ b/lib/mutant/subject/method/instance.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Subject
     class Method
diff --git a/lib/mutant/subject/method/singleton.rb b/lib/mutant/subject/method/singleton.rb
index b69b21fd..c3bc3593 100644
--- a/lib/mutant/subject/method/singleton.rb
+++ b/lib/mutant/subject/method/singleton.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   class Subject
     class Method
diff --git a/lib/mutant/test.rb b/lib/mutant/test.rb
index 37fbc7d7..c71d5aa2 100644
--- a/lib/mutant/test.rb
+++ b/lib/mutant/test.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # Abstract base class for test that might kill a mutation
   class Test
diff --git a/lib/mutant/util.rb b/lib/mutant/util.rb
index 8740ee9b..847144bc 100644
--- a/lib/mutant/util.rb
+++ b/lib/mutant/util.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # Utility methods
   module Util
diff --git a/lib/mutant/version.rb b/lib/mutant/version.rb
index 151aee97..fc492072 100644
--- a/lib/mutant/version.rb
+++ b/lib/mutant/version.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # Current mutant version
   VERSION = '0.8.16'.freeze
diff --git a/lib/mutant/warning_filter.rb b/lib/mutant/warning_filter.rb
index 26dcbef7..7eed16df 100644
--- a/lib/mutant/warning_filter.rb
+++ b/lib/mutant/warning_filter.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # Stream filter for warnings
   class WarningFilter
diff --git a/lib/mutant/zombifier.rb b/lib/mutant/zombifier.rb
index 51c604b1..0c53e604 100644
--- a/lib/mutant/zombifier.rb
+++ b/lib/mutant/zombifier.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Mutant
   # Zombifier namespace
   class Zombifier
diff --git a/meta/and.rb b/meta/and.rb
index 6628febf..e7f5b31a 100644
--- a/meta/and.rb
+++ b/meta/and.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :and do
   source 'true and false'
 
diff --git a/meta/and_asgn.rb b/meta/and_asgn.rb
index fd731e5a..33f43b4f 100644
--- a/meta/and_asgn.rb
+++ b/meta/and_asgn.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :and_asgn do
   source 'a &&= 1'
 
diff --git a/meta/array.rb b/meta/array.rb
index c7ecca05..03a70dfe 100644
--- a/meta/array.rb
+++ b/meta/array.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :array do
   source '[true]'
 
diff --git a/meta/begin.rb b/meta/begin.rb
index 896d827d..d325f9a0 100644
--- a/meta/begin.rb
+++ b/meta/begin.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :begin do
 
   source 'true; false'
diff --git a/meta/block.rb b/meta/block.rb
index faa15e6f..06817801 100644
--- a/meta/block.rb
+++ b/meta/block.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :block do
   source 'foo { a; b }'
 
diff --git a/meta/block_pass.rb b/meta/block_pass.rb
index 2c835db6..715e1899 100644
--- a/meta/block_pass.rb
+++ b/meta/block_pass.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :block_pass do
   source 'foo(&bar)'
 
diff --git a/meta/blockarg.rb b/meta/blockarg.rb
index 3e582a07..267a0c7c 100644
--- a/meta/blockarg.rb
+++ b/meta/blockarg.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :blockarg do
   source 'foo { |&bar| }'
 
diff --git a/meta/break.rb b/meta/break.rb
index fb71048a..1848bc25 100644
--- a/meta/break.rb
+++ b/meta/break.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :break do
   source 'break true'
 
diff --git a/meta/case.rb b/meta/case.rb
index 5ff2ab86..24b86da3 100644
--- a/meta/case.rb
+++ b/meta/case.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :case do
   source <<-RUBY
     case
diff --git a/meta/casgn.rb b/meta/casgn.rb
index 9ad300d7..bca5d649 100644
--- a/meta/casgn.rb
+++ b/meta/casgn.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :casgn do
   source 'A = true'
 
diff --git a/meta/cbase.rb b/meta/cbase.rb
index a2b996cc..3a1ef0c6 100644
--- a/meta/cbase.rb
+++ b/meta/cbase.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :cbase do
   source '::A'
 
diff --git a/meta/class.rb b/meta/class.rb
index 275eb9b6..2266ded8 100644
--- a/meta/class.rb
+++ b/meta/class.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :class do
   source 'class Foo; bar; end'
 
diff --git a/meta/const.rb b/meta/const.rb
index 79fe9caa..893ceae9 100644
--- a/meta/const.rb
+++ b/meta/const.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :const do
   source 'A::B::C'
 
diff --git a/meta/csend.rb b/meta/csend.rb
index 8d21098e..59931a75 100644
--- a/meta/csend.rb
+++ b/meta/csend.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :csend do
   source 'a&.b'
 
diff --git a/meta/cvar.rb b/meta/cvar.rb
index 05c4574a..82b11fba 100644
--- a/meta/cvar.rb
+++ b/meta/cvar.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :cvar do
   source '@@a'
 
diff --git a/meta/cvasgn.rb b/meta/cvasgn.rb
index c52d9bee..c32af925 100644
--- a/meta/cvasgn.rb
+++ b/meta/cvasgn.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :cvasgn do
   source '@@a = true'
 
diff --git a/meta/date.rb b/meta/date.rb
index 78c15c07..64bf3f80 100644
--- a/meta/date.rb
+++ b/meta/date.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :send do
   source 'Date.parse(nil)'
 
diff --git a/meta/def.rb b/meta/def.rb
index b51689ce..ae973d34 100644
--- a/meta/def.rb
+++ b/meta/def.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :def do
   source 'def foo; end'
 
diff --git a/meta/defined.rb b/meta/defined.rb
index 6bce9192..689ca206 100644
--- a/meta/defined.rb
+++ b/meta/defined.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :defined? do
   source 'defined?(foo)'
 
diff --git a/meta/dstr.rb b/meta/dstr.rb
index d23c31dc..3849bacf 100644
--- a/meta/dstr.rb
+++ b/meta/dstr.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :dstr do
   source '"foo#{bar}baz"'
 
diff --git a/meta/dsym.rb b/meta/dsym.rb
index 49a73107..146798d3 100644
--- a/meta/dsym.rb
+++ b/meta/dsym.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :dsym do
   source ':"foo#{bar}baz"'
 
diff --git a/meta/ensure.rb b/meta/ensure.rb
index a69ccb5d..1cd16fda 100644
--- a/meta/ensure.rb
+++ b/meta/ensure.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :ensure do
   source 'begin; rescue; ensure; true; end'
 
diff --git a/meta/false.rb b/meta/false.rb
index 83f07276..f1907cf6 100644
--- a/meta/false.rb
+++ b/meta/false.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :false do
   source 'false'
 
diff --git a/meta/float.rb b/meta/float.rb
index 4c4533c4..5ec02b2b 100644
--- a/meta/float.rb
+++ b/meta/float.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :float do
   source '10.0'
 
diff --git a/meta/gvar.rb b/meta/gvar.rb
index 2293a3ea..b58075a3 100644
--- a/meta/gvar.rb
+++ b/meta/gvar.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :gvar do
   source '$a'
 
diff --git a/meta/gvasgn.rb b/meta/gvasgn.rb
index 0a95bef0..d536e4a8 100644
--- a/meta/gvasgn.rb
+++ b/meta/gvasgn.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :gvasgn do
   source '$a = true'
 
diff --git a/meta/hash.rb b/meta/hash.rb
index 32b3267f..7a1c512a 100644
--- a/meta/hash.rb
+++ b/meta/hash.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :hash do
   source '{true => true, false => false}'
 
diff --git a/meta/if.rb b/meta/if.rb
index 42941841..1137b637 100644
--- a/meta/if.rb
+++ b/meta/if.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :if do
   source 'if condition; true; else false; end'
 
diff --git a/meta/int.rb b/meta/int.rb
index a68e847e..abf24281 100644
--- a/meta/int.rb
+++ b/meta/int.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :int do
   source '10'
 
diff --git a/meta/ivar.rb b/meta/ivar.rb
index e3959a96..86a51397 100644
--- a/meta/ivar.rb
+++ b/meta/ivar.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :ivar do
   source '@foo'
 
