Could you share warnings you saw?

It seems that the added test without your fix don't show any warnings.

Sorry, I was ahead of myself. The warning comes from Rails with Minitest.

I set up this Rails app with an index controller:

class IndexController < ApplicationController
  module Text
    def self.text
      "hello, world"
    end
  end

  def index
    render plain: Text.text
  end
end

class IndexControllerTest < ActionDispatch::IntegrationTest
  test "should get index" do
    stub(IndexController::Text).text { "stub" }

    get root_url
    assert_response 200
    assert_equal "stub", response.body
  end
end

Expected

Test works without warnings:

> rails test test/controllers/index_controller_test.rb
Running 1 tests in a single process (parallelization threshold is 50)
Run options: --seed 12192

# Running:

.

Finished in 0.043967s, 22.7443 runs/s, 45.4887 assertions/s.
1 runs, 2 assertions, 0 failures, 0 errors, 0 skips

When I use RR, that is using the assert_received without other assertions:

class IndexControllerTest < ActionDispatch::IntegrationTest
  test "should get index" do
    stub(IndexController::Text).text { "stub" }

    get root_url
    assert_received(IndexController::Text) do |expect|
      expect.text
    end
  end
end

I should get 1 runs, 1 assertions, and Rails should not warn me about missing assertions.

Actual

rails test test/controllers/index_controller_test.rb
Running 1 tests in a single process (parallelization threshold is 50)
Run options: --seed 22002

# Running:

Test is missing assertions: `test_should_get_index` testapp/test/controllers/index_controller_test.rb:4
.

Finished in 0.057891s, 17.2738 runs/s, 0.0000 assertions/s.
1 runs, 0 assertions, 0 failures, 0 errors, 0 skips

With this patch

rails test test/controllers/index_controller_test.rb
Running 1 tests in a single process (parallelization threshold is 50)
Run options: --seed 38290

# Running:

.

Finished in 0.060685s, 16.4785 runs/s, 16.4785 assertions/s.
1 runs, 1 assertions, 0 failures, 0 errors, 0 skips

Thanks. It seems that the following change is enough:

diff --git a/lib/rr/integrations/minitest_4.rb b/lib/rr/integrations/minitest_4.rb
index 8e73aa8..90bacfc 100644
--- a/lib/rr/integrations/minitest_4.rb
+++ b/lib/rr/integrations/minitest_4.rb
@@ -3,6 +3,7 @@ module RR
     class MiniTest4
       module Mixin
         def assert_received(subject, &block)
+          self.assertions += 1
           block.call(received(subject)).call
         end
       end

What do you think about this?

Of course. I will move the change to minitest4.

Updated. I kept the test case to test the assertions count on minitest5.

(We'll use the PR description for commit message.)

Thanks for the context! Updated.

Would you want another PR to support open struct?

RR looks to be quite backward compatible to me. If you want to keep the backwards compatible promise, I suggest the 'defined?' check for now.

Would you want another PR to support open struct?

I've already fixed it :-)
29ce3b2