How to generate a random string of a fixed length in golang?

//random_string.go
package main

import (
	"fmt"
	"math/rand"
	"time"
)

var letters = []rune("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ")

func init() {
	rand.Seed(time.Now().UnixNano())
}

func RandStringRunes(n int) string {
	b := make([]rune, n)
	for i := range b {
		b[i] = letters[rand.Intn(len(letters))]
	}
	return string(b)
}

func main() {
	var tmp string
	for i := 0; i < 10; i++ {
		tmp = RandStringRunes(8)
		fmt.Println(tmp)
	}

}