4 files changed, 135 insertions, 5 deletions
diff --git a/rubocop/cop/rspec/env_assignment.rb b/rubocop/cop/rspec/env_assignment.rb
new file mode 100644
index 00000000000..257454af0e1
--- /dev/null
+++ b/rubocop/cop/rspec/env_assignment.rb
@@ -0,0 +1,58 @@
+require 'rubocop-rspec'
+require_relative '../../spec_helpers'
+
+module RuboCop
+  module Cop
+    module RSpec
+      # This cop checks for ENV assignment in specs
+      #
+      # @example
+      #
+      #   # bad
+      #   before do
+      #     ENV['FOO'] = 'bar'
+      #   end
+      #
+      #   # good
+      #   before do
+      #     stub_env('FOO', 'bar')
+      #   end
+      class EnvAssignment < Cop
+        include SpecHelpers
+
+        MESSAGE = "Don't assign to ENV, use `stub_env` instead.".freeze
+
+        def_node_search :env_assignment?, <<~PATTERN
+          (send (const nil? :ENV) :[]= ...)
+        PATTERN
+
+        # Following is what node.children looks like on a match
+        # [s(:const, nil, :ENV), :[]=, s(:str, "key"), s(:str, "value")]
+        def on_send(node)
+          return unless in_spec?(node)
+          return unless env_assignment?(node)
+
+          add_offense(node, :expression, MESSAGE)
+        end
+
+        def autocorrect(node)
+          lambda do |corrector|
+            corrector.replace(node.loc.expression, stub_env(env_key(node), env_value(node)))
+          end
+        end
+
+        def env_key(node)
+          node.children[2].source
+        end
+
+        def env_value(node)
+          node.children[3].source
+        end
+
+        def stub_env(key, value)
+          "stub_env(#{key}, #{value})"
+        end
+      end
+    end
+  end
+end
diff --git a/rubocop/rubocop.rb b/rubocop/rubocop.rb
index 1df23899efb..4ebbe010e90 100644
--- a/rubocop/rubocop.rb
+++ b/rubocop/rubocop.rb
@@ -1,11 +1,11 @@
+require_relative 'cop/active_record_dependent'
+require_relative 'cop/active_record_serialize'
 require_relative 'cop/custom_error_class'
 require_relative 'cop/gem_fetcher'
-require_relative 'cop/active_record_serialize'
-require_relative 'cop/redirect_with_status'
+require_relative 'cop/in_batches'
 require_relative 'cop/polymorphic_associations'
 require_relative 'cop/project_path_helper'
-require_relative 'cop/active_record_dependent'
-require_relative 'cop/in_batches'
+require_relative 'cop/redirect_with_status'
 require_relative 'cop/migration/add_column'
 require_relative 'cop/migration/add_column_with_default_to_large_table'
 require_relative 'cop/migration/add_concurrent_foreign_key'
@@ -13,12 +13,13 @@ require_relative 'cop/migration/add_concurrent_index'
 require_relative 'cop/migration/add_index'
 require_relative 'cop/migration/add_timestamps'
 require_relative 'cop/migration/datetime'
-require_relative 'cop/migration/safer_boolean_column'
 require_relative 'cop/migration/hash_index'
 require_relative 'cop/migration/remove_concurrent_index'
 require_relative 'cop/migration/remove_index'
 require_relative 'cop/migration/reversible_add_column_with_default'
+require_relative 'cop/migration/safer_boolean_column'
 require_relative 'cop/migration/timestamps'
 require_relative 'cop/migration/update_column_in_batches'
+require_relative 'cop/rspec/env_assignment'
 require_relative 'cop/rspec/single_line_hook'
 require_relative 'cop/rspec/verbose_include_metadata'
diff --git a/rubocop/spec_helpers.rb b/rubocop/spec_helpers.rb
new file mode 100644
index 00000000000..a702a083958
--- /dev/null
+++ b/rubocop/spec_helpers.rb
@@ -0,0 +1,12 @@
+module RuboCop
+  module SpecHelpers
+    SPEC_HELPERS = %w[spec_helper.rb rails_helper.rb].freeze
+
+    # Returns true if the given node originated from the spec directory.
+    def in_spec?(node)
+      path = node.location.expression.source_buffer.name
+
+      !SPEC_HELPERS.include?(File.basename(path)) && path.start_with?(File.join(Dir.pwd, 'spec'))
+    end
+  end
+end
diff --git a/spec/rubocop/cop/rspec/env_assignment_spec.rb b/spec/rubocop/cop/rspec/env_assignment_spec.rb
new file mode 100644
index 00000000000..4e859b6f6fa
--- /dev/null
+++ b/spec/rubocop/cop/rspec/env_assignment_spec.rb
@@ -0,0 +1,59 @@
+require 'spec_helper'
+
+require 'rubocop'
+require 'rubocop/rspec/support'
+
+require_relative '../../../../rubocop/cop/rspec/env_assignment'
+
+describe RuboCop::Cop::RSpec::EnvAssignment do
+  include CopHelper
+
+  OFFENSE_CALL_SINGLE_QUOTES_KEY = %(ENV['FOO'] = 'bar').freeze
+  OFFENSE_CALL_DOUBLE_QUOTES_KEY = %(ENV["FOO"] = 'bar').freeze
+
+  let(:source_file) { 'spec/foo_spec.rb' }
+
+  subject(:cop) { described_class.new }
+
+  shared_examples 'an offensive ENV#[]= call' do |content|
+    it "registers an offense for `#{content}`" do
+      inspect_source(cop, content, source_file)
+
+      expect(cop.offenses.size).to eq(1)
+      expect(cop.offenses.map(&:line)).to eq([1])
+      expect(cop.highlights).to eq([content])
+    end
+  end
+
+  shared_examples 'an autocorrected ENV#[]= call' do |content, autocorrected_content|
+    it "registers an offense for `#{content}` and autocorrects it to `#{autocorrected_content}`" do
+      autocorrected = autocorrect_source(cop, content, source_file)
+
+      expect(autocorrected).to eql(autocorrected_content)
+    end
+  end
+
+  context 'in a spec file' do
+    before do
+      allow(cop).to receive(:in_spec?).and_return(true)
+    end
+
+    context 'with a key using single quotes' do
+      it_behaves_like 'an offensive ENV#[]= call', OFFENSE_CALL_SINGLE_QUOTES_KEY
+      it_behaves_like 'an autocorrected ENV#[]= call', OFFENSE_CALL_SINGLE_QUOTES_KEY, %(stub_env('FOO', 'bar'))
+    end
+
+    context 'with a key using double quotes' do
+      it_behaves_like 'an offensive ENV#[]= call', OFFENSE_CALL_DOUBLE_QUOTES_KEY
+      it_behaves_like 'an autocorrected ENV#[]= call', OFFENSE_CALL_DOUBLE_QUOTES_KEY, %(stub_env("FOO", 'bar'))
+    end
+  end
+
+  context 'outside of a spec file' do
+    it "does not register an offense for `#{OFFENSE_CALL_SINGLE_QUOTES_KEY}` in a non-spec file" do
+      inspect_source(cop, OFFENSE_CALL_SINGLE_QUOTES_KEY)
+
+      expect(cop.offenses.size).to eq(0)
+    end
+  end
+end