Add new RuboCop for GraphQL::Types::JSON

This cop backs up our GraphQL API Styleguide advice to avoid the use of
GraphQL::Types::JSON.

.rubocop.yml

@@ -336,6 +336,15 @@ Graphql/AuthorizeTypes:
     - 'spec/**/*.rb'
     - 'ee/spec/**/*.rb'
 
+Graphql/JSONType:
+  Enabled: true
+  Include:
+    - 'app/graphql/types/**/*'
+    - 'ee/app/graphql/types/**/*'
+  Exclude:
+    - 'spec/**/*.rb'
+    - 'ee/spec/**/*.rb'
+
 RSpec/EnvAssignment:
   Enable: true
   Include:

app/graphql/types/alert_management/alert_type.rb

@@ -71,7 +71,7 @@ module Types
             description: 'Number of events of this alert',
             method: :events
 
-      field :details,
+      field :details, # rubocop:disable Graphql/JSONType
             GraphQL::Types::JSON,
             null: true,
             description: 'Alert details'

rubocop/cop/graphql/json_type.rb

+# frozen_string_literal: true
+
+# This cop checks for use of GraphQL::Types::JSON types in GraphQL fields
+# and arguments.
+#
+# @example
+#
+#   # bad
+#   class AwfulClass
+#     field :some_field, GraphQL::Types::JSON
+#   end
+#
+#   # good
+#   class GreatClass
+#     field :some_field, GraphQL::STRING_TYPE
+#   end
+
+module RuboCop
+  module Cop
+    module Graphql
+      class JSONType < RuboCop::Cop::Cop
+        MSG = 'Avoid using GraphQL::Types::JSON. See: ' \
+              'https://docs.gitlab.com/ee/development/api_graphql_styleguide.html#json'.freeze
+
+        def_node_matcher :has_json_type?, <<~PATTERN
+          (send nil? {:field :argument}
+            (sym _)
+            (const
+              (const
+                (const nil? :GraphQL) :Types) :JSON)
+            (...)?)
+        PATTERN
+
+        def on_send(node)
+          add_offense(node, location: :expression) if has_json_type?(node)
+        end
+      end
+    end
+  end
+end

spec/rubocop/cop/graphql/json_type_spec.rb

+# frozen_string_literal: true
+
+require 'fast_spec_helper'
+require 'rubocop'
+require_relative '../../../../rubocop/cop/graphql/json_type'
+
+RSpec.describe RuboCop::Cop::Graphql::JSONType, type: :rubocop do
+  include CopHelper
+
+  subject(:cop) { described_class.new }
+
+  context 'fields' do
+    it 'adds an offense when GraphQL::Types::JSON is used' do
+      inspect_source(<<~RUBY.strip)
+        class MyType
+          field :some_field, GraphQL::Types::JSON
+        end
+      RUBY
+
+      expect(cop.offenses.size).to eq(1)
+    end
+
+    it 'adds an offense when GraphQL::Types::JSON is used with other keywords' do
+      inspect_source(<<~RUBY.strip)
+        class MyType
+          field :some_field, GraphQL::Types::JSON, null: true, description: 'My description'
+        end
+      RUBY
+
+      expect(cop.offenses.size).to eq(1)
+    end
+
+    it 'does not add an offense for other types' do
+      expect_no_offenses(<<~RUBY.strip)
+        class MyType
+          field :some_field, GraphQL::STRING_TYPE
+        end
+      RUBY
+    end
+  end
+
+  context 'arguments' do
+    it 'adds an offense when GraphQL::Types::JSON is used' do
+      inspect_source(<<~RUBY.strip)
+        class MyType
+          argument :some_arg, GraphQL::Types::JSON
+        end
+      RUBY
+
+      expect(cop.offenses.size).to eq(1)
+    end
+
+    it 'adds an offense when GraphQL::Types::JSON is used with other keywords' do
+      inspect_source(<<~RUBY.strip)
+        class MyType
+          argument :some_arg, GraphQL::Types::JSON, null: true, description: 'My description'
+        end
+      RUBY
+
+      expect(cop.offenses.size).to eq(1)
+    end
+
+    it 'does not add an offense for other types' do
+      expect_no_offenses(<<~RUBY.strip)
+        class MyType
+          argument :some_arg, GraphQL::STRING_TYPE
+        end
+      RUBY
+    end
+  end
+
+  it 'does not add an offense for uses outside of field or argument' do
+    expect_no_offenses(<<~RUBY.strip)
+      class MyType
+        foo :some_field, GraphQL::Types::JSON
+      end
+    RUBY
+  end
+end