What is the zero value of an interface type and how does it differ from a zero-value struct?

// Zero value of a struct — all fields zeroed
type Config struct{ Host string; Port int; Debug bool }
var cfg Config // zero value: Config{Host:"", Port:0, Debug:false}
fmt.Println(cfg.Host)  // ""  — valid, ready to use
fmt.Println(cfg.Debug) // false

// Zero value of an interface — nil (both words zeroed)
var r io.Reader // nil interface
fmt.Println(r == nil) // true
// r.Read(buf)         // PANIC: nil pointer dereference — no concrete impl

// Nil check before using interface
func useReader(r io.Reader) {
    if r == nil {
        log.Println("no reader provided")
        return
    }
    io.Copy(os.Stdout, r)
}

// Structs designed to be useful at zero value (sync.Mutex, bytes.Buffer)
var mu sync.Mutex  // zero value is a valid, unlocked mutex
mu.Lock()
defer mu.Unlock()

var buf bytes.Buffer // zero value is a valid empty buffer
buf.WriteString("hello")

// Interface containing a zero-value struct (NOT nil interface)
var c Config
var any interface{} = c  // any is non-nil (type=Config, data=*Config zero)
fmt.Println(any == nil)  // FALSE — interface holds a value