time: make Duration.String inlineable

Perform the [32]byte to string conversion in an inlinable method.
Thus, if the result does not escape in the context of the caller,
we can entirely avoid a call to runtime.slicebytetostring.

Change-Id: Iae8ec2a532776ed6cf99597f19e3f7f21c694c3a
Reviewed-on: https://go-review.googlesource.com/c/go/+/520602
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/time/time.go

@@ -650,8 +650,17 @@ const (
 // second format use a smaller unit (milli-, micro-, or nanoseconds) to ensure
 // that the leading digit is non-zero. The zero duration formats as 0s.
 func (d Duration) String() string {
+	// This is inlinable to take advantage of "function outlining".
+	// Thus, the caller can decide whether a string must be heap allocated.
+	var arr [32]byte
+	n := d.format(&arr)
+	return string(arr[n:])
+}
+
+// format formats the representation of d into the end of buf and
+// returns the offset of the first character.
+func (d Duration) format(buf *[32]byte) int {
 	// Largest time is 2540400h10m10.000000000s
-	var buf [32]byte
 	w := len(buf)
 
 	u := uint64(d)
@@ -669,7 +678,8 @@ func (d Duration) String() string {
 		w--
 		switch {
 		case u == 0:
-			return "0s"
+			buf[w] = '0'
+			return w
 		case u < uint64(Microsecond):
 			// print nanoseconds
 			prec = 0
@@ -719,7 +729,7 @@ func (d Duration) String() string {
 		buf[w] = '-'
 	}
 
-	return string(buf[w:])
+	return w
 }
 
 // fmtFrac formats the fraction of v/10**prec (e.g., ".12345") into the