Add ValueMockRegistry

Support injecting mock functionality into concrete structs (value types).

spec/spectator/mocks/mock_spec.cr

@@ -16,6 +16,22 @@ class MockedClass
   end
 end
 
+struct MockedStruct
+  getter method1 = 42
+
+  def method2
+    :original
+  end
+
+  def method3
+    "original"
+  end
+
+  def instance_variables
+    [{{@type.instance_vars.map(&.name.symbolize).splat}}]
+  end
+end
+
 Spectator.describe Spectator::Mock do
   describe "#define_subclass" do
     class Thing
@@ -106,5 +122,43 @@ Spectator.describe Spectator::Mock do
         expect(mock.instance_variables).to eq([:method1])
       end
     end
+
+    context "with a struct" do
+      Spectator::Mock.inject(MockedStruct, :mock_name, method1: 123) do
+        stub def method2
+          :stubbed
+        end
+      end
+
+      let(mock) { MockedStruct.new }
+
+      it "overrides responses from methods with keyword arguments" do
+        expect(mock.method1).to eq(123)
+      end
+
+      it "overrides responses from methods defined in the block" do
+        expect(mock.method2).to eq(:stubbed)
+      end
+
+      it "allows methods to be stubbed" do
+        stub1 = Spectator::ValueStub.new(:method1, 777)
+        stub2 = Spectator::ValueStub.new(:method2, :override)
+        stub3 = Spectator::ValueStub.new(:method3, "stubbed")
+
+        aggregate_failures do
+          expect { mock._spectator_define_stub(stub1) }.to change { mock.method1 }.to(777)
+          expect { mock._spectator_define_stub(stub2) }.to change { mock.method2 }.to(:override)
+          expect { mock._spectator_define_stub(stub3) }.to change { mock.method3 }.from("original").to("stubbed")
+        end
+      end
+
+      it "doesn't change the size of an instance" do
+        expect(sizeof(MockedStruct)).to eq(4) # sizeof(Int32)
+      end
+
+      it "doesn't affect instance variables" do
+        expect(mock.instance_variables).to eq([:method1])
+      end
+    end
   end
 end