diff --git a/meta/ivasgn.rb b/meta/ivasgn.rb
index 0fd5ab7b..f79aaf42 100644
--- a/meta/ivasgn.rb
+++ b/meta/ivasgn.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :ivasgn do
   source '@a = true'
 
diff --git a/meta/kwarg.rb b/meta/kwarg.rb
index 78784e51..82b6f0fb 100644
--- a/meta/kwarg.rb
+++ b/meta/kwarg.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :kwarg do
   source 'def foo(bar:); end'
 
diff --git a/meta/kwbegin.rb b/meta/kwbegin.rb
index 0dc8ea50..3f8d7dd9 100644
--- a/meta/kwbegin.rb
+++ b/meta/kwbegin.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :kwbegin do
   source 'begin; true; end'
 
diff --git a/meta/kwoptarg.rb b/meta/kwoptarg.rb
index 695c20af..28b2619b 100644
--- a/meta/kwoptarg.rb
+++ b/meta/kwoptarg.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :kwarg do
   source 'def foo(bar: baz); end'
 
diff --git a/meta/lvar.rb b/meta/lvar.rb
index 2e658e2e..718d18aa 100644
--- a/meta/lvar.rb
+++ b/meta/lvar.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :lvar do
   source 'a = nil; a'
 
diff --git a/meta/lvasgn.rb b/meta/lvasgn.rb
index da7de9f2..ef4772e0 100644
--- a/meta/lvasgn.rb
+++ b/meta/lvasgn.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :lvasgn do
   source 'a = true'
 
diff --git a/meta/masgn.rb b/meta/masgn.rb
index bf408311..85ded623 100644
--- a/meta/masgn.rb
+++ b/meta/masgn.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :masgn do
   source 'a, b = c, d'
 
diff --git a/meta/match_current_line.rb b/meta/match_current_line.rb
index 4b2b812d..c7215ceb 100644
--- a/meta/match_current_line.rb
+++ b/meta/match_current_line.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :match_current_line do
   source 'true if /foo/'
 
diff --git a/meta/next.rb b/meta/next.rb
index 4407938c..3206be8d 100644
--- a/meta/next.rb
+++ b/meta/next.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :next do
   source 'next true'
 
diff --git a/meta/nil.rb b/meta/nil.rb
index 235ae1d4..73a829fd 100644
--- a/meta/nil.rb
+++ b/meta/nil.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :nil do
   source 'nil'
 end
diff --git a/meta/nthref.rb b/meta/nthref.rb
index 8d182175..4f33c1aa 100644
--- a/meta/nthref.rb
+++ b/meta/nthref.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :nth_ref do
   source '$1'
 
diff --git a/meta/op_assgn.rb b/meta/op_assgn.rb
index dabe5763..4238c369 100644
--- a/meta/op_assgn.rb
+++ b/meta/op_assgn.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :op_asgn do
   source '@a.b += 1'
 
diff --git a/meta/or.rb b/meta/or.rb
index 85999a0c..9f1ee0db 100644
--- a/meta/or.rb
+++ b/meta/or.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :or do
   source 'true or false'
 
diff --git a/meta/or_asgn.rb b/meta/or_asgn.rb
index f1a9156a..49622347 100644
--- a/meta/or_asgn.rb
+++ b/meta/or_asgn.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :or_asgn do
   source 'a ||= 1'
 
diff --git a/meta/range.rb b/meta/range.rb
index c06b57a1..dc451047 100644
--- a/meta/range.rb
+++ b/meta/range.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :irange do
   source '1..100'
 
diff --git a/meta/redo.rb b/meta/redo.rb
index 0554edbf..12268dc3 100644
--- a/meta/redo.rb
+++ b/meta/redo.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :redo do
   source 'redo'
 end
diff --git a/meta/regexp.rb b/meta/regexp.rb
index 3acda5d2..c868e3a5 100644
--- a/meta/regexp.rb
+++ b/meta/regexp.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :regexp do
   source '/foo/'
 
diff --git a/meta/regexp/character_types.rb b/meta/regexp/character_types.rb
index 4fc5e39e..ecb3e283 100644
--- a/meta/regexp/character_types.rb
+++ b/meta/regexp/character_types.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 mutations = {
   [:regexp_digit_type,           '/\d/'] => [:regexp_nondigit_type,           '/\D/'],
   [:regexp_hex_type,             '/\h/'] => [:regexp_nonhex_type,             '/\H/'],
diff --git a/meta/regexp/regexp_alternation_meta.rb b/meta/regexp/regexp_alternation_meta.rb
index d1e82a18..76c239af 100644
--- a/meta/regexp/regexp_alternation_meta.rb
+++ b/meta/regexp/regexp_alternation_meta.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :regexp_alternation_meta do
   source '/\A(foo|bar|baz)\z/'
 
diff --git a/meta/regexp/regexp_bol_anchor.rb b/meta/regexp/regexp_bol_anchor.rb
index b0487d66..7cd51e93 100644
--- a/meta/regexp/regexp_bol_anchor.rb
+++ b/meta/regexp/regexp_bol_anchor.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :regexp_bol_anchor do
   source '/^/'
 
diff --git a/meta/regexp/regexp_bos_anchor.rb b/meta/regexp/regexp_bos_anchor.rb
index 0e09cf8b..b1dce505 100644
--- a/meta/regexp/regexp_bos_anchor.rb
+++ b/meta/regexp/regexp_bos_anchor.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :regexp_bos_anchor do
   source '/\A/'
 
diff --git a/meta/regexp/regexp_capture_group.rb b/meta/regexp/regexp_capture_group.rb
index e207c4dd..24d28fff 100644
--- a/meta/regexp/regexp_capture_group.rb
+++ b/meta/regexp/regexp_capture_group.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :regexp_capture_group do
   source '/()/'
 
diff --git a/meta/regexp/regexp_eol_anchor.rb b/meta/regexp/regexp_eol_anchor.rb
index f17aa835..f34de4a3 100644
--- a/meta/regexp/regexp_eol_anchor.rb
+++ b/meta/regexp/regexp_eol_anchor.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :regexp_eol_anchor do
   source '/$/'
 
diff --git a/meta/regexp/regexp_eos_anchor.rb b/meta/regexp/regexp_eos_anchor.rb
index f70a8278..b0ca0ff2 100644
--- a/meta/regexp/regexp_eos_anchor.rb
+++ b/meta/regexp/regexp_eos_anchor.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :regexp_eos_anchor do
   source '/\z/'
 
diff --git a/meta/regexp/regexp_eos_ob_eol_anchor.rb b/meta/regexp/regexp_eos_ob_eol_anchor.rb
index e091b4bd..7ef8b2f2 100644
--- a/meta/regexp/regexp_eos_ob_eol_anchor.rb
+++ b/meta/regexp/regexp_eos_ob_eol_anchor.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :regexp_eos_ob_eol_anchor do
   source '/\Z/'
 
diff --git a/meta/regexp/regexp_greedy_zero_or_more.rb b/meta/regexp/regexp_greedy_zero_or_more.rb
index eaf6be69..21f49b7e 100644
--- a/meta/regexp/regexp_greedy_zero_or_more.rb
+++ b/meta/regexp/regexp_greedy_zero_or_more.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :regexp_greedy_zero_or_more do
   source '/\d*/'
 
diff --git a/meta/regexp/regexp_root_expression.rb b/meta/regexp/regexp_root_expression.rb
index 737fc708..004c8aed 100644
--- a/meta/regexp/regexp_root_expression.rb
+++ b/meta/regexp/regexp_root_expression.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :regexp_root_expression do
   source '/^/'
 
diff --git a/meta/regopt.rb b/meta/regopt.rb
index 56c8bb21..85df9dfc 100644
--- a/meta/regopt.rb
+++ b/meta/regopt.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :regopt do
   source '/foo/ixom'
 
diff --git a/meta/rescue.rb b/meta/rescue.rb
index a5da4b7b..af5bf660 100644
--- a/meta/rescue.rb
+++ b/meta/rescue.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :rescue do
   source 'begin; rescue ExceptionA, ExceptionB => error; true; end'
 
