time: change formatting of microseconds duration to SI modifier

'u' is not micro, µ (U+00B5) is.

LGTM=gri, bradfitz
R=golang-codereviews, bradfitz, gri
CC=golang-codereviews
https://golang.org/cl/105030046

src/pkg/time/example_test.go

@@ -122,7 +122,7 @@ func ExampleTime_Round() {
 	}
 	// Output:
 	// t.Round(   1ns) = 12:15:30.918273645
-	// t.Round(   1us) = 12:15:30.918274
+	// t.Round(   1µs) = 12:15:30.918274
 	// t.Round(   1ms) = 12:15:30.918
 	// t.Round(    1s) = 12:15:31
 	// t.Round(    2s) = 12:15:30
@@ -150,7 +150,7 @@ func ExampleTime_Truncate() {
 
 	// Output:
 	// t.Truncate(   1ns) = 12:15:30.918273645
-	// t.Truncate(   1us) = 12:15:30.918273
+	// t.Truncate(   1µs) = 12:15:30.918273
 	// t.Truncate(   1ms) = 12:15:30.918
 	// t.Truncate(    1s) = 12:15:30
 	// t.Truncate(    2s) = 12:15:30

src/pkg/time/time.go

@@ -475,29 +475,28 @@ func (d Duration) String() string {
 	if u < uint64(Second) {
 		// Special case: if duration is smaller than a second,
 		// use smaller units, like 1.2ms
-		var (
-			prec int
-			unit byte
-		)
+		var prec int
+		w--
+		buf[w] = 's'
+		w--
 		switch {
 		case u == 0:
 			return "0"
 		case u < uint64(Microsecond):
 			// print nanoseconds
 			prec = 0
-			unit = 'n'
+			buf[w] = 'n'
 		case u < uint64(Millisecond):
 			// print microseconds
 			prec = 3
-			unit = 'u'
+			// U+00B5 'µ' micro sign == 0xC2 0xB5
+			w-- // Need room for two bytes.
+			copy(buf[w:], "µ")
 		default:
 			// print milliseconds
 			prec = 6
-			unit = 'm'
+			buf[w] = 'm'
 		}
-		w -= 2
-		buf[w] = unit
-		buf[w+1] = 's'
 		w, u = fmtFrac(buf[:w], u, prec)
 		w = fmtInt(buf[:w], u)
 	} else {

src/pkg/time/time_test.go

@@ -535,7 +535,7 @@ var durationTests = []struct {
 }{
 	{"0", 0},
 	{"1ns", 1 * Nanosecond},
-	{"1.1us", 1100 * Nanosecond},
+	{"1.1µs", 1100 * Nanosecond},
 	{"2.2ms", 2200 * Microsecond},
 	{"3.3s", 3300 * Millisecond},
 	{"4m5s", 4*Minute + 5*Second},