Add new Rails/RootPathnameMethods cop

`Rails.root` is an instance of `Pathname`. So instead of

```ruby
File.open(Rails.root.join('db', 'schema.rb'))
File.open(Rails.root.join('db', 'schema.rb'), 'w')
File.read(Rails.root.join('db', 'schema.rb'))
File.binread(Rails.root.join('db', 'schema.rb'))
File.write(Rails.root.join('db', 'schema.rb'), content)
File.binwrite(Rails.root.join('db', 'schema.rb'), content)
```

we can simply write

```ruby
Rails.root.join('db', 'schema.rb').open
Rails.root.join('db', 'schema.rb').open('w')
Rails.root.join('db', 'schema.rb').read
Rails.root.join('db', 'schema.rb').binread
Rails.root.join('db', 'schema.rb').write(content)
Rails.root.join('db', 'schema.rb').binwrite(content)
```

This cop works best when used together with
[`Style/FileRead`](rubocop/rubocop#10261),
[`Style/FileWrite`](rubocop/rubocop#10260),
and [`Rails/RootJoinChain`](rubocop#586).

changelog/new_add_new_railsrootpathnamemethods_cop.md

@@ -0,0 +1 @@
+* [#587](https://github.com/rubocop/rubocop-rails/pull/587): Add new `Rails/RootPathnameMethods` cop. ([@leoarnold][])

config/default.yml

@@ -788,6 +788,11 @@ Rails/RootJoinChain:
   Enabled: pending
   VersionAdded: '2.13'
 
+Rails/RootPathnameMethods:
+  Description: 'Use `Rails.root` IO methods instead of passing it to `File`.'
+  Enabled: pending
+  VersionAdded: '<<next>>'
+
 Rails/RootPublicPath:
   Description: "Favor `Rails.public_path` over `Rails.root` with `'public'`."
   Enabled: pending

lib/rubocop/cop/rails/root_pathname_methods.rb

@@ -0,0 +1,101 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Rails
+      # Use `Rails.root` IO methods instead of passing it to `File`.
+      #
+      # `Rails.root` is an instance of `Pathname`
+      # so we can apply many IO methods directly.
+      #
+      # This cop works best when used together with
+      # `Style/FileRead`, `Style/FileWrite` and `Rails/RootJoinChain`.
+      #
+      # @example
+      #   # bad
+      #   File.open(Rails.root.join('db', 'schema.rb'))
+      #   File.open(Rails.root.join('db', 'schema.rb'), 'w')
+      #   File.read(Rails.root.join('db', 'schema.rb'))
+      #   File.binread(Rails.root.join('db', 'schema.rb'))
+      #   File.write(Rails.root.join('db', 'schema.rb'), content)
+      #   File.binwrite(Rails.root.join('db', 'schema.rb'), content)
+      #
+      #   # good
+      #   Rails.root.join('db', 'schema.rb').open
+      #   Rails.root.join('db', 'schema.rb').open('w')
+      #   Rails.root.join('db', 'schema.rb').read
+      #   Rails.root.join('db', 'schema.rb').binread
+      #   Rails.root.join('db', 'schema.rb').write(content)
+      #   Rails.root.join('db', 'schema.rb').binwrite(content)
+      #
+      class RootPathnameMethods < Base
+        extend AutoCorrector
+
+        MSG = '`Rails.root` is a `Pathname` so you can just append `#%<method>s`.'
+
+        RESTRICT_ON_SEND = %i[binread binwrite open read write].to_set.freeze
+
+        def_node_matcher :file_open, <<~PATTERN
+          (send (const {nil? cbase} :File) :open $_ $...)
+        PATTERN
+
+        def_node_matcher :file_read, <<~PATTERN
+          (send (const {nil? cbase} :File) ${:binread :read} $_ $...)
+        PATTERN
+
+        def_node_matcher :file_write, <<~PATTERN
+          (send (const {nil? cbase} :File) ${:binwrite :write} $_ $...)
+        PATTERN
+
+        def_node_matcher :rails_root_pathname?, <<~PATTERN
+          {
+            #rails_root?
+            (send #rails_root? :join ...)
+          }
+        PATTERN
+
+        def_node_matcher :rails_root?, <<~PATTERN
+          (send (const {nil? cbase} :Rails) {:root :public_path})
+        PATTERN
+
+        def on_send(node)
+          investigate_file_open(node) || investigate_file_read(node) || investigate_file_write(node)
+        end
+
+        private
+
+        def investigate_file_open(node)
+          return if node.parent&.send_type?
+          return unless (rails_root, args = file_open(node)) && rails_root_pathname?(rails_root)
+
+          add_offense(node, message: format(MSG, method: :open)) do |corrector|
+            replacement = "#{rails_root.source}.open"
+            replacement += "(#{args.map(&:source).join(', ')})" unless args.empty?
+
+            corrector.replace(node, replacement)
+          end
+        end
+
+        def investigate_file_read(node)
+          return unless (method, rails_root, args = file_read(node)) && rails_root_pathname?(rails_root)
+
+          add_offense(node, message: format(MSG, method: method)) do |corrector|
+            replacement = "#{rails_root.source}.#{method}"
+            replacement += "(#{args.map(&:source).join(', ')})" unless args.empty?
+
+            corrector.replace(node, replacement)
+          end
+        end
+
+        def investigate_file_write(node)
+          return unless (method, rails_root, args = file_write(node)) && rails_root_pathname?(rails_root)
+
+          add_offense(node, message: format(MSG, method: method)) do |corrector|
+            replacement = "#{rails_root.source}.#{method}(#{args.map(&:source).join(', ')})"
+            corrector.replace(node, replacement)
+          end
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rails_cops.rb