diff --git a/meta/restarg.rb b/meta/restarg.rb
index 7eb608ca..91d1a3ca 100644
--- a/meta/restarg.rb
+++ b/meta/restarg.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :restarg do
   source 'def foo(*bar); end'
 
diff --git a/meta/return.rb b/meta/return.rb
index 6df54f79..c1ed9bc2 100644
--- a/meta/return.rb
+++ b/meta/return.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :return do
   source 'return'
 
diff --git a/meta/self.rb b/meta/self.rb
index cae8f232..ffef2e99 100644
--- a/meta/self.rb
+++ b/meta/self.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :self do
   source 'self'
 
diff --git a/meta/send.rb b/meta/send.rb
index ef9837d8..e58b7a90 100644
--- a/meta/send.rb
+++ b/meta/send.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :send do
   source 'a > b'
 
diff --git a/meta/str.rb b/meta/str.rb
index 45fda7a0..4624cef5 100644
--- a/meta/str.rb
+++ b/meta/str.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :str do
   source '"foo"'
 
diff --git a/meta/super.rb b/meta/super.rb
index 1989357e..2a31d0ff 100644
--- a/meta/super.rb
+++ b/meta/super.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :super do
   source 'super'
 
diff --git a/meta/sym.rb b/meta/sym.rb
index 94ab31bf..d304da3c 100644
--- a/meta/sym.rb
+++ b/meta/sym.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :sym do
   source ':foo'
 
diff --git a/meta/true.rb b/meta/true.rb
index 64681e4c..85a1fe9d 100644
--- a/meta/true.rb
+++ b/meta/true.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :true do
   source 'true'
 
diff --git a/meta/until.rb b/meta/until.rb
index 3edd1f1f..eabef533 100644
--- a/meta/until.rb
+++ b/meta/until.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :until do
   source 'until true; foo; bar; end'
 
diff --git a/meta/while.rb b/meta/while.rb
index cc41bfd2..1deac1be 100644
--- a/meta/while.rb
+++ b/meta/while.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :while do
   source 'while true; foo; bar; end'
 
diff --git a/meta/yield.rb b/meta/yield.rb
index b8485dc0..582b2bc7 100644
--- a/meta/yield.rb
+++ b/meta/yield.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example.add :yield do
   source 'yield true'
 
diff --git a/mutant-rspec.gemspec b/mutant-rspec.gemspec
index 4d1ce0c9..0e32b5cf 100644
--- a/mutant-rspec.gemspec
+++ b/mutant-rspec.gemspec
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require File.expand_path('../lib/mutant/version', __FILE__)
 
 Gem::Specification.new do |gem|
diff --git a/mutant.gemspec b/mutant.gemspec
index b8d4cab9..39c56043 100644
--- a/mutant.gemspec
+++ b/mutant.gemspec
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require File.expand_path('../lib/mutant/version', __FILE__)
 
 Gem::Specification.new do |gem|
diff --git a/spec/integration/mutant/corpus_spec.rb b/spec/integration/mutant/corpus_spec.rb
index 18385ba1..ceeb42f9 100644
--- a/spec/integration/mutant/corpus_spec.rb
+++ b/spec/integration/mutant/corpus_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe 'Mutant on ruby corpus', mutant: false do
 
   before do
diff --git a/spec/integration/mutant/isolation/fork_spec.rb b/spec/integration/mutant/isolation/fork_spec.rb
index 32b60e60..27da10e2 100644
--- a/spec/integration/mutant/isolation/fork_spec.rb
+++ b/spec/integration/mutant/isolation/fork_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Isolation::Fork, mutant: false do
   specify do
     a = 1
diff --git a/spec/integration/mutant/null_spec.rb b/spec/integration/mutant/null_spec.rb
index 409b0398..eaafafa4 100644
--- a/spec/integration/mutant/null_spec.rb
+++ b/spec/integration/mutant/null_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe 'null integration', mutant: false do
 
   let(:base_cmd) { 'bundle exec mutant -I lib --require test_app "TestApp*"' }
diff --git a/spec/integration/mutant/rspec_spec.rb b/spec/integration/mutant/rspec_spec.rb
index 08053446..120cca37 100644
--- a/spec/integration/mutant/rspec_spec.rb
+++ b/spec/integration/mutant/rspec_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe 'rspec integration', mutant: false do
 
   let(:base_cmd) { 'bundle exec mutant -I lib --require test_app --use rspec' }
diff --git a/spec/integration/mutant/test_mutator_handles_types_spec.rb b/spec/integration/mutant/test_mutator_handles_types_spec.rb
index 65769fe2..a7a607cb 100644
--- a/spec/integration/mutant/test_mutator_handles_types_spec.rb
+++ b/spec/integration/mutant/test_mutator_handles_types_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe 'AST type coverage', mutant: false do
   specify 'mutant should not crash for any node parser can generate' do
     Mutant::AST::Types::ALL.each do |type|
diff --git a/spec/shared/framework_integration_behavior.rb b/spec/shared/framework_integration_behavior.rb
index 70df4d8a..36f3de40 100644
--- a/spec/shared/framework_integration_behavior.rb
+++ b/spec/shared/framework_integration_behavior.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.shared_examples_for 'framework integration' do
   def system_with_gemfile(*command)
     Kernel.system({ 'BUNDLE_GEMFILE' => gemfile }, *command)
diff --git a/spec/shared/method_matcher_behavior.rb b/spec/shared/method_matcher_behavior.rb
index a63c7d2b..d03669ac 100644
--- a/spec/shared/method_matcher_behavior.rb
+++ b/spec/shared/method_matcher_behavior.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.shared_examples_for 'a method matcher' do
   let(:node)              { mutation_subject.node    }
   let(:context)           { mutation_subject.context }
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 73ca93b1..d5157948 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 if ENV['COVERAGE'] == 'true'
   require 'simplecov'
 
diff --git a/spec/support/compress_helper.rb b/spec/support/compress_helper.rb
index c67abeeb..2f841bb3 100644
--- a/spec/support/compress_helper.rb
+++ b/spec/support/compress_helper.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module CompressHelper
   def strip_indent(string)
     lines       = string.lines
diff --git a/spec/support/corpus.rb b/spec/support/corpus.rb
index d5ee2609..3eae41e8 100644
--- a/spec/support/corpus.rb
+++ b/spec/support/corpus.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require 'morpher'
 require 'anima'
 require 'mutant'
diff --git a/spec/support/fake_actor.rb b/spec/support/fake_actor.rb
index bf827898..cb6929fc 100644
--- a/spec/support/fake_actor.rb
+++ b/spec/support/fake_actor.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require 'mutant/actor'
 
 # A fake actor used from specs
@@ -14,7 +15,7 @@ module FakeActor
       unless eql?(other)
         fail "Got:\n#{other.inspect}\nExpected:\n#{inspect}"
       end
-      block.call(other.message) if block
+      block&.call(other.message)
     end
   end # Expectation
 
diff --git a/spec/support/file_system.rb b/spec/support/file_system.rb
index 1b59d6bb..f81f5511 100644
--- a/spec/support/file_system.rb
+++ b/spec/support/file_system.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module MutantSpec
   class FileState
     DEFAULTS = IceNine.deep_freeze(
diff --git a/spec/support/ice_nine_config.rb b/spec/support/ice_nine_config.rb
index 54ee42a0..dd557ed1 100644
--- a/spec/support/ice_nine_config.rb
+++ b/spec/support/ice_nine_config.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require 'ice_nine'
 
 module IceNine
diff --git a/spec/support/rb_bug.rb b/spec/support/rb_bug.rb
index d0e40bc8..a2b068aa 100644
--- a/spec/support/rb_bug.rb
+++ b/spec/support/rb_bug.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require 'ffi'
 
