time: add Time.AppendFormat

This is a version of Time.Format that doesn't require allocation.

This is an updated version of 0af302f5
submitted by @bradfitz which was later rolled back.

Fixes #5192
Updates #5195

Change-Id: I4e6255bee1cf3914a6cc8d9d2a881cfeb273c08e
Reviewed-on: https://go-review.googlesource.com/1760Reviewed-by: Rob Pike <r@golang.org>

src/time/format.go

@@ -410,6 +410,22 @@ func (t Time) String() string {
 // about the formats and the definition of the reference time, see the
 // documentation for ANSIC and the other constants defined by this package.
 func (t Time) Format(layout string) string {
+	const bufSize = 64
+	var b []byte
+	max := len(layout) + 10
+	if max < bufSize {
+		var buf [bufSize]byte
+		b = buf[:0]
+	} else {
+		b = make([]byte, 0, max)
+	}
+	b = t.AppendFormat(b, layout)
+	return string(b)
+}
+
+// AppendFormat is like Format but appends the textual
+// representation to b and returns the extended buffer.
+func (t Time) AppendFormat(b []byte, layout string) []byte {
 	var (
 		name, offset, abs = t.locabs()
 
@@ -419,16 +435,7 @@ func (t Time) Format(layout string) string {
 		hour  int = -1
 		min   int
 		sec   int
-
-		b   []byte
-		buf [64]byte
 	)
-	max := len(layout) + 10
-	if max <= len(buf) {
-		b = buf[:0]
-	} else {
-		b = make([]byte, 0, max)
-	}
 	// Each iteration generates one std value.
 	for layout != "" {
 		prefix, std, suffix := nextStdChunk(layout)
@@ -568,7 +575,7 @@ func (t Time) Format(layout string) string {
 			b = formatNano(b, uint(t.Nanosecond()), std>>stdArgShift, std&stdMask == stdFracSecond9)
 		}
 	}
-	return string(b)
+	return b
 }
 
 var errBad = errors.New("bad value for field") // placeholder not passed to user