raise ArgumentError if object cannot be proxied

lib/rspec/mocks/proxy.rb

@@ -21,6 +21,8 @@ def ensure_implemented(*_args)
 
       # @private
       def initialize(object, order_group, options={})
+        ensure_can_be_proxied!(object)
+
         @object = object
         @order_group = order_group
         @error_generator = ErrorGenerator.new(object)
@@ -31,6 +33,20 @@ def initialize(object, order_group, options={})
         @method_doubles = Hash.new { |h, k| h[k] = MethodDouble.new(@object, k, self) }
       end
 
+      # @private
+      def ensure_can_be_proxied!(object)
+        return unless object.is_a?(Symbol) || object.frozen?
+        return if object.nil?
+
+        msg = "Cannot proxy frozen objects"
+        if Symbol === object
+          msg << ". Symbols such as #{object} cannot be mocked or stubbed."
+        else
+          msg << ", rspec-mocks relies on proxies for method stubbing and expectations."
+        end
+        raise ArgumentError, msg
+      end
+
       # @private
       attr_reader :object
 

spec/rspec/mocks/space_spec.rb

@@ -232,6 +232,22 @@ def create_generations
       }.to raise_error(RSpec::Expectations::ExpectationNotMetError, /Diff/)
     end
 
+    it 'raises ArgumentError with message if object is symbol' do
+      space1 = Space.new
+      object = :subject
+      expected_message = "Cannot proxy frozen objects. Symbols such as #{object} cannot be mocked or stubbed."
+
+      expect { space1.proxy_for(object) }.to raise_error(ArgumentError, expected_message)
+    end
+
+    it 'raises ArgumentError with message if object is frozen' do
+      space1 = Space.new
+      object = "subject".freeze
+      expected_message = "Cannot proxy frozen objects, rspec-mocks relies on proxies for method stubbing and expectations."
+
+      expect { space1.proxy_for(object) }.to raise_error(ArgumentError, expected_message)
+    end
+
     def in_new_space_scope
       RSpec::Mocks.setup
       yield

spec/rspec/mocks/syntax_agnostic_message_matchers_spec.rb

@@ -42,6 +42,12 @@ module Mocks
 
         reset subject
       end
+
+      context 'when target cannot be proxied' do
+        it 'raises ArgumentError with message' do
+          expect { ::RSpec::Mocks.allow_message(:subject, :foo) { :a } }.to raise_error(ArgumentError)
+        end
+      end
     end
 
     RSpec.describe ".expect_message" do
@@ -91,6 +97,12 @@ module Mocks
 
         verify subject
       end
+
+      context 'when target cannot be proxied' do
+        it 'raises ArgumentError with message' do
+          expect { ::RSpec::Mocks.expect_message(:subject, :foo) { :a } }.to raise_error(ArgumentError)
+        end
+      end
     end
   end
 end