 # @api private
diff --git a/spec/support/ruby_vm.rb b/spec/support/ruby_vm.rb
index 09f22c60..516aa60c 100644
--- a/spec/support/ruby_vm.rb
+++ b/spec/support/ruby_vm.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module MutantSpec
   # Not a real VM, just kidding. It connects the require / eval triggers
   # require semantics Zombifier relies on in a way we can avoid having to
diff --git a/spec/support/shared_context.rb b/spec/support/shared_context.rb
index 20c5772f..df5f553f 100644
--- a/spec/support/shared_context.rb
+++ b/spec/support/shared_context.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 # rubocop:disable ModuleLength
 module SharedContext
   # Prepend an anonymous module with the new `with` method
diff --git a/spec/support/test_app.rb b/spec/support/test_app.rb
index 5b207f6f..5987b4eb 100644
--- a/spec/support/test_app.rb
+++ b/spec/support/test_app.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module TestApp
   def self.root
     File.expand_path('../../../test_app', __FILE__)
diff --git a/spec/support/warning.rb b/spec/support/warning.rb
index d2054846..9183984f 100644
--- a/spec/support/warning.rb
+++ b/spec/support/warning.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require 'yaml'
 require 'equalizer'
 require 'memoizable'
diff --git a/spec/support/xspec.rb b/spec/support/xspec.rb
index 2638dcca..8e7f7615 100644
--- a/spec/support/xspec.rb
+++ b/spec/support/xspec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module XSpec
   class MessageReaction
     include Concord.new(:event_list)
diff --git a/spec/unit/mutant/actor/binding_spec.rb b/spec/unit/mutant/actor/binding_spec.rb
index 0b53d4f6..e9a2750b 100644
--- a/spec/unit/mutant/actor/binding_spec.rb
+++ b/spec/unit/mutant/actor/binding_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Actor::Binding do
   let(:actor_a)    { instance_double(Mutant::Actor::Mailbox, sender: sender_a, receiver: receiver_a) }
   let(:sender_a)   { instance_double(Mutant::Actor::Sender)                                          }
diff --git a/spec/unit/mutant/actor/env_spec.rb b/spec/unit/mutant/actor/env_spec.rb
index cb0d98dd..c7f088e6 100644
--- a/spec/unit/mutant/actor/env_spec.rb
+++ b/spec/unit/mutant/actor/env_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Actor::Env do
   let(:thread)      { instance_double(Thread) }
   let(:thread_root) { class_double(Thread)    }
diff --git a/spec/unit/mutant/actor/mailbox_spec.rb b/spec/unit/mutant/actor/mailbox_spec.rb
index 9f4c864e..ca1a38cc 100644
--- a/spec/unit/mutant/actor/mailbox_spec.rb
+++ b/spec/unit/mutant/actor/mailbox_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Actor::Mailbox do
   let(:mutex)              { instance_double(Mutex)             }
   let(:condition_variable) { instance_double(ConditionVariable) }
diff --git a/spec/unit/mutant/actor/message_spec.rb b/spec/unit/mutant/actor/message_spec.rb
index 9834b8c3..c570ce3d 100644
--- a/spec/unit/mutant/actor/message_spec.rb
+++ b/spec/unit/mutant/actor/message_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Actor::Message do
 
   let(:type)    { instance_double(Symbol) }
diff --git a/spec/unit/mutant/actor/receiver_spec.rb b/spec/unit/mutant/actor/receiver_spec.rb
index bb1ec215..24e5cf16 100644
--- a/spec/unit/mutant/actor/receiver_spec.rb
+++ b/spec/unit/mutant/actor/receiver_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Actor::Receiver do
   let(:messages)           { instance_double(Array)             }
   let(:mutex)              { instance_double(Mutex)             }
diff --git a/spec/unit/mutant/actor/sender_spec.rb b/spec/unit/mutant/actor/sender_spec.rb
index 8ef6ec13..7e1d077a 100644
--- a/spec/unit/mutant/actor/sender_spec.rb
+++ b/spec/unit/mutant/actor/sender_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Actor::Sender do
   let(:object) { described_class.new(condition_variable, mutex, messages) }
 
diff --git a/spec/unit/mutant/ast/meta/optarg_spec.rb b/spec/unit/mutant/ast/meta/optarg_spec.rb
index 82f59cca..4553b04e 100644
--- a/spec/unit/mutant/ast/meta/optarg_spec.rb
+++ b/spec/unit/mutant/ast/meta/optarg_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::AST::Meta::Optarg do
   subject(:object) { described_class.new(node) }
 
diff --git a/spec/unit/mutant/ast/meta/send/proc_predicate_spec.rb b/spec/unit/mutant/ast/meta/send/proc_predicate_spec.rb
index a06c3d92..2b99be54 100644
--- a/spec/unit/mutant/ast/meta/send/proc_predicate_spec.rb
+++ b/spec/unit/mutant/ast/meta/send/proc_predicate_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::AST::Meta::Send, '#proc?' do
   subject { described_class.new(node).proc? }
 
diff --git a/spec/unit/mutant/ast/meta/send/receiver_possible_top_level_const_predicate_spec.rb b/spec/unit/mutant/ast/meta/send/receiver_possible_top_level_const_predicate_spec.rb
index cf9c93e9..29709195 100644
--- a/spec/unit/mutant/ast/meta/send/receiver_possible_top_level_const_predicate_spec.rb
+++ b/spec/unit/mutant/ast/meta/send/receiver_possible_top_level_const_predicate_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::AST::Meta::Send, '#receiver_possible_top_level_const?' do
   subject { described_class.new(node).receiver_possible_top_level_const? }
 
diff --git a/spec/unit/mutant/ast/meta/send_spec.rb b/spec/unit/mutant/ast/meta/send_spec.rb
index f8fa6186..43ae88a4 100644
--- a/spec/unit/mutant/ast/meta/send_spec.rb
+++ b/spec/unit/mutant/ast/meta/send_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::AST::Meta::Send do
   let(:object) { described_class.new(node) }
 
diff --git a/spec/unit/mutant/ast/named_children_spec.rb b/spec/unit/mutant/ast/named_children_spec.rb
index 512a40e6..53b599a1 100644
--- a/spec/unit/mutant/ast/named_children_spec.rb
+++ b/spec/unit/mutant/ast/named_children_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::AST::NamedChildren do
   describe '.included' do
     let(:klass) do
diff --git a/spec/unit/mutant/ast/regexp/parse_spec.rb b/spec/unit/mutant/ast/regexp/parse_spec.rb
index 2e22f456..69a76345 100644
--- a/spec/unit/mutant/ast/regexp/parse_spec.rb
+++ b/spec/unit/mutant/ast/regexp/parse_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::AST::Regexp, '.parse' do
   it 'parses using minor ruby version' do
     expect(described_class.parse(/foo/).to_re).to eql(/foo/)
diff --git a/spec/unit/mutant/ast/regexp/supported_predicate_spec.rb b/spec/unit/mutant/ast/regexp/supported_predicate_spec.rb
index 78596d59..13275166 100644
--- a/spec/unit/mutant/ast/regexp/supported_predicate_spec.rb
+++ b/spec/unit/mutant/ast/regexp/supported_predicate_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::AST::Regexp, '.supported?' do
   subject { described_class.supported?(expression) }
 
diff --git a/spec/unit/mutant/ast/regexp/transformer/lookup_table/table_spec.rb b/spec/unit/mutant/ast/regexp/transformer/lookup_table/table_spec.rb
index 8a5c8257..673cbf3b 100644
--- a/spec/unit/mutant/ast/regexp/transformer/lookup_table/table_spec.rb
+++ b/spec/unit/mutant/ast/regexp/transformer/lookup_table/table_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::AST::Regexp::Transformer::LookupTable::Table do
   subject { table.lookup(:regexp_fake_thing) }
 
diff --git a/spec/unit/mutant/ast/regexp/transformer/lookup_table_spec.rb b/spec/unit/mutant/ast/regexp/transformer/lookup_table_spec.rb
index dcfad4bb..8c541d2a 100644
--- a/spec/unit/mutant/ast/regexp/transformer/lookup_table_spec.rb
+++ b/spec/unit/mutant/ast/regexp/transformer/lookup_table_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::AST::Regexp::Transformer::LookupTable do
   subject(:pair) { mapper.new(s(:regexp_fake)).pair }
 
