bencode: Remove string allocation when parsing strings

bencode/decode.go

@@ -10,6 +10,7 @@ import (
 	"runtime"
 	"strconv"
 	"strings"
+	"unsafe"
 )
 
 type Decoder struct {
@@ -404,7 +405,11 @@ func (d *Decoder) readOneValue() bool {
 		if b >= '0' && b <= '9' {
 			start := d.buf.Len() - 1
 			d.readUntil(':')
-			length, err := strconv.ParseInt(d.buf.String()[start:], 10, 64)
+			s := reflect.StringHeader{
+				uintptr(unsafe.Pointer(&d.buf.Bytes()[start])),
+				d.buf.Len() - start,
+			}
+			length, err := strconv.ParseInt(*(*string)(unsafe.Pointer(&s)), 10, 64)
 			checkForIntParseError(err, d.Offset-1)
 
 			d.buf.WriteString(":")