Add new RedundantContext cop

To check context with single example.

Closes rubocop#2001

config/default.yml

@@ -794,6 +794,12 @@ RSpec/RedundantAround:
   VersionAdded: '2.19'
   Reference: https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/RedundantAround
 
+RSpec/RedundantContext:
+  Description: Detect redundant context with single example
+  Enabled: true
+  VersionAdded: '3.4.0'
+  Reference: https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/RedundantContext
+
 RSpec/RedundantPredicateMatcher:
   Description: Checks for redundant predicate matcher.
   Enabled: true

lib/rubocop/cop/rspec/redundant_context.rb

@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module RSpec
+      # Remove redundant `around` hook.
+      #
+      # @example
+      #   # bad
+      #   context 'when condition' do
+      #     it 'tests something' do
+      #     end
+      #   end
+      #
+      #   # good
+      #   it 'tests something when condition' do
+      #   end
+      #
+      class RedundantContext < Base
+        MSG = 'Redundant context with single example.'
+
+        # @!method redundant_context?(node)
+        def_node_matcher :redundant_context?, <<~PATTERN
+          (block
+            (send #rspec? :context _)
+            _
+            (block (send _ :it ...) ...))
+        PATTERN
+
+        def on_block(node)
+          return unless redundant_context?(node)
+
+          add_offense(node)
+        end
+        alias on_numblock on_block
+      end
+    end
+  end
+end

lib/rubocop/cop/rspec_cops.rb

@@ -81,6 +81,7 @@
 require_relative 'rspec/receive_messages'
 require_relative 'rspec/receive_never'
 require_relative 'rspec/redundant_around'
+require_relative 'rspec/redundant_context'
 require_relative 'rspec/redundant_predicate_matcher'
 require_relative 'rspec/remove_const'
 require_relative 'rspec/repeated_description'

spec/rubocop/cop/rspec/redundant_context_spec.rb

@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::RSpec::RedundantContext do
+  it 'registers an offense when single example inside context' do
+    expect_offense(<<~RUBY)
+      context 'when condition' do
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^ Redundant context with single example.
+        it 'does something' do
+        end
+      end
+    RUBY
+  end
+
+  it 'does not register offense when multiple examples inside context' do
+    expect_no_offenses(<<~RUBY)
+      context 'when condition' do
+        it 'does something' do
+        end
+
+        it 'does something else' do
+        end
+      end
+    RUBY
+  end
+end