Fix debug platform inclusion for Windows

lib/ruby_lsp/setup_bundler.rb

@@ -152,7 +152,13 @@ def write_custom_gemfile
       end
 
       unless @dependencies["debug"]
-        parts << 'gem "debug", require: false, group: :development, platforms: :mri'
+        # The `mri` platform excludes Windows. We want to install the debug gem only on MRI for any operating system,
+        # but that constraint doesn't yet exist in Bundler. On Windows, we are manually checking if the engine is MRI
+        parts << if Gem.win_platform?
+          'gem "debug", require: false, group: :development, install_if: -> { RUBY_ENGINE == "ruby" }'
+        else
+          'gem "debug", require: false, group: :development, platforms: :mri'
+        end
       end
 
       if @rails_app && !@dependencies["ruby-lsp-rails"]

test/setup_bundler_test.rb

@@ -823,6 +823,38 @@ def test_is_resilient_to_gemfile_changes_in_the_middle_of_setup
     end
   end
 
+  def test_composed_bundle_includes_debug
+    Dir.mktmpdir do |dir|
+      Dir.chdir(dir) do
+        File.write(File.join(dir, "Gemfile"), <<~GEMFILE)
+          source "https://rubygems.org"
+        GEMFILE
+
+        env = T.let(nil, T.nilable(T::Hash[String, String]))
+
+        Bundler.with_unbundled_env do
+          capture_subprocess_io do
+            # Run bundle install to generate the lockfile
+            system("bundle install")
+            env = RubyLsp::SetupBundler.new(dir, launcher: true).setup!
+          end
+        end
+
+        gemfile_path = File.join(dir, ".ruby-lsp", "Gemfile")
+        assert_match("debug", File.read(gemfile_path))
+        assert_match("ruby-lsp", File.read(gemfile_path))
+
+        _stdout, stderr = capture_subprocess_io do
+          Bundler.with_unbundled_env do
+            system(T.must(env), "bundle exec ruby -e 'require \"debug\"'")
+          end
+        end
+
+        assert_empty(stderr)
+      end
+    end
+  end
+
   private
 
   def with_default_external_encoding(encoding, &block)