Honor custom formatters on runtimes without Ractor

lib/did_you_mean.rb

@@ -118,14 +118,16 @@ def self.formatter
     if defined?(Ractor)
       Ractor.current[:__did_you_mean_formatter__] || Formatter
     else
-      Formatter
+      @formatter || Formatter
     end
   end
 
   # Updates the primary formatter used to format the suggestions.
   def self.formatter=(formatter)
     if defined?(Ractor)
       Ractor.current[:__did_you_mean_formatter__] = formatter
+    else
+      @formatter = formatter
     end
   end
 end

test/test_formatter.rb

@@ -0,0 +1,37 @@
+require_relative './helper'
+
+class FormatterTest < Test::Unit::TestCase
+  class CustomFormatter
+    def self.message_for(corrections)
+      " custom suggestions: #{corrections.join(', ')}"
+    end
+  end
+
+  def setup
+    @original_formatter = DidYouMean.formatter
+  end
+
+  def teardown
+    DidYouMean.formatter = @original_formatter
+  end
+
+  def test_custom_formatter_is_used_for_error_messages
+    DidYouMean.formatter = CustomFormatter
+    assert_same CustomFormatter, DidYouMean.formatter
+    error = assert_raise(KeyError) { { book: 1 }.fetch(:boook) }
+    assert_include DidYouMean::TestHelper.get_message(error), 'custom suggestions: :book'
+  end
+
+  def test_nil_formatter_restores_default
+    DidYouMean.formatter = CustomFormatter
+    DidYouMean.formatter = nil
+    assert_same DidYouMean::Formatter, DidYouMean.formatter
+  end
+
+  unless defined?(Ractor)
+    def test_formatter_is_shared_between_threads_without_ractors
+      DidYouMean.formatter = CustomFormatter
+      assert_same CustomFormatter, Thread.new { DidYouMean.formatter }.value
+    end
+  end
+end