Why can't I duplicate a slice with `copy()`?

Question

I need to make a copy of a slice in Go and reading the docs there is a copy function at my disposal.

The copy built-in function copies elements from a so

Answer 1

The Go Programming Language Specification

Appending to and copying slices

The function copy copies slice elements from a source src to a destination dst and returns the number of elements copied. Both arguments must have identical element type T and must be assignable to a slice of type []T. The number of elements copied is the minimum of len(src) and len(dst). As a special case, copy also accepts a destination argument assignable to type []byte with a source argument of a string type. This form copies the bytes from the string into the byte slice.

copy(dst, src []T) int
copy(dst []byte, src string) int

tmp needs enough room for arr. For example,

package main

import "fmt"

func main() {
    arr := []int{1, 2, 3}
    tmp := make([]int, len(arr))
    copy(tmp, arr)
    fmt.Println(tmp)
    fmt.Println(arr)
}

Output:

[1 2 3]
[1 2 3]