[Fix #562] Add new Rails/CompactBlank cop

Fixes #562.

This cop checks for places where custom logic on rejection blanks
from arrays and hashes can be replaced with `compact_blank`.

```ruby
# bad
collection.reject { |e| e.blank? }
collection.reject { |e| e.empty? }

# good
collection.compact_blank

# bad
collection.reject! { |e| e.blank? }
collection.reject! { |e| e.empty? }

# good
collection.compact_blank!
```

config/default.yml

@@ -180,6 +180,11 @@ Rails/BulkChangeTable:
   Include:
     - db/migrate/*.rb
 
+Rails/CompactBlank:
+  Description: 'Checks if collection can be blank-compacted with `compact_blank`.'
+  Enabled: pending
+  VersionAdded: '<<next>>'
+
 Rails/ContentTag:
   Description: 'Use `tag.something` instead of `tag(:something)`.'
   Reference:

lib/rubocop/cop/rails/compact_blank.rb

@@ -0,0 +1,90 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Rails
+      # Checks if collection can be blank-compacted with `compact_blank`.
+      #
+      # @example
+      #
+      #   # bad
+      #   collection.reject(&:blank?)
+      #   collection.reject(&:empty?)
+      #   collection.reject { |_k, v| v.blank? }
+      #   collection.reject { |_k, v| v.empty? }
+      #
+      #   # good
+      #   collection.compact_blank
+      #
+      #   # bad
+      #   collection.reject!(&:blank?)
+      #   collection.reject!(&:empty?)
+      #   collection.reject! { |_k, v| v.blank? }
+      #   collection.reject! { |_k, v| v.empty? }
+      #
+      #   # good
+      #   collection.compact_blank!
+      #
+      class CompactBlank < Base
+        include RangeHelp
+        extend AutoCorrector
+        extend TargetRailsVersion
+
+        MSG = 'Use `%<preferred_method>s` instead.'
+        RESTRICT_ON_SEND = %i[reject reject!].freeze
+
+        minimum_target_rails_version 6.1
+
+        def_node_matcher :reject_with_block?, <<~PATTERN
+          (block
+            (send _ {:reject :reject!})
+            $(args ...)
+            (send
+              $(lvar _) {:blank? :empty?}))
+        PATTERN
+
+        def_node_matcher :reject_with_block_pass?, <<~PATTERN
+          (send _ {:reject :reject!}
+            (block_pass
+              (sym {:blank? :empty?})))
+        PATTERN
+
+        def on_send(node)
+          return unless bad_method?(node)
+
+          range = offense_range(node)
+          preferred_method = preferred_method(node)
+          add_offense(range, message: format(MSG, preferred_method: preferred_method)) do |corrector|
+            corrector.replace(range, preferred_method)
+          end
+        end
+
+        private
+
+        def bad_method?(node)
+          return true if reject_with_block_pass?(node)
+
+          if (arguments, receiver_in_block = reject_with_block?(node.parent))
+            return arguments.length == 1 || use_hash_value_block_argument?(arguments, receiver_in_block)
+          end
+
+          false
+        end
+
+        def use_hash_value_block_argument?(arguments, receiver_in_block)
+          arguments.length == 2 && arguments[1].source == receiver_in_block.source
+        end
+
+        def offense_range(node)
+          end_pos = node.parent&.block_type? ? node.parent.loc.expression.end_pos : node.loc.expression.end_pos
+
+          range_between(node.loc.selector.begin_pos, end_pos)
+        end
+
+        def preferred_method(node)
+          node.method?(:reject) ? 'compact_blank' : 'compact_blank!'
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rails_cops.rb

@@ -23,6 +23,7 @@
 require_relative 'rails/belongs_to'
 require_relative 'rails/blank'
 require_relative 'rails/bulk_change_table'
+require_relative 'rails/compact_blank'
 require_relative 'rails/content_tag'
 require_relative 'rails/create_table_with_timestamps'
 require_relative 'rails/date'

