log.go
func formatHeader(buf *[]byte, t time.Time, lvl, tag string, file string, line int) {
	year, month, day := t.Date()
	hour, min, sec := t.Clock()
	ms := t.Nanosecond() / 1e6

	itoa(buf, year, 4)
	*buf = append(*buf, '-')
	itoa(buf, int(month), 2)
	*buf = append(*buf, '-')
	itoa(buf, day, 2)
	*buf = append(*buf, ' ')
	itoa(buf, hour, 2)
	*buf = append(*buf, ':')
	itoa(buf, min, 2)
	*buf = append(*buf, ':')
	itoa(buf, sec, 2)
	*buf = append(*buf, '.')
	itoa(buf, ms, 3)
	*buf = append(*buf, " ["...)
	*buf = append(*buf, lvl...)
	*buf = append(*buf, "] "...)
	*buf = append(*buf, tag...)
	if file != "" {
		*buf = append(*buf, ' ')
		*buf = append(*buf, file...)
		*buf = append(*buf, ':')
		itoa(buf, line, -1)
	}
	*buf = append(*buf, ": "...)
}
func formatHeader(buf *[]byte, t time.Time, lvl, tag string, file string, line int) {
	year, month, day := t.Date()
	hour, min, sec := t.Clock()
	ms := t.Nanosecond() / 1e6

	itoa(buf, year, 4)
	*buf = append(*buf, '-')
	itoa(buf, int(month), 2)
	*buf = append(*buf, '-')
	itoa(buf, day, 2)
	*buf = append(*buf, ' ')
	itoa(buf, hour, 2)
	*buf = append(*buf, ':')
	itoa(buf, min, 2)
	*buf = append(*buf, ':')
	itoa(buf, sec, 2)
	*buf = append(*buf, '.')
	itoa(buf, ms, 3)
	*buf = append(*buf, " ["...)
	*buf = append(*buf, lvl...)
	*buf = append(*buf, "] "...)
	*buf = append(*buf, tag...)
	if file != "" {
		*buf = append(*buf, ' ')
		*buf = append(*buf, file...)
		*buf = append(*buf, ':')
		itoa(buf, line, -1)
	}
	*buf = append(*buf, ": "...)
}
// Appends a header in the default format 'YYYY-MM-DD hh:mm:ss.sss [LVL] TAG: '.
// If either of the Lshortfile or Llongfile flags are specified, the file named
// and line number are included after the tag and before the final colon.