Fix compatibility with --enable-frozen-string-literal

.github/workflows/ruby.yml

@@ -18,6 +18,7 @@ jobs:
       fail-fast: false
       matrix:
         ruby_version: [head, 3.4, 3.3, 3.2, 3.1, '3.0', jruby]
+        rubyopt: [""]
         gemfile:
           - Gemfile
           - gemfiles/Gemfile.rails-6.0.x
@@ -48,6 +49,10 @@ jobs:
             gemfile: gemfiles/Gemfile.rails-8.0.x
           - ruby_version: jruby
             gemfile: gemfiles/Gemfile.rails-main
+        include:
+          - ruby_version: 3.4
+            gemfile: Gemfile
+            rubyopt: "--enable-frozen-string-literal --debug-frozen-string-literal"
 
     runs-on: ubuntu-latest
 

lib/i18n/backend/interpolation_compiler.rb

@@ -25,6 +25,7 @@ module Compiler
 
         def compile_if_an_interpolation(string)
           if interpolated_str?(string)
+            string = +string
             string.instance_eval <<-RUBY_EVAL, __FILE__, __LINE__
               def i18n_interpolate(v = {})
                 "#{compiled_interpolation_body(string)}"

lib/i18n/tests/interpolation.rb

@@ -89,14 +89,14 @@ module Interpolation
         end
 
         test "interpolation: ASCII strings in the backend should be encoded to UTF8 if interpolation options are in UTF8" do
-          I18n.backend.store_translations 'en', 'encoding' => ('%{who} let me go'.force_encoding("ASCII"))
+          I18n.backend.store_translations 'en', 'encoding' => ('%{who} let me go'.dup.force_encoding(Encoding::US_ASCII))
           result = I18n.t 'encoding', :who => "måmmå miå"
           assert_equal Encoding::UTF_8, result.encoding
         end
 
         test "interpolation: UTF8 strings in the backend are still returned as UTF8 with ASCII interpolation" do
           I18n.backend.store_translations 'en', 'encoding' => 'måmmå miå %{what}'
-          result = I18n.t 'encoding', :what => 'let me go'.force_encoding("ASCII")
+          result = I18n.t 'encoding', :what => 'let me go'.dup.force_encoding(Encoding::US_ASCII)
           assert_equal Encoding::UTF_8, result.encoding
         end
 
@@ -172,7 +172,7 @@ def capture(stream)
       end
 
       def euc_jp(string)
-        string.encode!(Encoding::EUC_JP)
+        string.encode(Encoding::EUC_JP)
       end
 
       def interpolate(*args)

test/backend/metadata_test.rb

@@ -38,7 +38,7 @@ def setup
   end
 
   test "pluralization adds the count to metadata on Strings" do
-    assert_equal(1, I18n.t(:missing, :count => 1, :default => { :one => 'foo' }).translation_metadata[:count])
+    assert_equal(1, I18n.t(:missing, :count => 1, :default => { :one => +'foo' }).translation_metadata[:count])
   end
 
   test "metadata works with frozen values" do