How to use Go slices

Question content

I am looking at the sha1 related code https://cs.opensource.google/go/go/ /refs/tags/go1. 21.5:src/crypto/sha1/sha1.go;l=146-152

Especially this line append(in, hash[:]...)

I'm not sure why hash[:]... is used, while hash... seems to be enough.

This is a test code https://go.dev/play/p/DaIa0X4KyeD

func main() {
    s := make([]int, 2, 10)
    s[0] = 1
    s[1] = 2

    d := []int{88}
    d = append(d, s[:]...) // d = append(d, s...) seems to work the same
    fmt.Printf("d is: (%v)\n", d)
    fmt.Printf("d len is: (%v)\n", len(d))
    fmt.Printf("d cap is: (%v)\n", cap(d))
}

So my question is what does [:] mean for slicing? Thanks!

Correct answer

hash is an array (type [Size]byte), not a slice. hash[:] is a slice — equivalent to hash[0:len(hash)]. The ... notation requires a slice, so it applies to the slice hash[:] rather than the array hash.

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