Rspec mock argument capture

capture.rb

# just an example of what it could be
argument = capture('label')
widget = double

# argument acts like an argument matcher that always matches but stores a reference to the real value
allow(widget).to receive(:do_something).with(argument)

# contrived, I know this could be done using the instance_of argument matcher
expect(argument.value).to be_instance_of(Something)

expect(foo).to receive(:publish_doc) do |doc|
  expect(doc.category).to eq "Code red"
  expect(doc.message).to eq "The monkeys have escaped"
end