diff --git a/spec/unit/mutant/ast/regexp/transformer_spec.rb b/spec/unit/mutant/ast/regexp/transformer_spec.rb
index e969c9cf..a1feb8db 100644
--- a/spec/unit/mutant/ast/regexp/transformer_spec.rb
+++ b/spec/unit/mutant/ast/regexp/transformer_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::AST::Regexp::Transformer do
   before do
     stub_const("#{described_class}::REGISTRY", Mutant::Registry.new)
diff --git a/spec/unit/mutant/ast/regexp_spec.rb b/spec/unit/mutant/ast/regexp_spec.rb
index 4815101a..3212ec9f 100644
--- a/spec/unit/mutant/ast/regexp_spec.rb
+++ b/spec/unit/mutant/ast/regexp_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module RegexpSpec
   class Expression < SimpleDelegator
     NO_EXPRESSIONS = Object.new.freeze
diff --git a/spec/unit/mutant/ast/sexp_spec.rb b/spec/unit/mutant/ast/sexp_spec.rb
index 0c689795..e2ec424f 100644
--- a/spec/unit/mutant/ast/sexp_spec.rb
+++ b/spec/unit/mutant/ast/sexp_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::AST::Sexp do
   let(:object) do
     Class.new do
diff --git a/spec/unit/mutant/ast_spec.rb b/spec/unit/mutant/ast_spec.rb
index e50c5ee2..d4da4ab7 100644
--- a/spec/unit/mutant/ast_spec.rb
+++ b/spec/unit/mutant/ast_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::AST do
   let(:object) { described_class }
 
diff --git a/spec/unit/mutant/cli_spec.rb b/spec/unit/mutant/cli_spec.rb
index d6bbcc5b..e8f4e4f9 100644
--- a/spec/unit/mutant/cli_spec.rb
+++ b/spec/unit/mutant/cli_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::CLI do
   let(:object) { described_class }
 
diff --git a/spec/unit/mutant/context_spec.rb b/spec/unit/mutant/context_spec.rb
index 636bfd27..c287e620 100644
--- a/spec/unit/mutant/context_spec.rb
+++ b/spec/unit/mutant/context_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Context do
   describe '.wrap' do
     subject { described_class.wrap(scope, node) }
diff --git a/spec/unit/mutant/diff_spec.rb b/spec/unit/mutant/diff_spec.rb
index fe43dc00..b55b0c6f 100644
--- a/spec/unit/mutant/diff_spec.rb
+++ b/spec/unit/mutant/diff_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Diff do
   let(:object) { described_class }
 
diff --git a/spec/unit/mutant/env/bootstrap_spec.rb b/spec/unit/mutant/env/bootstrap_spec.rb
index 2606fe08..31d50284 100644
--- a/spec/unit/mutant/env/bootstrap_spec.rb
+++ b/spec/unit/mutant/env/bootstrap_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 # This spec is a good example for:
 #
 # If test look that ugly the class under test sucks.
diff --git a/spec/unit/mutant/env_spec.rb b/spec/unit/mutant/env_spec.rb
index a808d87e..3c3a99bb 100644
--- a/spec/unit/mutant/env_spec.rb
+++ b/spec/unit/mutant/env_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Env do
   context '#kill' do
     let(:object) do
diff --git a/spec/unit/mutant/expression/method_spec.rb b/spec/unit/mutant/expression/method_spec.rb
index fb780c48..b9ecbf71 100644
--- a/spec/unit/mutant/expression/method_spec.rb
+++ b/spec/unit/mutant/expression/method_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Expression::Method do
   let(:object)           { parse_expression(input)   }
   let(:env)              { Fixtures::TEST_ENV        }
diff --git a/spec/unit/mutant/expression/methods_spec.rb b/spec/unit/mutant/expression/methods_spec.rb
index 835bd2c2..7c9dcd91 100644
--- a/spec/unit/mutant/expression/methods_spec.rb
+++ b/spec/unit/mutant/expression/methods_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Expression::Methods do
   let(:object) { described_class.new(attributes) }
 
diff --git a/spec/unit/mutant/expression/namespace/exact_spec.rb b/spec/unit/mutant/expression/namespace/exact_spec.rb
index 24c3d3b7..dc2704db 100644
--- a/spec/unit/mutant/expression/namespace/exact_spec.rb
+++ b/spec/unit/mutant/expression/namespace/exact_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Expression::Namespace::Exact do
   let(:object) { parse_expression(input) }
   let(:input)  { 'TestApp::Literal'      }
diff --git a/spec/unit/mutant/expression/namespace/recursive_spec.rb b/spec/unit/mutant/expression/namespace/recursive_spec.rb
index d32bfecb..c744c40f 100644
--- a/spec/unit/mutant/expression/namespace/recursive_spec.rb
+++ b/spec/unit/mutant/expression/namespace/recursive_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Expression::Namespace::Recursive do
   let(:object) { parse_expression(input) }
   let(:input)  { 'TestApp::Literal*'     }
diff --git a/spec/unit/mutant/expression/parser_spec.rb b/spec/unit/mutant/expression/parser_spec.rb
index ec2df51d..71a0d1ab 100644
--- a/spec/unit/mutant/expression/parser_spec.rb
+++ b/spec/unit/mutant/expression/parser_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Expression::Parser do
   let(:object) { Mutant::Config::DEFAULT.expression_parser }
 
diff --git a/spec/unit/mutant/expression_spec.rb b/spec/unit/mutant/expression_spec.rb
index 7bd9bebf..fd39f39e 100644
--- a/spec/unit/mutant/expression_spec.rb
+++ b/spec/unit/mutant/expression_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Expression do
   let(:parser) { Mutant::Config::DEFAULT.expression_parser }
 
diff --git a/spec/unit/mutant/integration/rspec_spec.rb b/spec/unit/mutant/integration/rspec_spec.rb
index eb3adc6c..ce0472eb 100644
--- a/spec/unit/mutant/integration/rspec_spec.rb
+++ b/spec/unit/mutant/integration/rspec_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require 'mutant/integration/rspec'
 
 RSpec.describe Mutant::Integration::Rspec do
diff --git a/spec/unit/mutant/integration_spec.rb b/spec/unit/mutant/integration_spec.rb
index b6411d91..d8858113 100644
--- a/spec/unit/mutant/integration_spec.rb
+++ b/spec/unit/mutant/integration_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Integration do
 
   let(:class_under_test) do
diff --git a/spec/unit/mutant/isolation/fork_spec.rb b/spec/unit/mutant/isolation/fork_spec.rb
index b6cdc419..cfd09832 100644
--- a/spec/unit/mutant/isolation/fork_spec.rb
+++ b/spec/unit/mutant/isolation/fork_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 # The fork isolation is all about managing a series of systemcalls with proper error handling
 #
 # So creating a unit spec for this is challenging. Especially under mutation testing.
diff --git a/spec/unit/mutant/isolation/none_spec.rb b/spec/unit/mutant/isolation/none_spec.rb
index f0f69928..0b159324 100644
--- a/spec/unit/mutant/isolation/none_spec.rb
+++ b/spec/unit/mutant/isolation/none_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Isolation::None do
   describe '.call' do
     let(:object) { described_class.new }
diff --git a/spec/unit/mutant/loader_spec.rb b/spec/unit/mutant/loader_spec.rb
index b896e216..54897daf 100644
--- a/spec/unit/mutant/loader_spec.rb
+++ b/spec/unit/mutant/loader_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Loader, '.call' do
   subject do
     described_class.call(
diff --git a/spec/unit/mutant/matcher/chain_spec.rb b/spec/unit/mutant/matcher/chain_spec.rb
index ff3408d0..ad2806ba 100644
--- a/spec/unit/mutant/matcher/chain_spec.rb
+++ b/spec/unit/mutant/matcher/chain_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Matcher::Chain, '#call' do
   subject { object.call(env) }
 
diff --git a/spec/unit/mutant/matcher/compiler/subject_prefix_spec.rb b/spec/unit/mutant/matcher/compiler/subject_prefix_spec.rb
index c6c81bef..4727536a 100644
--- a/spec/unit/mutant/matcher/compiler/subject_prefix_spec.rb
+++ b/spec/unit/mutant/matcher/compiler/subject_prefix_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Matcher::Compiler::SubjectPrefix, '#call' do
   let(:object)   { described_class.new(parse_expression('Foo*')) }
 
