这在golang中意味着什么

Question

type Serializer struct {       
    meta    MetaFactory        
    creater runtime.ObjectCreater
    typer   runtime.ObjectTyper
    yaml    bool               
    pretty  bool               
} 

// Serializer implements Serializer
var _ runtime.Serializer = &Serializer{}
var _ recognizer.RecognizingDecoder = &Serializer{}

最后一行是什么意思？ runtime.Serilazer是一个接口，为什么在这里创建一个null存在的变量？