Treat string literals as frozen

.travis.yml

@@ -4,7 +4,9 @@ branches:
   only:
     - master
 script:
-  - "RUBYOPT='-w' bundle exec rake $TASK"
+  - bundle exec rake $TASK
+env:
+  - RUBYOPT='-w'
 matrix:
   include:
     - rvm: 2.3.8
@@ -17,6 +19,8 @@ matrix:
       env: TASK=test
     - rvm: 2.7.0
       env: TASK=test
+    - rvm: 2.7.0
+      env: TASK=test RUBYOPT='-w --enable-frozen-string-literal'
     - rvm: ruby-head
       env: TASK=test
     - rvm: jruby-9.2.8.0

lib/hamlit/compiler/comment_compiler.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Hamlit
   class Compiler
     class CommentCompiler

lib/hamlit/filters/markdown.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Hamlit
   class Filters
     class Markdown < TiltBase

lib/hamlit/string_splitter.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require 'ripper'
 require 'hamlit/ruby_expression'
 

test/haml/engine_test.rb

@@ -1783,7 +1783,7 @@ def test_ruby_multiline_with_punctuated_methods_is_continuation
 <p>bar</p>
 HTML
 = ["bar",
-   "  ".strip!,
+   "  ".strip,
    "".empty?,
    "bang"].join(", ")
 %p foo
@@ -2000,7 +2000,7 @@ def test_convert_template_def_method
   end
 
   def test_encoding_error # encoding
-    render("foo\nbar\nb\xFEaz".force_encoding("utf-8"))
+    render("foo\nbar\nb\xFEaz".dup.force_encoding("utf-8"))
     assert(false, "Expected exception")
   rescue Hamlit::Error => e
     assert_equal(3, e.line)

test/hamlit/engine/attributes_test.rb

@@ -13,7 +13,7 @@
       it { assert_haml(%q|#b{ id: 'a' }|) }
       it { assert_haml(%q|%a{ 'id' => 60 }|) }
       if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('2.4.0')
-        it { assert_haml(%q|%p{class: "a #{"1".concat "2", "3"}"} foo|) }
+        it { assert_haml(%q|%p{class: "a #{["1", "2", "3"].join}"} foo|) }
       end
 
       it { assert_haml(%q|#a{ id: 'b' }(id=id)|,       locals: { id: 'c' }) }