[Fix rubocop#2731] Customise method-creating methods.

This addresses the issue raised in rubocop#2731 where methods (often from outside of Ruby itself) may be called within the context of a module that in turn define methods. The `Lint/UselessAccessModifier` wasn’t taking these into account.

Much like the existing approach taken with blocks/context-creating methods, there’s now a configuration option `MethodCreatingMethods` for the cop. This setting is an array of methods which, when called, are known to create other methods in the module’s current access context.

CHANGELOG.md

@@ -20,6 +20,7 @@
 * Renamed `MethodCallParentheses` to `MethodCallWithoutArgsParentheses`. ([@dominh][])
 * [#3854](https://github.com/bbatsov/rubocop/pull/3854): Add new `Rails/ReversibleMigration` cop. ([@sue445][])
 * [#3872](https://github.com/bbatsov/rubocop/pull/3872): Detect `String#%` with hash literal. ([@backus][])
+* [#2731](https://github.com/bbatsov/rubocop/issues/2731): Allow configuration of method calls that create methods for `Lint/UselessAccessModifier`. ([@pat][])
 
 ### Changes
 
@@ -2584,3 +2585,4 @@
 [@sue445]: https://github.com/sue445
 [@zverok]: https://github.com/zverok
 [@backus]: https://github.com/backus
+[@pat]: https://github.com/pat

config/enabled.yml

@@ -1279,6 +1279,7 @@ Lint/UselessAccessModifier:
   Description: 'Checks for useless access modifiers.'
   Enabled: true
   ContextCreatingMethods: []
+  MethodCreatingMethods: []
 
 Lint/UselessAssignment:
   Description: 'Checks for useless assignment to a local variable.'

lib/rubocop/cop/lint/useless_access_modifier.rb

@@ -78,6 +78,18 @@ module Lint
       #     def some_other_private_method
       #     end
       #   end
+      #
+      # @example
+      #   # Lint/UselessAccessModifier:
+      #   #   MethodCreatingMethods:
+      #   #     - delegate
+      #   require 'active_support/core_ext/module/delegation'
+      #   class Foo
+      #     # this is not redundant because `delegate` creates methods
+      #     private
+      #
+      #     delegate :method_a, to: :method_b
+      #   end
       class UselessAccessModifier < Cop
         MSG = 'Useless `%s` access modifier.'.freeze
 
@@ -170,7 +182,22 @@ def check_new_visibility(node, unused, new_vis, cur_vis)
         end
 
         def method_definition?(child)
-          static_method_definition?(child) || dynamic_method_definition?(child)
+          static_method_definition?(child) ||
+            dynamic_method_definition?(child) ||
+            any_method_definition?(child)
+        end
+
+        def any_method_definition?(child)
+          cop_config.fetch('MethodCreatingMethods', []).any? do |m|
+            matcher_name = "#{m}_method?".to_sym
+            unless respond_to?(matcher_name)
+              self.class.def_node_matcher matcher_name, <<-PATTERN
+                {def (send nil :#{m} ...)}
+              PATTERN
+            end
+
+            send(matcher_name, child)
+          end
         end
 
         def start_of_new_scope?(child)

manual/cops_lint.md

@@ -1784,12 +1784,25 @@ class Foo
   end
 end
 ```
+```ruby
+# Lint/UselessAccessModifier:
+#   MethodCreatingMethods:
+#     - delegate
+require 'active_support/core_ext/module/delegation'
+class Foo
+  # this is not redundant because `delegate` creates methods
+  private
+
+  delegate :method_a, to: :method_b
+end
+```
 
 ### Important attributes
 
 Attribute | Value
 --- | ---
 ContextCreatingMethods | 
+MethodCreatingMethods | 
 
 
 ## Lint/UselessAssignment

spec/rubocop/cop/lint/useless_access_modifier_spec.rb

@@ -347,6 +347,36 @@
     end
   end
 
+  context 'when using a known method-creating method' do
+    let(:config) do
+      RuboCop::Config.new(
+        'Lint/UselessAccessModifier' => {
+          'MethodCreatingMethods' => ['delegate']
+        }
+      )
+    end
+    it 'is aware that this creates a new method' do
+      src = ['class SomeClass',
+             '  private',
+             '  ',
+             '  delegate :foo, to: :bar',
+             'end']
+      inspect_source(cop, src)
+      expect(cop.offenses).to be_empty
+    end
+
+    it 'still points out redundant uses within the module' do
+      src = ['class SomeClass',
+             '  delegate :foo, to: :bar',
+             '  ',
+             '  private',
+             'end']
+      inspect_source(cop, src)
+      expect(cop.offenses.size).to eq(1)
+      expect(cop.offenses.first.line).to eq(4)
+    end
+  end
+
   shared_examples 'at the top of the body' do |keyword|
     it 'registers an offense for `public`' do
       src = ["#{keyword} A",