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

The builtin copy(dst, src) copies min(len(dst), len(src)) elements.

So if your dst is empty (len(dst) == 0), nothing will be copied.

Try tmp := make([]int, len(arr)) (Go Playground):

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

Output (as expected):

[1 2 3]
[1 2 3]

Unfortunately this is not documented in the builtin package, but it is documented in the Go Language Specification: Appending to and copying slices:

The number of elements copied is the minimum of len(src) and len(dst).

Edit:

Finally the documentation of copy() has been updated and it now contains the fact that the minimum length of source and destination will be copied:

Copy returns the number of elements copied, which will be the minimum of len(src) and len(dst).

Another simple way to do this is by using append which will allocate the slice in the process.

arr := []int{1, 2, 3}
tmp := append([]int(nil), arr...)  // Notice the ... splat
fmt.Println(tmp)
fmt.Println(arr)

Output (as expected):

[1 2 3]
[1 2 3]

So a shorthand for copying array arr would be append([]int(nil), arr...)

https://play.golang.org/p/sr_4ofs5GW

If your slices were of the same size, it would work:

arr := []int{1, 2, 3}
tmp := []int{0, 0, 0}
i := copy(tmp, arr)
fmt.Println(i)
fmt.Println(tmp)
fmt.Println(arr)

Would give:

3
[1 2 3]
[1 2 3]

From "Go Slices: usage and internals":

The copy function supports copying between slices of different lengths (it will copy only up to the smaller number of elements)

The usual example is:

t := make([]byte, len(s), (cap(s)+1)*2)
copy(t, s)
s = t