diff --git a/spec/unit/mutant/matcher/compiler_spec.rb b/spec/unit/mutant/matcher/compiler_spec.rb
index cf0d1176..b2fced2c 100644
--- a/spec/unit/mutant/matcher/compiler_spec.rb
+++ b/spec/unit/mutant/matcher/compiler_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Matcher::Compiler, '#call' do
   let(:object)         { described_class                  }
   let(:matcher_config) { Mutant::Matcher::Config::DEFAULT }
diff --git a/spec/unit/mutant/matcher/config_spec.rb b/spec/unit/mutant/matcher/config_spec.rb
index 119a229f..dc825cfb 100644
--- a/spec/unit/mutant/matcher/config_spec.rb
+++ b/spec/unit/mutant/matcher/config_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Matcher::Config do
   describe '#inspect' do
     subject { object.inspect }
diff --git a/spec/unit/mutant/matcher/filter_spec.rb b/spec/unit/mutant/matcher/filter_spec.rb
index 066729bd..e9f7f081 100644
--- a/spec/unit/mutant/matcher/filter_spec.rb
+++ b/spec/unit/mutant/matcher/filter_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Matcher::Filter, '#call' do
   subject { object.call(env) }
 
diff --git a/spec/unit/mutant/matcher/method/instance_spec.rb b/spec/unit/mutant/matcher/method/instance_spec.rb
index 9ae77128..e5d7b0bf 100644
--- a/spec/unit/mutant/matcher/method/instance_spec.rb
+++ b/spec/unit/mutant/matcher/method/instance_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Matcher::Method::Instance, '#call' do
   subject { object.call(env) }
 
diff --git a/spec/unit/mutant/matcher/method/singleton_spec.rb b/spec/unit/mutant/matcher/method/singleton_spec.rb
index b22861ad..d9564bf8 100644
--- a/spec/unit/mutant/matcher/method/singleton_spec.rb
+++ b/spec/unit/mutant/matcher/method/singleton_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Matcher::Method::Singleton, '#call' do
   subject { object.call(env) }
 
diff --git a/spec/unit/mutant/matcher/methods/instance_spec.rb b/spec/unit/mutant/matcher/methods/instance_spec.rb
index 4b93b6c3..efdd2782 100644
--- a/spec/unit/mutant/matcher/methods/instance_spec.rb
+++ b/spec/unit/mutant/matcher/methods/instance_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Matcher::Methods::Instance, '#call' do
   let(:object) { described_class.new(class_under_test) }
   let(:env)    { Fixtures::TEST_ENV                    }
diff --git a/spec/unit/mutant/matcher/methods/singleton_spec.rb b/spec/unit/mutant/matcher/methods/singleton_spec.rb
index ac5e2682..d223f423 100644
--- a/spec/unit/mutant/matcher/methods/singleton_spec.rb
+++ b/spec/unit/mutant/matcher/methods/singleton_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Matcher::Methods::Singleton, '#call' do
   let(:object) { described_class.new(class_under_test) }
   let(:env)    { Fixtures::TEST_ENV                    }
diff --git a/spec/unit/mutant/matcher/namespace_spec.rb b/spec/unit/mutant/matcher/namespace_spec.rb
index 21ee19ac..e15ec3da 100644
--- a/spec/unit/mutant/matcher/namespace_spec.rb
+++ b/spec/unit/mutant/matcher/namespace_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Matcher::Namespace, '#call' do
   let(:object)      { described_class.new(parse_expression('TestApp*')) }
   let(:env)         { instance_double(Mutant::Env)                      }
diff --git a/spec/unit/mutant/matcher/null_spec.rb b/spec/unit/mutant/matcher/null_spec.rb
index 4320a182..ad469188 100644
--- a/spec/unit/mutant/matcher/null_spec.rb
+++ b/spec/unit/mutant/matcher/null_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Matcher::Null, '#call' do
   let(:object) { described_class.new          }
   let(:env)    { instance_double(Mutant::Env) }
diff --git a/spec/unit/mutant/matcher/scope_spec.rb b/spec/unit/mutant/matcher/scope_spec.rb
index 909c6cca..d6eaaec7 100644
--- a/spec/unit/mutant/matcher/scope_spec.rb
+++ b/spec/unit/mutant/matcher/scope_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Matcher::Scope, '#call' do
   let(:scope)     { TestApp }
   let(:object)    { described_class.new(scope)       }
diff --git a/spec/unit/mutant/matcher/static_spec.rb b/spec/unit/mutant/matcher/static_spec.rb
index 1299c5dd..3a14f3e1 100644
--- a/spec/unit/mutant/matcher/static_spec.rb
+++ b/spec/unit/mutant/matcher/static_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Matcher::Static, '#call' do
   let(:object)   { described_class.new(subjects) }
   let(:env)      { instance_double(Mutant::Env)  }
diff --git a/spec/unit/mutant/meta/example/dsl_spec.rb b/spec/unit/mutant/meta/example/dsl_spec.rb
index 450923cb..d474ea58 100644
--- a/spec/unit/mutant/meta/example/dsl_spec.rb
+++ b/spec/unit/mutant/meta/example/dsl_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Meta::Example::DSL do
   describe '.call' do
     subject { described_class.call(file, type, block) }
diff --git a/spec/unit/mutant/meta/example/verification_spec.rb b/spec/unit/mutant/meta/example/verification_spec.rb
index 98ecb055..5470e922 100644
--- a/spec/unit/mutant/meta/example/verification_spec.rb
+++ b/spec/unit/mutant/meta/example/verification_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Meta::Example::Verification do
   let(:object) { described_class.new(example, mutations) }
 
diff --git a/spec/unit/mutant/meta/example_spec.rb b/spec/unit/mutant/meta/example_spec.rb
index 0f18e5a7..5036dd90 100644
--- a/spec/unit/mutant/meta/example_spec.rb
+++ b/spec/unit/mutant/meta/example_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Meta::Example do
   let(:object) do
     described_class.new(
diff --git a/spec/unit/mutant/mutation_spec.rb b/spec/unit/mutant/mutation_spec.rb
index 16f1e3b2..2f63985c 100644
--- a/spec/unit/mutant/mutation_spec.rb
+++ b/spec/unit/mutant/mutation_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Mutation do
   let(:mutation_class) do
     Class.new(Mutant::Mutation) do
diff --git a/spec/unit/mutant/mutator/node_spec.rb b/spec/unit/mutant/mutator/node_spec.rb
index ff8945e5..cc6326f8 100644
--- a/spec/unit/mutant/mutator/node_spec.rb
+++ b/spec/unit/mutant/mutator/node_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 Mutant::Meta::Example::ALL.each.group_by(&:node_type).each do |type, examples|
   RSpec.describe Mutant::Mutator::REGISTRY.lookup(type) do
     toplevel_nodes = examples.map { |example| example.node.type }.uniq
diff --git a/spec/unit/mutant/mutator_spec.rb b/spec/unit/mutant/mutator_spec.rb
index 0ac5d59e..262455d8 100644
--- a/spec/unit/mutant/mutator_spec.rb
+++ b/spec/unit/mutant/mutator_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Mutator do
   describe '.handle' do
     subject do
diff --git a/spec/unit/mutant/parallel/driver_spec.rb b/spec/unit/mutant/parallel/driver_spec.rb
index ff6b7b7f..349e8000 100644
--- a/spec/unit/mutant/parallel/driver_spec.rb
+++ b/spec/unit/mutant/parallel/driver_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Parallel::Driver do
   let(:object) { described_class.new(binding) }
 
diff --git a/spec/unit/mutant/parallel/master_spec.rb b/spec/unit/mutant/parallel/master_spec.rb
index 278de25c..d63b1338 100644
--- a/spec/unit/mutant/parallel/master_spec.rb
+++ b/spec/unit/mutant/parallel/master_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Parallel::Master do
   let(:message_sequence)     { FakeActor::MessageSequence.new                                             }
   let(:actor_names)          { %i[master worker_a worker_b]                                               }
diff --git a/spec/unit/mutant/parallel/source/array_spec.rb b/spec/unit/mutant/parallel/source/array_spec.rb
index 82e621b6..902c6e9a 100644
--- a/spec/unit/mutant/parallel/source/array_spec.rb
+++ b/spec/unit/mutant/parallel/source/array_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Parallel::Source::Array do
   let(:object) { described_class.new(jobs) }
 