@@ -97,6 +97,7 @@
 require_relative 'rails/reversible_migration'
 require_relative 'rails/reversible_migration_method_definition'
 require_relative 'rails/root_join_chain'
+require_relative 'rails/root_pathname_methods'
 require_relative 'rails/root_public_path'
 require_relative 'rails/safe_navigation'
 require_relative 'rails/safe_navigation_with_blank'

spec/rubocop/cop/rails/root_pathname_methods_spec.rb

@@ -0,0 +1,137 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Rails::RootPathnameMethods, :config do
+  it 'registers an offense when using `::File.open(::Rails.root.join(...))`' do
+    expect_offense(<<~RUBY)
+      ::File.open(::Rails.root.join('db', 'schema.rb'))
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `Rails.root` is a `Pathname` so you can just append `#open`.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      ::Rails.root.join('db', 'schema.rb').open
+    RUBY
+  end
+
+  it 'registers an offense when using `File.open(Rails.root.join(...))`' do
+    expect_offense(<<~RUBY)
+      File.open(Rails.root.join('db', 'schema.rb'))
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `Rails.root` is a `Pathname` so you can just append `#open`.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      Rails.root.join('db', 'schema.rb').open
+    RUBY
+  end
+
+  it "registers an offense when using `File.open(Rails.root.join(...), 'w')`" do
+    expect_offense(<<~RUBY)
+      File.open(Rails.root.join('db', 'schema.rb'), 'w')
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `Rails.root` is a `Pathname` so you can just append `#open`.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      Rails.root.join('db', 'schema.rb').open('w')
+    RUBY
+  end
+
+  # This is handled by `Rails/RootJoinChain`
+  it 'does not register an offense when using `File.read(Rails.root.join(...).join(...))`' do
+    expect_no_offenses(<<~RUBY)
+      File.read(Rails.root.join('db').join('schema.rb'))
+    RUBY
+  end
+
+  # This is handled by `Style/FileRead`
+  it 'does not register an offense when using `File.open(Rails.root.join(...)).read`' do
+    expect_no_offenses(<<~RUBY)
+      File.open(Rails.root.join('db', 'schema.rb')).read
+    RUBY
+  end
+
+  # This is handled by `Style/FileRead`
+  it 'does not register an offense when using `File.open(Rails.root.join(...)).binread`' do
+    expect_no_offenses(<<~RUBY)
+      File.open(Rails.root.join('db', 'schema.rb')).binread
+    RUBY
+  end
+
+  # This is handled by `Style/FileWrite`
+  it 'does not register an offense when using `File.open(Rails.root.join(...)).write(content)`' do
+    expect_no_offenses(<<~RUBY)
+      File.open(Rails.root.join('db', 'schema.rb')).write(content)
+    RUBY
+  end
+
+  # This is handled by `Style/FileWrite`
+  it 'does not register an offense when using `File.open(Rails.root.join(...)).binwrite(content)`' do
+    expect_no_offenses(<<~RUBY)
+      File.open(Rails.root.join('db', 'schema.rb')).binwrite(content)
+    RUBY
+  end
+
+  it 'registers an offense when using `File.read(Rails.root.join(...))`' do
+    expect_offense(<<~RUBY)
+      File.read(Rails.root.join('db', 'schema.rb'))
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `Rails.root` is a `Pathname` so you can just append `#read`.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      Rails.root.join('db', 'schema.rb').read
+    RUBY
+  end
+
+  it 'registers an offense when using `File.read(Rails.root.join(...), ...)`' do
+    expect_offense(<<~RUBY)
+      File.read(Rails.root.join('db', 'schema.rb'), 20, 5)
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `Rails.root` is a `Pathname` so you can just append `#read`.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      Rails.root.join('db', 'schema.rb').read(20, 5)
+    RUBY
+  end
+
+  it 'registers an offense when using `File.binread(Rails.root.join(...))`' do
+    expect_offense(<<~RUBY)
+      File.binread(Rails.root.join('db', 'schema.rb'))
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `Rails.root` is a `Pathname` so you can just append `#binread`.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      Rails.root.join('db', 'schema.rb').binread
+    RUBY
+  end
+
+  it 'registers an offense when using `File.write(Rails.root.join(...), ...)`' do
+    expect_offense(<<~RUBY)
+      File.write(Rails.root.join('db', 'schema.rb'), content, 20)
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `Rails.root` is a `Pathname` so you can just append `#write`.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      Rails.root.join('db', 'schema.rb').write(content, 20)
+    RUBY
+  end
+
+  it 'registers an offense when using `File.binwrite(Rails.root.join(...), content)`' do
+    expect_offense(<<~RUBY)
+      File.binwrite(Rails.root.join('db', 'schema.rb'), content)
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `Rails.root` is a `Pathname` so you can just append `#binwrite`.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      Rails.root.join('db', 'schema.rb').binwrite(content)
+    RUBY
+  end
+
+  it 'registers an offense when using `File.open(Rails.root.join(...), ...)` inside an iterator' do
+    expect_offense(<<~RUBY)
+      files.map { |file| File.open(Rails.root.join('db', file), 'wb') }
+                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `Rails.root` is a `Pathname` so you can just append `#open`.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      files.map { |file| Rails.root.join('db', file).open('wb') }
+    RUBY
+  end
+end