Which is faster: `s += str` or `buf.WriteString(str) ... buf.String()`?

results.txt

PASS
BenchmarkStringConcat-4	 2000000	      3901 ns/op
BenchmarkByteBuffer-4  	10000000	       966 ns/op

strings_test.go

package strings_test

import (
	"bytes"
	"testing"
)
import "strings"

var alphabet = strings.Split("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ", "")

var result string

func TestNothing(t *testing.T) {}

func BenchmarkStringConcat(b *testing.B) {
	for i := 0; i < b.N; i++ {
		s := ""
		for _, c := range alphabet {
			s += c
		}
		result = s
	}
	if result != strings.Join(alphabet, "") {
		b.Fatalf("bug")
	}
}

func BenchmarkByteBuffer(b *testing.B) {
	for i := 0; i < b.N; i++ {
		var b bytes.Buffer
		for _, c := range alphabet {
			_, _ = b.WriteString(c)
		}
		result = b.String()
	}
	if result != strings.Join(alphabet, "") {
		b.Fatalf("bug")
	}
}