How to convert slice to fixed size array? [duplicate]

Edit: as of Go 1.17+, you may be able to use new support for slice-to-array conversions, https://tip.golang.org/ref/spec#Conversions_from_slice_to_array_pointer:

s := make([]byte, 2, 4)
s0 := (*[0]byte)(s)      // s0 != nil
s1 := (*[1]byte)(s[1:])  // &s1[0] == &s[1]
s2 := (*[2]byte)(s)      // &s2[0] == &s[0]
s4 := (*[4]byte)(s)      // panics: len([4]byte) > len(s)

Previous answer for Go 1.16 and below:

You need to use copy:

slice := []byte("abcdefgh")

var arr [4]byte

copy(arr[:], slice[:4])

fmt.Println(arr)

As Aedolon notes you can also just use

copy(arr[:], slice)

as copy will always only copy the minimum of len(src) and len(dst) bytes.