diff --git a/spec/unit/mutant/parallel/worker_spec.rb b/spec/unit/mutant/parallel/worker_spec.rb
index 42b4d1c6..6ecbf942 100644
--- a/spec/unit/mutant/parallel/worker_spec.rb
+++ b/spec/unit/mutant/parallel/worker_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Parallel::Worker do
   let(:actor_env) do
     FakeActor::Env.new(message_sequence, actor_names)
diff --git a/spec/unit/mutant/parallel_spec.rb b/spec/unit/mutant/parallel_spec.rb
index 4eae123c..b0fe11ea 100644
--- a/spec/unit/mutant/parallel_spec.rb
+++ b/spec/unit/mutant/parallel_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Parallel do
   describe '.async' do
     subject { described_class.async(config) }
diff --git a/spec/unit/mutant/parser_spec.rb b/spec/unit/mutant/parser_spec.rb
index 1c6d4a24..cf44191c 100644
--- a/spec/unit/mutant/parser_spec.rb
+++ b/spec/unit/mutant/parser_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Parser do
   let(:object) { described_class.new }
 
diff --git a/spec/unit/mutant/registry_spec.rb b/spec/unit/mutant/registry_spec.rb
index dbd28ba4..054527d8 100644
--- a/spec/unit/mutant/registry_spec.rb
+++ b/spec/unit/mutant/registry_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Registry do
   let(:lookup)  { object.lookup(type)           }
   let(:object)  { described_class.new           }
diff --git a/spec/unit/mutant/reporter/cli/printer/config_spec.rb b/spec/unit/mutant/reporter/cli/printer/config_spec.rb
index a23ce225..76b7340d 100644
--- a/spec/unit/mutant/reporter/cli/printer/config_spec.rb
+++ b/spec/unit/mutant/reporter/cli/printer/config_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Reporter::CLI::Printer::Config do
   setup_shared_context
 
diff --git a/spec/unit/mutant/reporter/cli/printer/env_progress_spec.rb b/spec/unit/mutant/reporter/cli/printer/env_progress_spec.rb
index 332d7639..6cd0bd1a 100644
--- a/spec/unit/mutant/reporter/cli/printer/env_progress_spec.rb
+++ b/spec/unit/mutant/reporter/cli/printer/env_progress_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Reporter::CLI::Printer::EnvProgress do
   setup_shared_context
 
diff --git a/spec/unit/mutant/reporter/cli/printer/env_result_spec.rb b/spec/unit/mutant/reporter/cli/printer/env_result_spec.rb
index a62e2dec..2f951053 100644
--- a/spec/unit/mutant/reporter/cli/printer/env_result_spec.rb
+++ b/spec/unit/mutant/reporter/cli/printer/env_result_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Reporter::CLI::Printer::EnvResult do
   setup_shared_context
 
diff --git a/spec/unit/mutant/reporter/cli/printer/mutation_progress_result_spec.rb b/spec/unit/mutant/reporter/cli/printer/mutation_progress_result_spec.rb
index 0e31d0c1..a3773571 100644
--- a/spec/unit/mutant/reporter/cli/printer/mutation_progress_result_spec.rb
+++ b/spec/unit/mutant/reporter/cli/printer/mutation_progress_result_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Reporter::CLI::Printer::MutationProgressResult do
   setup_shared_context
 
diff --git a/spec/unit/mutant/reporter/cli/printer/mutation_result_spec.rb b/spec/unit/mutant/reporter/cli/printer/mutation_result_spec.rb
index a95d0dad..a256bc67 100644
--- a/spec/unit/mutant/reporter/cli/printer/mutation_result_spec.rb
+++ b/spec/unit/mutant/reporter/cli/printer/mutation_result_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Reporter::CLI::Printer::MutationResult do
   setup_shared_context
 
diff --git a/spec/unit/mutant/reporter/cli/printer/status_progressive_spec.rb b/spec/unit/mutant/reporter/cli/printer/status_progressive_spec.rb
index 31e63625..684117fd 100644
--- a/spec/unit/mutant/reporter/cli/printer/status_progressive_spec.rb
+++ b/spec/unit/mutant/reporter/cli/printer/status_progressive_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Reporter::CLI::Printer::StatusProgressive do
   setup_shared_context
 
diff --git a/spec/unit/mutant/reporter/cli/printer/status_spec.rb b/spec/unit/mutant/reporter/cli/printer/status_spec.rb
index e888f8f4..8f503f4e 100644
--- a/spec/unit/mutant/reporter/cli/printer/status_spec.rb
+++ b/spec/unit/mutant/reporter/cli/printer/status_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Reporter::CLI::Printer::Status do
   setup_shared_context
 
diff --git a/spec/unit/mutant/reporter/cli/printer/subject_progress_spec.rb b/spec/unit/mutant/reporter/cli/printer/subject_progress_spec.rb
index 041ba1d4..a525879a 100644
--- a/spec/unit/mutant/reporter/cli/printer/subject_progress_spec.rb
+++ b/spec/unit/mutant/reporter/cli/printer/subject_progress_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Reporter::CLI::Printer::SubjectProgress do
   setup_shared_context
 
diff --git a/spec/unit/mutant/reporter/cli/printer/subject_result_spec.rb b/spec/unit/mutant/reporter/cli/printer/subject_result_spec.rb
index a0525282..ae5e3aa8 100644
--- a/spec/unit/mutant/reporter/cli/printer/subject_result_spec.rb
+++ b/spec/unit/mutant/reporter/cli/printer/subject_result_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Reporter::CLI::Printer::SubjectResult do
   setup_shared_context
 
diff --git a/spec/unit/mutant/reporter/cli/printer/test_result_spec.rb b/spec/unit/mutant/reporter/cli/printer/test_result_spec.rb
index 10b2a25c..b06a23e6 100644
--- a/spec/unit/mutant/reporter/cli/printer/test_result_spec.rb
+++ b/spec/unit/mutant/reporter/cli/printer/test_result_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Reporter::CLI::Printer::TestResult do
   setup_shared_context
 
diff --git a/spec/unit/mutant/reporter/cli/printer_spec.rb b/spec/unit/mutant/reporter/cli/printer_spec.rb
index 880f36e1..825e7c81 100644
--- a/spec/unit/mutant/reporter/cli/printer_spec.rb
+++ b/spec/unit/mutant/reporter/cli/printer_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Reporter::CLI::Printer do
   let(:output) { StringIO.new }
 
@@ -62,12 +63,12 @@ RSpec.describe Mutant::Reporter::CLI::Printer do
 
     context 'on tty' do
       context 'on success' do
-        it_reports Mutant::Color::GREEN.format('foo bar') << "\n"
+        it_reports Mutant::Color::GREEN.format('foo bar') + "\n"
       end
 
       context 'on failure' do
         let(:success?) { false }
-        it_reports Mutant::Color::RED.format('foo bar') << "\n"
+        it_reports Mutant::Color::RED.format('foo bar') + "\n"
       end
     end
 
@@ -150,7 +151,7 @@ RSpec.describe Mutant::Reporter::CLI::Printer do
     end
 
     context 'when output is a tty?' do
-      it_reports Mutant::Color::RED.format('foo') << "\n"
+      it_reports Mutant::Color::RED.format('foo') + "\n"
     end
 
     context 'when output is NOT a tty?' do
diff --git a/spec/unit/mutant/reporter/cli/tput_spec.rb b/spec/unit/mutant/reporter/cli/tput_spec.rb
index 37135ac7..e39044fe 100644
--- a/spec/unit/mutant/reporter/cli/tput_spec.rb
+++ b/spec/unit/mutant/reporter/cli/tput_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Reporter::CLI::Tput do
   describe '.detect' do
     subject { described_class.detect }
diff --git a/spec/unit/mutant/reporter/cli_spec.rb b/spec/unit/mutant/reporter/cli_spec.rb
index 6f3d287a..aae8e874 100644
--- a/spec/unit/mutant/reporter/cli_spec.rb
+++ b/spec/unit/mutant/reporter/cli_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Reporter::CLI do
   setup_shared_context
 
