[Fix rubocop#157] Add new Minitest/EmptyLinesBeforeAssertionMethod cop

Fixes rubocop#157.

This PR adds new `Minitest/EmptyLinesBeforeAssertionMethod` cop.

This cop enforces empty line before assertion method.

```ruby
# bad
do_something
assert_equal(expected, actual)

# good
do_something

assert_equal(expected, actual)
```

changelog/new_add_new_minitest_empty_line_before_assertion_method_cop.md

@@ -0,0 +1 @@
+* [#157](https://github.com/rubocop/rubocop-minitest/issues/157): Add new `Minitest/EmptyLinesBeforeAssertionMethod` cop. ([@koic][])

config/default.yml

@@ -99,6 +99,11 @@ Minitest/AssertWithExpectedArgument:
   Safe: false
   VersionAdded: '0.11'
 
+Minitest/EmptyLinesBeforeAssertionMethod:
+  Description: 'Add empty line after assertion method.'
+  Enabled: pending
+  VersionAdded: '<<next>>'
+
 Minitest/GlobalExpectations:
   Description: 'This cop checks for deprecated global expectations.'
   StyleGuide: 'https://minitest.rubystyle.guide#global-expectations'

lib/rubocop/cop/minitest/empty_lines_before_assertion_method.rb

@@ -0,0 +1,76 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Minitest
+      # This cop enforces empty line before assertion method.
+      #
+      # @example
+      #
+      #   # bad
+      #   do_something
+      #   assert_equal(expected, actual)
+      #
+      #   # good
+      #   do_something
+      #
+      #   assert_equal(expected, actual)
+      #
+      class EmptyLinesBeforeAssertionMethod < Base
+        include MinitestExplorationHelpers
+        extend AutoCorrector
+
+        MSG = 'Add empty line before assertion.'
+
+        def on_send(node)
+          return unless assertion_method?(node)
+          return unless (previous_line_node = node.left_sibling)
+          return if accept_previous_line?(previous_line_node, node)
+
+          previous_line_node = previous_line_node.arguments.last if use_heredoc_argument?(previous_line_node)
+          return unless no_empty_line?(previous_line_node, node)
+
+          register_offense(node, previous_line_node)
+        end
+
+        private
+
+        def accept_previous_line?(previous_line_node, node)
+          return true if previous_line_node.args_type? || node.parent.basic_conditional?
+
+          previous_line_node.send_type? && assertion_method?(previous_line_node)
+        end
+
+        def use_heredoc_argument?(node)
+          node.respond_to?(:arguments) && heredoc?(node.arguments.last)
+        end
+
+        def heredoc?(last_argument)
+          last_argument.respond_to?(:heredoc?) && last_argument.heredoc?
+        end
+
+        def no_empty_line?(previous_line_node, node)
+          previous_line = if heredoc?(previous_line_node)
+                            previous_line_node.loc.heredoc_end.line
+                          else
+                            previous_line_node.loc.last_line
+                          end
+
+          previous_line + 1 == node.loc.line
+        end
+
+        def register_offense(node, previous_line_node)
+          add_offense(node) do |corrector|
+            range = if heredoc?(previous_line_node)
+                      previous_line_node.loc.heredoc_end
+                    else
+                      previous_line_node
+                    end
+
+            corrector.insert_after(range, "\n")
+          end
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/minitest_cops.rb

@@ -21,6 +21,7 @@
 require_relative 'minitest/assert_respond_to'
 require_relative 'minitest/assert_silent'
 require_relative 'minitest/assert_truthy'
+require_relative 'minitest/empty_lines_before_assertion_method'
 require_relative 'minitest/global_expectations'
 require_relative 'minitest/literal_as_actual_argument'
 require_relative 'minitest/multiple_assertions'

test/project_test.rb

@@ -52,8 +52,8 @@ def test_entry_has_an_issue_number_prefixed_with_sharp
   def test_entry_has_a_valid_url
     @issues.each do |issue|
       number = issue[:number].gsub(/\D/, '')
-      pattern =
-        %r{^https://github\.com/rubocop/rubocop-minitest/(?:issues|pull)/#{number}$}
+      pattern = %r{^https://github\.com/rubocop/rubocop-minitest/(?:issues|pull)/#{number}$}
+
       assert_match(pattern, issue[:url])
     end
   end

