How to generate *unique* random number in Go using standard library

You should absolutely go with approach 2. Let's assume you're running on a 64-bit machine, and thus generating 63-bit integers (64 bits, but rand.Int never returns negative numbers). Even if you generate 4 billion numbers, there's still only a 1 in 4 billion chance that any given number will be a duplicate. Thus, you'll almost never have to regenerate, and almost never never have to regenerate twice.

Try, for example:

type UniqueRand struct {
    generated map[int]bool
}

func (u *UniqueRand) Int() int {
    for {
        i := rand.Int()
        if !u.generated[i] {
            u.generated[i] = true
            return i
        }
    }
}