@@ -191,7 +192,7 @@ RSpec.describe Mutant::Reporter::CLI do
 
         let(:tty?) { true }
 
-        it_reports Mutant::Color::GREEN.format('(00/02) 100% - killtime: 0.00s runtime: 4.00s overhead: 4.00s') << "\n"
+        it_reports Mutant::Color::GREEN.format('(00/02) 100% - killtime: 0.00s runtime: 4.00s overhead: 4.00s') + "\n"
       end
 
       context 'with last mutation present' do
diff --git a/spec/unit/mutant/reporter/null_spec.rb b/spec/unit/mutant/reporter/null_spec.rb
index 59d2ca62..43dcd155 100644
--- a/spec/unit/mutant/reporter/null_spec.rb
+++ b/spec/unit/mutant/reporter/null_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Reporter::Null do
   let(:object) { described_class.new     }
   let(:value)  { instance_double(Object) }
diff --git a/spec/unit/mutant/reporter/sequence_spec.rb b/spec/unit/mutant/reporter/sequence_spec.rb
index c5c98b46..cc84a4dd 100644
--- a/spec/unit/mutant/reporter/sequence_spec.rb
+++ b/spec/unit/mutant/reporter/sequence_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Reporter::Sequence do
   let(:object)     { described_class.new([reporter_a, reporter_b]) }
   let(:value)      { instance_double(Object)                       }
diff --git a/spec/unit/mutant/repository/diff_spec.rb b/spec/unit/mutant/repository/diff_spec.rb
index a7957d17..2b460a20 100644
--- a/spec/unit/mutant/repository/diff_spec.rb
+++ b/spec/unit/mutant/repository/diff_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 describe Mutant::Repository::Diff do
   describe '#touches?' do
     let(:object) do
diff --git a/spec/unit/mutant/repository/subject_filter_spec.rb b/spec/unit/mutant/repository/subject_filter_spec.rb
index 80fa2bc5..4c5b2e69 100644
--- a/spec/unit/mutant/repository/subject_filter_spec.rb
+++ b/spec/unit/mutant/repository/subject_filter_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Repository::SubjectFilter do
   context '#call' do
     subject { object.call(mutant_subject) }
diff --git a/spec/unit/mutant/require_highjack_spec.rb b/spec/unit/mutant/require_highjack_spec.rb
index dbb766e1..356aac42 100644
--- a/spec/unit/mutant/require_highjack_spec.rb
+++ b/spec/unit/mutant/require_highjack_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::RequireHighjack do
   let(:highjacked_calls) { [] }
   let(:require_calls)    { [] }
diff --git a/spec/unit/mutant/result/class_methods_spec.rb b/spec/unit/mutant/result/class_methods_spec.rb
index 1cf3aaf3..be6e6f87 100644
--- a/spec/unit/mutant/result/class_methods_spec.rb
+++ b/spec/unit/mutant/result/class_methods_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 # The effect of this private module is done at boot time.
 # Hence there is no way to kill the mutations at runtime
 # so we have to directly hook into the private moduel via
diff --git a/spec/unit/mutant/result/env_spec.rb b/spec/unit/mutant/result/env_spec.rb
index 22d9bd78..cb1b71fb 100644
--- a/spec/unit/mutant/result/env_spec.rb
+++ b/spec/unit/mutant/result/env_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Result::Env do
   let(:object) do
     described_class.new(
diff --git a/spec/unit/mutant/result/mutation_spec.rb b/spec/unit/mutant/result/mutation_spec.rb
index baef27d1..2c1c4625 100644
--- a/spec/unit/mutant/result/mutation_spec.rb
+++ b/spec/unit/mutant/result/mutation_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Result::Mutation do
   let(:object) do
     described_class.new(
diff --git a/spec/unit/mutant/result/subject_spec.rb b/spec/unit/mutant/result/subject_spec.rb
index b337032f..2c3b9035 100644
--- a/spec/unit/mutant/result/subject_spec.rb
+++ b/spec/unit/mutant/result/subject_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Result::Subject do
   let(:object) do
     described_class.new(
diff --git a/spec/unit/mutant/result_spec.rb b/spec/unit/mutant/result_spec.rb
index 25350a5f..e79546d0 100644
--- a/spec/unit/mutant/result_spec.rb
+++ b/spec/unit/mutant/result_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Result do
   let(:object) do
     Class.new do
diff --git a/spec/unit/mutant/runner/sink_spec.rb b/spec/unit/mutant/runner/sink_spec.rb
index e49aa988..a144e72a 100644
--- a/spec/unit/mutant/runner/sink_spec.rb
+++ b/spec/unit/mutant/runner/sink_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 describe Mutant::Runner::Sink do
   setup_shared_context
 
diff --git a/spec/unit/mutant/runner_spec.rb b/spec/unit/mutant/runner_spec.rb
index 5ac9a49b..53932952 100644
--- a/spec/unit/mutant/runner_spec.rb
+++ b/spec/unit/mutant/runner_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Runner do
   describe '.call' do
     let(:reporter)    { instance_double(Mutant::Reporter, delay: delay) }
diff --git a/spec/unit/mutant/selector/expression_spec.rb b/spec/unit/mutant/selector/expression_spec.rb
index 38aebd0b..06cd3f30 100644
--- a/spec/unit/mutant/selector/expression_spec.rb
+++ b/spec/unit/mutant/selector/expression_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Selector::Expression do
   describe '#call' do
     let(:object) { described_class.new(integration) }
diff --git a/spec/unit/mutant/subject/method/instance_spec.rb b/spec/unit/mutant/subject/method/instance_spec.rb
index 441c291b..68f4739b 100644
--- a/spec/unit/mutant/subject/method/instance_spec.rb
+++ b/spec/unit/mutant/subject/method/instance_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Subject::Method::Instance do
   let(:object)  { described_class.new(context, node) }
 
diff --git a/spec/unit/mutant/subject/method/singleton_spec.rb b/spec/unit/mutant/subject/method/singleton_spec.rb
index f61f10f4..88861338 100644
--- a/spec/unit/mutant/subject/method/singleton_spec.rb
+++ b/spec/unit/mutant/subject/method/singleton_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Subject::Method::Singleton do
 
   let(:object)  { described_class.new(context, node) }
diff --git a/spec/unit/mutant/subject_spec.rb b/spec/unit/mutant/subject_spec.rb
index 92e85342..a4830c37 100644
--- a/spec/unit/mutant/subject_spec.rb
+++ b/spec/unit/mutant/subject_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Subject do
   let(:class_under_test) do
     Class.new(described_class) do
diff --git a/spec/unit/mutant/util/one_spec.rb b/spec/unit/mutant/util/one_spec.rb
index 5f4acdec..a3907708 100644
--- a/spec/unit/mutant/util/one_spec.rb
+++ b/spec/unit/mutant/util/one_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Util, '.one' do
   let(:first) { instance_double(Object)                          }
   let(:array) { instance_double(Array, one?: true, first: first) }
diff --git a/spec/unit/mutant/warning_filter_spec.rb b/spec/unit/mutant/warning_filter_spec.rb
index 7d1d069b..d6b78f75 100644
--- a/spec/unit/mutant/warning_filter_spec.rb
+++ b/spec/unit/mutant/warning_filter_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::WarningFilter do
   before do
     if RUBY_ENGINE.eql?('rbx')
diff --git a/spec/unit/mutant/zombifier_spec.rb b/spec/unit/mutant/zombifier_spec.rb
index ee14daa6..1d623778 100644
--- a/spec/unit/mutant/zombifier_spec.rb
+++ b/spec/unit/mutant/zombifier_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant::Zombifier do
   let(:root_require) { Pathname.new('project') }
   let(:pathname)     { class_double(Pathname)  }
diff --git a/spec/unit/mutant_spec.rb b/spec/unit/mutant_spec.rb
index b6d1bdd4..1a1046ea 100644
--- a/spec/unit/mutant_spec.rb
+++ b/spec/unit/mutant_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe Mutant do
   let(:object) { described_class }