mark gomock functions as test helpers

Use Go 1.9's new "testing.T".Helper method to mark gomock's functions as
test helpers so that test failures print the caller's line and file
information instead of gomock's information.

gomock/call.go

@@ -80,6 +80,10 @@ func (c *Call) Do(f interface{}) *Call {
 
 // Return declares the values to be returned by the mocked function call.
 func (c *Call) Return(rets ...interface{}) *Call {
+	if h, ok := c.t.(testHelper); ok {
+		h.Helper()
+	}
+
 	mt := c.methodType
 	if len(rets) != mt.NumOut() {
 		c.t.Fatalf("wrong number of arguments to Return for %T.%v: got %d, want %d [%s]",
@@ -123,6 +127,10 @@ func (c *Call) Times(n int) *Call {
 // indirected through a pointer. Or, in the case of a slice, SetArg
 // will copy value's elements into the nth argument.
 func (c *Call) SetArg(n int, value interface{}) *Call {
+	if h, ok := c.t.(testHelper); ok {
+		h.Helper()
+	}
+
 	if c.setArgs == nil {
 		c.setArgs = make(map[int]reflect.Value)
 	}
@@ -167,6 +175,10 @@ func (c *Call) isPreReq(other *Call) bool {
 
 // After declares that the call may only match after preReq has been exhausted.
 func (c *Call) After(preReq *Call) *Call {
+	if h, ok := c.t.(testHelper); ok {
+		h.Helper()
+	}
+
 	if c == preReq {
 		c.t.Fatalf("A call isn't allowed to be its own prerequisite")
 	}

gomock/call_test.go

@@ -18,6 +18,8 @@ func (o *mockTestReporter) Fatalf(format string, args ...interface{}) {
 	o.fatalCalls++
 }
 
+func (o *mockTestReporter) Helper() {}
+
 func TestCall_After(t *testing.T) {
 	t.Run("SelfPrereqCallsFatalf", func(t *testing.T) {
 		tr1 := &mockTestReporter{}
@@ -53,6 +55,7 @@ func TestCall_SetArg(t *testing.T) {
 	t.Run("SetArgSlice", func(t *testing.T) {
 		c := &Call{
 			methodType: reflect.TypeOf(func([]byte) {}),
+			t:          &mockTestReporter{},
 		}
 		c.SetArg(0, []byte{1, 2, 3})
 
@@ -67,6 +70,7 @@ func TestCall_SetArg(t *testing.T) {
 	t.Run("SetArgPointer", func(t *testing.T) {
 		c := &Call{
 			methodType: reflect.TypeOf(func(*int) {}),
+			t:          &mockTestReporter{},
 		}
 		c.SetArg(0, 42)
 

gomock/controller.go

@@ -86,6 +86,10 @@ func NewController(t TestReporter) *Controller {
 }
 
 func (ctrl *Controller) RecordCall(receiver interface{}, method string, args ...interface{}) *Call {
+	if h, ok := ctrl.t.(testHelper); ok {
+		h.Helper()
+	}
+
 	recv := reflect.ValueOf(receiver)
 	for i := 0; i < recv.Type().NumMethod(); i++ {
 		if recv.Type().Method(i).Name == method {
@@ -123,6 +127,10 @@ func (ctrl *Controller) RecordCallWithMethodType(receiver interface{}, method st
 }
 
 func (ctrl *Controller) Call(receiver interface{}, method string, args ...interface{}) []interface{} {
+	if h, ok := ctrl.t.(testHelper); ok {
+		h.Helper()
+	}
+
 	ctrl.mu.Lock()
 	defer ctrl.mu.Unlock()
 
@@ -159,6 +167,10 @@ func (ctrl *Controller) Call(receiver interface{}, method string, args ...interf
 }
 
 func (ctrl *Controller) Finish() {
+	if h, ok := ctrl.t.(testHelper); ok {
+		h.Helper()
+	}
+
 	ctrl.mu.Lock()
 	defer ctrl.mu.Unlock()
 
@@ -191,3 +203,8 @@ func callerInfo(skip int) string {
 	}
 	return "unknown file"
 }
+
+type testHelper interface {
+	TestReporter
+	Helper()
+}