DEV: Output failing MF keys when compilation fails

Currently, when the MessageFormat compiler fails on some translations,
we just have the raw output from the compiler in the logs and that’s not
always very helpful.

Now, when there is an error, we iterate over the translation keys and
try to compile them one by one. When we detect one that is failing, it’s
added to a list that is now outputted in the logs. That way, it’s easier
to know which keys are not properly translated, and the problems can be
addressed quicker.

lib/js_locale_helper.rb

@@ -163,7 +163,17 @@ module JsLocaleHelper
       require("discourse-mf");
     JS
   rescue => e
-    Rails.logger.error("Failed to compile message formats for #{locale} '#{e}'")
+    message_formats[locale.to_s]
+      .filter_map do |key, value|
+        next if MessageFormat.compile(locale, value, strict: false)
+      rescue StandardError
+        key
+      end
+      .then do |strings|
+        Rails.logger.error(
+          "Failed to compile message formats for #{locale}.\n\nBroken strings are: #{strings.join(", ")}\n\nError: #{e}",
+        )
+      end
     <<~JS
       console.error("Failed to compile message formats for #{locale}. Some translation strings will be missing.");
     JS

spec/lib/js_locale_helper_spec.rb

@@ -169,8 +169,10 @@ RSpec.describe JsLocaleHelper do
     let(:translated_message) do
       v8_ctx.eval("I18n._mfMessages.get('posts_likes_MF', {count: 3, ratio: 'med'})")
     end
+    let(:fake_logger) { FakeLogger.new }
 
     before do
+      Rails.logger.broadcast_to(fake_logger)
       overriden_translation_ja.update_columns(
         value: "{ count, plural, one {返信 # 件、} other {返信 # 件、} }",
       )
@@ -178,6 +180,8 @@ RSpec.describe JsLocaleHelper do
       v8_ctx.eval(output)
     end
 
+    after { Rails.logger.stop_broadcasting_to(fake_logger) }
+
     context "when locale is 'en'" do
       let(:locale) { "en" }
 
@@ -258,6 +262,11 @@ RSpec.describe JsLocaleHelper do
       it "raises an error" do
         expect(output).to match(/Failed to compile message formats/)
       end
+
+      it "logs which keys are problematic" do
+        output
+        expect(fake_logger.errors).to include(/posts_likes_MF/)
+      end
     end
   end