Compiler magic?

copying-stuff-to-array.go

func AppendDataNaive(packet1 []byte, packet2 []byte) uint16 {
    // Do some initial work done with packet1 and packet2...
    // Don't set any capacity, let Go auto-resize
    data := make([]byte, 0)
    data = append(data, packet1...)
    data = append(data, packet2...)
    // Do some calculations with 'data'
    return calculatedValue
}

func AppendDataSmart(packet1 []byte, packet2 []byte) uint16 {
    // Do some initial work done with packet1 and packet2...
    // Make a slice with len = 0 but underlying capacity of len1 + len2
    data := make([]byte, 0, len(packet1)+len(packet2))
    data = append(data, packet1...)
    data = append(data, packet2...)
    // Do some calculations with 'data'
    return calculatedValue
}