Add cop to forbid extend T::Sig in shims

config/default.yml

@@ -53,6 +53,13 @@ Sorbet/FalseSigil:
   - db/**/*.rb
   - script/**/*
 
+Sorbet/ForbidExtendTSigInShims:
+  Description: 'Forbid the use of `extend T::Sig` in RBI shims'
+  Enabled: false
+  VersionAdded: '0.86'
+  Include:
+  - "sorbet/rbi/shims/**/*.rbi"
+
 Sorbet/ForbidIncludeConstLiteral:
   Description: 'Forbids include of non-literal constants.'
   Enabled: false

lib/rubocop/cop/sorbet/forbid_extend_t_sig_in_shims.rb

@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Sorbet
+      # This cop ensures RBI shims do not include a call to extend T::Sig
+      #
+      # @example
+      #
+      #   # bad
+      #   module SomeModule
+      #     extend T::Sig
+      #
+      #     sig { returns(String) }
+      #     def foo; end
+      #   end
+      #
+      #   # good
+      #   module SomeModule
+      #     sig { returns(String) }
+      #     def foo; end
+      #   end
+      class ForbidExtendTSigInShims < RuboCop::Cop::Cop
+        include RangeHelp
+
+        MSG = 'Extending T::Sig in a shim is unnecessary'
+        RESTRICT_ON_SEND = [:extend]
+
+        def_node_matcher :extend_t_sig?, <<~PATTERN
+          (send nil? :extend (const (const nil? :T) :Sig))
+        PATTERN
+
+        def autocorrect(node)
+          -> (corrector) do
+            corrector.remove(
+              range_by_whole_lines(node.source_range, include_final_newline: true)
+            )
+          end
+        end
+
+        def on_send(node)
+          add_offense(node) if extend_t_sig?(node)
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/sorbet_cops.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 require_relative 'sorbet/binding_constants_without_type_alias'
 require_relative 'sorbet/constants_from_strings'
+require_relative 'sorbet/forbid_extend_t_sig_in_shims'
 require_relative 'sorbet/forbid_superclass_const_literal'
 require_relative 'sorbet/forbid_include_const_literal'
 require_relative 'sorbet/forbid_untyped_struct_props'

manual/cops.md

@@ -12,6 +12,7 @@ In the following section you find all available cops:
 * [Sorbet/EnforceSigilOrder](cops_sorbet.md#sorbetenforcesigilorder)
 * [Sorbet/EnforceSignatures](cops_sorbet.md#sorbetenforcesignatures)
 * [Sorbet/FalseSigil](cops_sorbet.md#sorbetfalsesigil)
+* [Sorbet/ForbidExtendTSigInShims](cops_sorbet.md#sorbetforbidextendtsiginshims)
 * [Sorbet/ForbidIncludeConstLiteral](cops_sorbet.md#sorbetforbidincludeconstliteral)
 * [Sorbet/ForbidSuperclassConstLiteral](cops_sorbet.md#sorbetforbidsuperclassconstliteral)
 * [Sorbet/ForbidUntypedStructProps](cops_sorbet.md#sorbetforbiduntypedstructprops)

manual/cops_sorbet.md

@@ -171,6 +171,38 @@ SuggestedStrictness | `true` | Boolean
 Include | `**/*.rb`, `**/*.rbi`, `**/*.rake`, `**/*.ru` | Array
 Exclude | `bin/**/*`, `db/**/*.rb`, `script/**/*` | Array
 
+## Sorbet/ForbidExtendTSigInShims
+
+Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged
+--- | --- | --- | --- | ---
+Disabled | Yes | Yes  | 0.86 | -
+
+This cop ensures RBI shims do not include a call to extend T::Sig
+
+### Examples
+
+```ruby
+# bad
+module SomeModule
+  extend T::Sig
+
+  sig { returns(String) }
+  def foo; end
+end
+
+# good
+module SomeModule
+  sig { returns(String) }
+  def foo; end
+end
+```
+
+### Configurable attributes
+
+Name | Default value | Configurable values
+--- | --- | ---
+Include | `sorbet/rbi/shims/**/*.rbi` | Array
+
 ## Sorbet/ForbidIncludeConstLiteral
 
 Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged

spec/rubocop/cop/sorbet/forbid_extend_t_sig_in_shims_spec.rb

@@ -0,0 +1,56 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe(RuboCop::Cop::Sorbet::ForbidExtendTSigInShims, :config) do
+  subject(:cop) { described_class.new(config) }
+
+  describe('offences') do
+    it 'adds an offence when a targeted class or module extends T::Sig' do
+      expect_offense(<<~RBI)
+        module MyModule
+          extend T::Sig
+          ^^^^^^^^^^^^^ Extending T::Sig in a shim is unnecessary
+
+          sig { returns(String) }
+          def foo; end
+        end
+      RBI
+    end
+  end
+
+  describe('no offences') do
+    it 'does not add an offence to uses of extend that are not T::Sig' do
+      expect_no_offenses(<<~RBI)
+        module MyModule
+          extend ActiverSupport::Concern
+
+          def foo; end
+        end
+      RBI
+    end
+  end
+
+  describe('autocorrect') do
+    it 'autocorrects usages of extend T::Sig by removing them' do
+      source = <<~RBI
+        module MyModule
+          extend T::Sig
+          extend ActiveSupport::Concern
+
+          sig { returns(String) }
+          def foo; end
+        end
+      RBI
+      expect(autocorrect_source(source))
+        .to(eq(<<~RBI))
+          module MyModule
+            extend ActiveSupport::Concern
+
+            sig { returns(String) }
+            def foo; end
+          end
+        RBI
+    end
+  end
+end