k8s 源码分析，语法提问

var _ generic.StorageDecorator = StorageWithCacher

// Creates a cacher based given storageConfig.
func StorageWithCacher(
    copier runtime.ObjectCopier,
    // 配置 etcd api version, server addr 等参数.
    storageConfig *storagebackend.Config,
    capacity int,
    objectType runtime.Object,
    resourcePrefix string,
    keyFunc func(obj runtime.Object) (string, error),
    newListFunc func() runtime.Object,
    getAttrsFunc storage.AttrFunc,
    triggerFunc storage.TriggerPublisherFunc) (storage.Interface, factory.DestroyFunc) {

    // 读取配置 etcd 的配置，如 API 版本，tls 认证等信息，创建 etcd 连接以及 Storage Interface
    s, d := generic.NewRawStorage(storageConfig)
    // TODO: we would change this later to make storage always have cacher and hide low level KV layer inside.
    // Currently it has two layers of same storage interface -- cacher and low level kv.
    cacherConfig := storage.CacherConfig{
        CacheCapacity:        capacity,
        Storage:              s,
        Versioner:            etcdstorage.APIObjectVersioner{},
        Copier:               copier,
        Type:                 objectType,
        ResourcePrefix:       resourcePrefix,
        KeyFunc:              keyFunc,
        NewListFunc:          newListFunc,
        GetAttrsFunc:         getAttrsFunc,
        TriggerPublisherFunc: triggerFunc,
        Codec:                storageConfig.Codec,
    }
    cacher := storage.NewCacherFromConfig(cacherConfig)
    destroyFunc := func() {
        cacher.Stop()
        d()
    }

    return cacher, destroyFunc
}


...
package  generic
type StorageDecorator func(
    copier runtime.ObjectCopier,
    config *storagebackend.Config,
    capacity int,
    objectType runtime.Object,
    resourcePrefix string,
    keyFunc func(obj runtime.Object) (string, error),
    newListFunc func() runtime.Object,
    getAttrsFunc storage.AttrFunc,
    trigger storage.TriggerPublisherFunc) (storage.Interface, factory.DestroyFunc)

提问： StorageWithCacher 这个函数，返回两个值，这个 var _ generic.StorageDecorator 中下划线_是什么意思， 它并不是_ , xxx :=