Make obtaining the Testable interface actually part of the interface.

impl/memory/taskqueue_data.go

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package memory
 
 import (
         "errors"
         "fmt"
         "net/http"
@@ skipping 14 matching lines @@
 
 //////////////////////////////// taskQueueData /////////////////////////////////
 
 type taskQueueData struct {
         sync.Mutex
 
         named    tq.QueueData
         archived tq.QueueData
 }
 
-var (
-        _ = memContextObj((*taskQueueData)(nil))
-        _ = tq.Testable((*taskQueueData)(nil))
-)
+var _ interface {
+        memContextObj
+        tq.Testable
+} = (*taskQueueData)(nil)
 
 func newTaskQueueData() memContextObj {
         return &taskQueueData{
                 named:    tq.QueueData{"default": {}},
                 archived: tq.QueueData{"default": {}},
         }
 }
 
 func (t *taskQueueData) canApplyTxn(obj memContextObj) bool { return true }
 func (t *taskQueueData) endTxn()                            {}
@@ skipping 135 matching lines @@
 
 type txnTaskQueueData struct {
         lock sync.Mutex
 
         // boolean 0 or 1, use atomic.*Int32 to access.
         closed int32
         anony  tq.AnonymousQueueData
         parent *taskQueueData
 }
 
-var (
-        _ = memContextObj((*txnTaskQueueData)(nil))
-        _ = tq.Testable((*txnTaskQueueData)(nil))
-)
+var _ interface {
+        memContextObj
+        tq.Testable
+} = (*txnTaskQueueData)(nil)
 
 func (t *txnTaskQueueData) canApplyTxn(obj memContextObj) bool         { return false }
 func (t *txnTaskQueueData) applyTxn(context.Context, memContextObj)    { panic("impossible") }
 func (t *txnTaskQueueData) mkTxn(*ds.TransactionOptions) memContextObj { panic("impossible") }
 
 func (t *txnTaskQueueData) endTxn() {
         if atomic.LoadInt32(&t.closed) == 1 {
                 panic("cannot end transaction twice")
         }
         atomic.StoreInt32(&t.closed, 1)
@@ skipping 39 matching lines @@
         return t.parent.GetTombstonedTasks()
 }
 
 func (t *txnTaskQueueData) GetScheduledTasks() tq.QueueData {
         return t.parent.GetScheduledTasks()
 }
 
 func (t *txnTaskQueueData) CreateQueue(queueName string) {
         t.parent.CreateQueue(queueName)
 }