Add test case

actionpack/test/controller/renderer_test.rb

 # frozen_string_literal: true
 
 require "abstract_unit"
+require "test_component"
 
 class RendererTest < ActiveSupport::TestCase
   test "action controller base has a renderer" do
@@ -65,6 +66,15 @@ class RendererTest < ActiveSupport::TestCase
     assert_equal "The secret is foo\n", content
   end
 
+  def test_render_component
+    renderer = ApplicationController.renderer
+
+    assert_equal(
+      %(<span title="my title">(Inline render)</span>),
+      renderer.render(TestComponent.new(title: "my title")).strip
+    )
+  end
+
   test "rendering with custom env" do
     renderer = ApplicationController.renderer.new method: "post"
     content  = renderer.render inline: "<%= request.post? %>"

actionpack/test/lib/test_component.rb

+# frozen_string_literal: true
+
+class TestComponent < ActionView::Base
+  include ActiveModel::Validations
+
+  validates :title, presence: true
+  delegate :render, to: :view_context
+
+  def initialize(title:)
+    @title = title
+  end
+
+  # Entrypoint for rendering. Called by ActionView::RenderingHelper#render.
+  #
+  # Returns ActionView::OutputBuffer.
+  def render_in(view_context)
+    self.class.compile
+    @view_context = view_context
+    validate!
+    rendered_template
+  end
+
+  def self.template
+    <<~'erb'
+    <span title="<%= title %>">(<%= render(plain: "Inline render") %>)</span>
+    erb
+  end
+
+  def self.compile
+    @compiled ||= nil
+    return if @compiled
+
+    class_eval(
+      "def rendered_template; @output_buffer = ActionView::OutputBuffer.new; " +
+      ActionView::Template::Handlers::ERB.erb_implementation.new(template, trim: true).src +
+      "; end"
+    )
+
+    @compiled = true
+  end
+
+private
+  attr_reader :title, :view_context
+end