Print warning when custom validators are detected

lib/graphql/schema/member/has_validators.rb

@@ -10,6 +10,12 @@ module HasValidators
         # @param validation_config [Hash{Symbol => Hash}]
         # @return [void]
         def validates(validation_config)
+          validation_config.each do |validator_name, _options|
+            if validator_name.is_a?(Class) || !GraphQL::Schema::Validator.all_validators.key?(validator_name)
+              GraphQL::Schema::Validator::CustomValidatorWarning.warn_for(validator_name)
+            end
+          end
+
           new_validators = GraphQL::Schema::Validator.from_config(self, validation_config)
           @own_validators ||= []
           @own_validators.concat(new_validators)

lib/graphql/schema/validator.rb

@@ -171,3 +171,4 @@ def self.validate!(validators, object, context, value, as: nil)
 GraphQL::Schema::Validator.install(:allow_blank, GraphQL::Schema::Validator::AllowBlankValidator)
 require "graphql/schema/validator/all_validator"
 GraphQL::Schema::Validator.install(:all, GraphQL::Schema::Validator::AllValidator)
+require "graphql/schema/validator/custom_validator_warning"

lib/graphql/schema/validator/custom_validator_warning.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+module GraphQL
+  class Schema
+    class Validator
+      module CustomValidatorWarning
+        def self.warn_for(validator_name)
+          border = "*" * 30
+          timestamp = "[#{Time.now.strftime('%Y-%m-%dT%H:%M:%S.%6N')} ##{Process.pid}]"
+          warn <<~WARN
+            #{border}
+            WARNING: GraphQL Custom validator registered: #{validator_name}
+            Note that custom validators with I/O operations may fail unexpectedly due to GraphQL's default validate_timeout setting. Long-running I/O operations may not be killed halfway through, resulting in unpredictable behavior. See https://graphql-ruby.org/queries/timeout.html#validation-and-analysis for more information.
+            #{caller(3,1).first}
+            #{timestamp}
+            #{border}
+          WARN
+        end
+      end
+    end
+  end
+end