Add a custom RSpec/SingleLineHook cop

This cop adds an offense when `before`, `after`, or `around` are used as
single-line blocks.

.rubocop.yml

@@ -1064,6 +1064,13 @@ RSpec/NotToNot:
 RSpec/RepeatedDescription:
   Enabled: false
 
+# Ensure RSpec hook blocks are always multi-line.
+RSpec/SingleLineHook:
+  Enabled: false
+  Exclude:
+    - 'spec/factories/*'
+    - 'spec/requests/api/v3/*'
+
 # Checks for stubbed test subjects.
 RSpec/SubjectStub:
   Enabled: false

rubocop/cop/rspec/single_line_hook.rb

@@ -0,0 +1,36 @@
+module RuboCop
+  module Cop
+    module RSpec
+      # This cop checks for single-line hook blocks
+      #
+      # @example
+      #
+      #   # bad
+      #   before { do_something }
+      #   after(:each) { undo_something }
+      #
+      #   # good
+      #   before do
+      #     do_something
+      #   end
+      #
+      #   after(:each) do
+      #     undo_something
+      #   end
+      class SingleLineHook < RuboCop::Cop::RSpec::Cop
+        MESSAGE = "Don't use single-line hook blocks.".freeze
+
+        def_node_search :rspec_hook?, <<~PATTERN
+          (send nil {:after :around :before} ...)
+        PATTERN
+
+        def on_block(node)
+          return unless rspec_hook?(node)
+          return unless node.single_line?
+
+          add_offense(node, :expression, MESSAGE)
+        end
+      end
+    end
+  end
+end

rubocop/rubocop.rb

@@ -15,3 +15,4 @@ require_relative 'cop/migration/remove_index'
 require_relative 'cop/migration/reversible_add_column_with_default'
 require_relative 'cop/migration/timestamps'
 require_relative 'cop/migration/update_column_in_batches'
+require_relative 'cop/rspec/single_line_hook'

spec/rubocop/cop/rspec/single_line_hook_spec.rb

@@ -0,0 +1,67 @@
+require 'spec_helper'
+
+require 'rubocop'
+require 'rubocop/rspec/support'
+require 'rubocop-rspec'
+
+require_relative '../../../../rubocop/cop/rspec/single_line_hook'
+
+describe RuboCop::Cop::RSpec::SingleLineHook do
+  include CopHelper
+
+  subject(:cop) { described_class.new }
+
+  # Override `CopHelper#inspect_source` to always appear to be in a spec file,
+  # so that our RSpec-only cop actually runs
+  def inspect_source(*args)
+    super(*args, 'foo_spec.rb')
+  end
+
+  it 'registers an offense for a single-line `before` block' do
+    inspect_source(cop, 'before { do_something }')
+
+    expect(cop.offenses.size).to eq(1)
+    expect(cop.offenses.map(&:line)).to eq([1])
+    expect(cop.highlights).to eq(['before { do_something }'])
+  end
+
+  it 'registers an offense for a single-line `after` block' do
+    inspect_source(cop, 'after(:each) { undo_something }')
+
+    expect(cop.offenses.size).to eq(1)
+    expect(cop.offenses.map(&:line)).to eq([1])
+    expect(cop.highlights).to eq(['after(:each) { undo_something }'])
+  end
+
+  it 'registers an offense for a single-line `around` block' do
+    inspect_source(cop, 'around { |ex| do_something_else }')
+
+    expect(cop.offenses.size).to eq(1)
+    expect(cop.offenses.map(&:line)).to eq([1])
+    expect(cop.highlights).to eq(['around { |ex| do_something_else }'])
+  end
+
+  it 'ignores a multi-line `before` block' do
+    inspect_source(cop, ['before do',
+                         '  do_something',
+                         'end'])
+
+    expect(cop.offenses.size).to eq(0)
+  end
+
+  it 'ignores a multi-line `after` block' do
+    inspect_source(cop, ['after(:each) do',
+                         '  undo_something',
+                         'end'])
+
+    expect(cop.offenses.size).to eq(0)
+  end
+
+  it 'ignores a multi-line `around` block' do
+    inspect_source(cop, ['around do |ex|',
+                         '  do_something_else',
+                         'end'])
+
+    expect(cop.offenses.size).to eq(0)
+  end
+end