Explicitly call I18n.exception_handler on missing translation

Fix: rails#54419

Since `TranslationHelper#translate` always sets a `default`,
missing translation is never considered like an error by `I18n`
and the error handler is never called.

This makes I18n's exception handler pretty much unusable.

The solution is to explictly call it ourselves, but the downside
is we need to allocate that exception, which isn't free, but
supposedly it's not expected to happen often.

Author: byroot

actionview/lib/action_view/helpers/translation_helper.rb

@@ -140,7 +140,12 @@ def scope_key_by_partial(key)
         end
 
         def missing_translation(key, options)
-          keys = I18n.normalize_keys(options[:locale] || I18n.locale, key, options[:scope])
+          locale = options[:locale] || I18n.locale
+
+          i18n_exception = I18n::MissingTranslation.new(locale, key, options)
+          I18n.exception_handler.call(i18n_exception, locale, key, options)
+
+          keys = I18n.normalize_keys(locale, key, options[:scope])
 
           title = +"translation missing: #{keys.join(".")}"
 

actionview/test/template/translation_helper_test.rb

@@ -172,6 +172,28 @@ def test_default_lookup_scoped_by_partial
     assert_equal "Foo", view.render(template: "translations/templates/default").strip
   end
 
+  def test_missing_translation_reported_to_i18n_exception_handler
+    previous_handler = I18n.exception_handler
+
+    calls = []
+    I18n.exception_handler = ->(*args) { calls << args }
+    view.render(template: "translations/templates/missing")
+
+    first_call = calls.first
+    assert_not_nil first_call
+    exception = first_call.first
+    assert_instance_of I18n::MissingTranslation, exception
+    assert_equal "translations.templates.missing.missing", exception.key
+
+    assert_equal 1, calls.size
+
+    assert_nothing_raised do
+      previous_handler.call(*first_call)
+    end
+  ensure
+    I18n.exception_handler = previous_handler
+  end
+
   def test_missing_translation_scoped_by_partial
     expected = '<span class="translation_missing" title="translation missing: en.translations.templates.missing.missing">Missing</span>'
     assert_equal expected, view.render(template: "translations/templates/missing").strip