test/rubocop/cop/minitest/empty_lines_before_assertion_method_test.rb

@@ -0,0 +1,156 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+
+class EmptyLinesBeforeAssertionMethodTest < Minitest::Test
+  def test_registers_offense_when_using_method_call_before_assertion_method
+    assert_offense(<<~RUBY)
+      def test_do_something
+        do_something
+        assert_equal(expected, actual)
+        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Add empty line before assertion.
+      end
+    RUBY
+
+    assert_correction(<<~RUBY)
+      def test_do_something
+        do_something
+
+        assert_equal(expected, actual)
+      end
+    RUBY
+  end
+
+  def test_registers_offense_when_using_method_call_with_line_breaked_args_before_assertion_method
+    assert_offense(<<~RUBY)
+      def test_do_something
+        do_something(
+          foo,
+          bar
+        )
+        assert_equal(expected, actual)
+        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Add empty line before assertion.
+      end
+    RUBY
+
+    assert_correction(<<~RUBY)
+      def test_do_something
+        do_something(
+          foo,
+          bar
+        )
+
+        assert_equal(expected, actual)
+      end
+    RUBY
+  end
+
+  def test_registers_offense_when_using_heredoc_before_assertion_method
+    assert_offense(<<~RUBY)
+      def test_do_something
+        <<~EOS
+          text
+        EOS
+        assert_equal(expected, actual)
+        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Add empty line before assertion.
+      end
+    RUBY
+
+    assert_correction(<<~RUBY)
+      def test_do_something
+        <<~EOS
+          text
+        EOS
+
+        assert_equal(expected, actual)
+      end
+    RUBY
+  end
+
+  def test_registers_offense_when_using_method_call_with_heredoc_arg_before_assertion_method
+    assert_offense(<<~RUBY)
+      def test_do_something
+        do_something(<<~EOS)
+          text
+        EOS
+        assert_equal(expected, actual)
+        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Add empty line before assertion.
+      end
+    RUBY
+
+    assert_correction(<<~RUBY)
+      def test_do_something
+        do_something(<<~EOS)
+          text
+        EOS
+
+        assert_equal(expected, actual)
+      end
+    RUBY
+  end
+
+  def test_does_not_register_offense_when_using_empty_line_before_assertion_method
+    assert_no_offenses(<<~RUBY)
+      def test_do_something
+        do_something
+
+        assert_equal(expected, actual)
+      end
+    RUBY
+  end
+
+  def test_does_not_register_offense_when_using_assertion_method_at_top_level
+    assert_no_offenses(<<~RUBY)
+      assert_equal(expected, actual)
+    RUBY
+  end
+
+  def test_does_not_register_offense_when_using_assertion_method_at_top_of_block_body
+    assert_no_offenses(<<~RUBY)
+      def test_do_something
+        do_something do
+          assert_equal(expected, actual)
+        end
+      end
+    RUBY
+  end
+
+  def test_does_not_register_offense_when_using_assertion_method_at_top_of_if_body
+    assert_no_offenses(<<~RUBY)
+      def test_do_something
+        if condition
+          assert_equal(expected, actual)
+        end
+      end
+    RUBY
+  end
+
+  def test_does_not_register_offense_when_using_assertion_method_at_top_of_while_body
+    assert_no_offenses(<<~RUBY)
+      def test_do_something
+        while condition
+          assert_equal(expected, actual)
+        end
+      end
+    RUBY
+  end
+
+  def test_does_not_register_offense_when_using_assertion_method_at_top_of_until_body
+    assert_no_offenses(<<~RUBY)
+      def test_do_something
+        until condition
+          assert_equal(expected, actual)
+        end
+      end
+    RUBY
+  end
+
+  def test_does_not_register_offense_when_using_assertion_methods_are_continuous_without_empty_line
+    assert_no_offenses(<<~RUBY)
+      def test_do_something
+        assert_not foo
+        assert bar
+      end
+    RUBY
+  end
+end