feat(cop): add Platanus/EnvInModule cop

config/default.yml

@@ -57,3 +57,11 @@ Platanus/SingularSerializer:
   Include:
     - "app/serializers/**/*.rb"
   VersionAdded: "<<next>>"
+
+Platanus/EnvInModule:
+  Description: 'Prefer retrieving environment variables in a dedicated module.'
+  Enabled: true
+  Include:
+    - 'app/**/*.rb'
+  VersionAdded: '<<next>>'
+

lib/rubocop/cop/platanus/env_in_module.rb

@@ -0,0 +1,63 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Platanus
+      # Environment variables should not be retrieved directly. Prefer retrieving them in a
+      # dedicated module. This centralizes their location, enhances their functionality and makes
+      # them easier to stub.
+      #
+      # @example
+      #
+      #   # bad
+      #   class Foo
+      #     BAR = ENV.fetch('BAR')
+      #
+      #     def foo_method
+      #       BAR
+      #     end
+      #   end
+      #
+      #   # bad
+      #   class Foo
+      #     BAR = ENV['BAR']
+      #
+      #     def foo_method
+      #       BAR
+      #     end
+      #   end
+      #
+      #   # good
+      #   module EnvironmentVariables
+      #     extend self
+      #
+      #     BAR = ENV.fetch('BAR')
+      #
+      #     def bar
+      #       BAR
+      #     end
+      #   end
+      #
+      #   class Foo
+      #     def foo_method
+      #       EnvironmentVariables.bar
+      #     end
+      #   end
+      #
+      class EnvInModule < Base
+        MSG = 'Use dedicated module for environment variables retrieval instead.'
+
+        # @!method env_retrieve?(node)
+        def_node_matcher :env_retrieve?, <<~PATTERN
+          (send (const nil? :ENV) { :fetch | :[] } ...)
+        PATTERN
+
+        def on_send(node)
+          return unless env_retrieve?(node)
+
+          add_offense(node)
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/platanus_cops.rb

@@ -8,3 +8,4 @@
 require_relative 'platanus/no_where_aasm_state'
 require_relative 'platanus/pundit_in_application_controller'
 require_relative 'platanus/singular_serializer'
+require_relative 'platanus/env_in_module'

spec/rubocop/cop/platanus/env_in_module_spec.rb

@@ -0,0 +1,41 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Platanus::EnvInModule, :config do
+  let(:config) { RuboCop::Config.new }
+
+  it 'registers an offense when using `ENV.fetch`' do
+    expect_offense <<~RUBY
+      class Foo
+        BAR = ENV.fetch('BAR')
+              ^^^^^^^^^^^^^^^^ Use dedicated module for environment variables retrieval instead.
+
+        def foo_method
+          BAR
+        end
+      end
+    RUBY
+  end
+
+  it 'registers an offense when using `ENV[]`' do
+    expect_offense <<~RUBY
+      class Foo
+        BAR = ENV['BAR']
+              ^^^^^^^^^^ Use dedicated module for environment variables retrieval instead.
+
+        def foo_method
+          BAR
+        end
+      end
+    RUBY
+  end
+
+  it 'does not register an offense when not using `ENV.fetch` or `ENV`' do
+    expect_no_offenses <<~RUBY
+      class Foo
+        def foo_method
+          EnviromentVariables.bar
+        end
+      end
+    RUBY
+  end
+end