spec/rubocop/cop/rails/compact_blank_spec.rb

@@ -0,0 +1,136 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Rails::CompactBlank, :config do
+  context 'Rails >= 6.1', :rails61 do
+    it 'registers and corrects an offense when using `reject { |e| e.blank? }`' do
+      expect_offense(<<~RUBY)
+        collection.reject { |e| e.blank? }
+                   ^^^^^^^^^^^^^^^^^^^^^^^ Use `compact_blank` instead.
+      RUBY
+
+      expect_correction(<<~RUBY)
+        collection.compact_blank
+      RUBY
+    end
+
+    it 'registers and corrects an offense when using `reject { |e| e.empty? }`' do
+      expect_offense(<<~RUBY)
+        collection.reject { |e| e.empty? }
+                   ^^^^^^^^^^^^^^^^^^^^^^^ Use `compact_blank` instead.
+      RUBY
+
+      expect_correction(<<~RUBY)
+        collection.compact_blank
+      RUBY
+    end
+
+    it 'registers and corrects an offense when using `reject(&:blank?)`' do
+      expect_offense(<<~RUBY)
+        collection.reject(&:blank?)
+                   ^^^^^^^^^^^^^^^^ Use `compact_blank` instead.
+      RUBY
+
+      expect_correction(<<~RUBY)
+        collection.compact_blank
+      RUBY
+    end
+
+    it 'registers and corrects an offense when using `reject(&:empty?)`' do
+      expect_offense(<<~RUBY)
+        collection.reject(&:empty?)
+                   ^^^^^^^^^^^^^^^^ Use `compact_blank` instead.
+      RUBY
+
+      expect_correction(<<~RUBY)
+        collection.compact_blank
+      RUBY
+    end
+
+    it 'registers and corrects an offense when using `reject! { |e| e.blank? }`' do
+      expect_offense(<<~RUBY)
+        collection.reject! { |e| e.blank? }
+                   ^^^^^^^^^^^^^^^^^^^^^^^^ Use `compact_blank!` instead.
+      RUBY
+
+      expect_correction(<<~RUBY)
+        collection.compact_blank!
+      RUBY
+    end
+
+    it 'registers and corrects an offense when using `reject! { |e| e.empty? }`' do
+      expect_offense(<<~RUBY)
+        collection.reject! { |e| e.empty? }
+                   ^^^^^^^^^^^^^^^^^^^^^^^^ Use `compact_blank!` instead.
+      RUBY
+
+      expect_correction(<<~RUBY)
+        collection.compact_blank!
+      RUBY
+    end
+
+    it 'registers and corrects an offense when using `reject!(&:blank?)`' do
+      expect_offense(<<~RUBY)
+        collection.reject!(&:blank?)
+                   ^^^^^^^^^^^^^^^^^ Use `compact_blank!` instead.
+      RUBY
+
+      expect_correction(<<~RUBY)
+        collection.compact_blank!
+      RUBY
+    end
+
+    it 'registers and corrects an offense when using `reject!(&:empty?)`' do
+      expect_offense(<<~RUBY)
+        collection.reject!(&:empty?)
+                   ^^^^^^^^^^^^^^^^^ Use `compact_blank!` instead.
+      RUBY
+
+      expect_correction(<<~RUBY)
+        collection.compact_blank!
+      RUBY
+    end
+
+    it 'registers and corrects an offense when using `reject { |k, v| v.empty? }`' do
+      expect_offense(<<~RUBY)
+        collection.reject { |k, v| v.empty? }
+                   ^^^^^^^^^^^^^^^^^^^^^^^^^^ Use `compact_blank` instead.
+      RUBY
+
+      expect_correction(<<~RUBY)
+        collection.compact_blank
+      RUBY
+    end
+
+    it 'does not register an offense when using `compact_blank`' do
+      expect_no_offenses(<<~RUBY)
+        collection.compact_blank
+      RUBY
+    end
+
+    it 'does not register an offense when using `compact_blank!`' do
+      expect_no_offenses(<<~RUBY)
+        collection.compact_blank!
+      RUBY
+    end
+
+    it 'does not register an offense when using `reject { |k, v| k.empty? }`' do
+      expect_no_offenses(<<~RUBY)
+        collection.reject { |k, v| k.empty? }
+      RUBY
+    end
+  end
+
+  context 'Rails <= 6.0', :rails60 do
+    it 'does not register an offense when using `reject { |e| e.blank? }`' do
+      expect_no_offenses(<<~RUBY)
+        collection.reject { |e| e.blank? }
+      RUBY
+    end
+
+    it 'does not register an offense when using `reject { |e| e.empty? }`' do
+      expect_no_offenses(<<~RUBY)
+        collection.reject { |e| e.empty? }
+      RUBY
+    end
+  end
+end