[Fix #10568] Add new Style/EnvHome cop

Fixes #10568.

This cop checks for consistent usage of `ENV['HOME']`. If `nil` is used as
the second argument of `ENV.fetch`, it is treated as a bad case like `ENV[]`.
This avoids conflicts with `Style/FetchEnvVar`'s autocorrection.

## Safety

The cop is unsafe because The result when `nil` is assigned to `ENV['HOME']` changes:

```ruby
ENV['HOME'] = nil
ENV['HOME'] # => nil
Dir.home    # => '/home/foo'
```

## Example

```ruby
# bad
ENV['HOME']
ENV.fetch('HOME', nil)

# good
Dir.home

# good
ENV.fetch('HOME', default)
```

changelog/new_add_new_style_env_home_cop.md

@@ -0,0 +1 @@
+* [#10568](https://github.com/rubocop/rubocop/issues/10568): Add new `Style/EnvHome` cop. ([@koic][])

config/default.yml

@@ -3504,6 +3504,12 @@ Style/EndlessMethod:
     - allow_always
     - disallow
 
+Style/EnvHome:
+  Description: "Checks for consistent usage of `ENV['HOME']`."
+  Enabled: pending
+  Safe: false
+  VersionAdded: '<<next>>'
+
 Style/EvalWithLocation:
   Description: 'Pass `__FILE__` and `__LINE__` to `eval` method, as they are used by backtraces.'
   Enabled: true

lib/rubocop.rb

@@ -480,6 +480,7 @@
 require_relative 'rubocop/cop/style/endless_method'
 require_relative 'rubocop/cop/style/encoding'
 require_relative 'rubocop/cop/style/end_block'
+require_relative 'rubocop/cop/style/env_home'
 require_relative 'rubocop/cop/style/eval_with_location'
 require_relative 'rubocop/cop/style/even_odd'
 require_relative 'rubocop/cop/style/expand_path_arguments'

lib/rubocop/cop/style/env_home.rb

@@ -0,0 +1,56 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Style
+      # This cop checks for consistent usage of `ENV['HOME']`. If `nil` is used as
+      # the second argument of `ENV.fetch`, it is treated as a bad case like `ENV[]`.
+      #
+      # @safety
+      #   The cop is unsafe because the result when `nil` is assigned to `ENV['HOME']` changes:
+      #
+      #   [source,ruby]
+      #   ----
+      #   ENV['HOME'] = nil
+      #   ENV['HOME'] # => nil
+      #   Dir.home    # => '/home/foo'
+      #   ----
+      #
+      # @example
+      #
+      #   # bad
+      #   ENV['HOME']
+      #   ENV.fetch('HOME', nil)
+      #
+      #   # good
+      #   Dir.home
+      #
+      #   # good
+      #   ENV.fetch('HOME', default)
+      #
+      class EnvHome < Base
+        extend AutoCorrector
+
+        MSG = 'Use `Dir.home` instead.'
+        RESTRICT_ON_SEND = %i[[] fetch].freeze
+
+        # @!method env_home?(node)
+        def_node_matcher :env_home?, <<~PATTERN
+          (send
+            (const {cbase nil?} :ENV) {:[] :fetch}
+            (str "HOME")
+            ...)
+        PATTERN
+
+        def on_send(node)
+          return unless env_home?(node)
+          return if node.arguments.count == 2 && !node.arguments[1].nil_type?
+
+          add_offense(node) do |corrector|
+            corrector.replace(node, 'Dir.home')
+          end
+        end
+      end
+    end
+  end
+end

lib/rubocop/result_cache.rb

@@ -81,7 +81,7 @@ def self.cache_root(config_store)
                  #
                  # To avoid raising warn log messages on FreeBSD, we retrieve
                  # the real path of the home folder.
-                 File.join(File.realpath(ENV.fetch('HOME')), '.cache')
+                 File.join(File.realpath(Dir.home), '.cache')
                end
       File.join(root, 'rubocop_cache')
     end

lib/rubocop/rspec/shared_contexts.rb

@@ -5,7 +5,7 @@
 RSpec.shared_context 'isolated environment', :isolated_environment do
   around do |example|
     Dir.mktmpdir do |tmpdir|
-      original_home = ENV.fetch('HOME', nil)
+      original_home = Dir.home
       original_xdg_config_home = ENV.fetch('XDG_CONFIG_HOME', nil)
 
       # Make sure to expand all symlinks in the path first. Otherwise we may

spec/rubocop/cop/style/env_home_spec.rb

@@ -0,0 +1,65 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Style::EnvHome, :config do
+  it "registers and corrects an offense when using `ENV['HOME']`" do
+    expect_offense(<<~RUBY)
+      ENV['HOME']
+      ^^^^^^^^^^^ Use `Dir.home` instead.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      Dir.home
+    RUBY
+  end
+
+  it "registers and corrects an offense when using `ENV.fetch('HOME')`" do
+    expect_offense(<<~RUBY)
+      ENV.fetch('HOME')
+      ^^^^^^^^^^^^^^^^^ Use `Dir.home` instead.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      Dir.home
+    RUBY
+  end
+
+  it "registers and corrects an offense when using `ENV.fetch('HOME', nil)`" do
+    expect_offense(<<~RUBY)
+      ENV.fetch('HOME', nil)
+      ^^^^^^^^^^^^^^^^^^^^^^ Use `Dir.home` instead.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      Dir.home
+    RUBY
+  end
+
+  it "registers and corrects an offense when using `::ENV['HOME']`" do
+    expect_offense(<<~RUBY)
+      ::ENV['HOME']
+      ^^^^^^^^^^^^^ Use `Dir.home` instead.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      Dir.home
+    RUBY
+  end
+
+  it 'does not register an offense when using `Dir.home`' do
+    expect_no_offenses(<<~RUBY)
+      Dir.home
+    RUBY
+  end
+
+  it "does not register an offense when using `ENV['HOME'] = '/home/foo'`" do
+    expect_no_offenses(<<~RUBY)
+      ENV['HOME'] = '/home/foo'
+    RUBY
+  end
+
+  it "does not register an offense when using `ENV.fetch('HOME', default)`" do
+    expect_no_offenses(<<~RUBY)
+      ENV.fetch('HOME', default)
+    RUBY
+  end
+end