Use Time.AppendFormat when possible (uber-go#786)

Fixes uber-go#783.

We can take advantage of Time.AppendFormat by adding a new exported
method to the JSON encoder, and upcasting when encoding time to use
the append-based method. This avoids an allocation to convert the time
to a string before appending to the buffer.

The benchmarks use the production config, which uses a nanos encoder
so to understand the performance difference, I tweaked
`BenchmarkZapJSON` to use RFC3339TimeEncoder and ran benchmarks:

```
> benchcmp -best old new
benchmark               old ns/op     new ns/op     delta
BenchmarkZapJSON-12     514           497           -3.31%

benchmark               old allocs     new allocs     delta
BenchmarkZapJSON-12     5              4              -20.00%

benchmark               old bytes     new bytes     delta
BenchmarkZapJSON-12     1297          1265          -2.47%
```

I also wrote a benchmark that only logs a simple message using the
RFC3339TimeEncoder,
```
func BenchmarkTimeEncoder(b *testing.B) {
        cfg := NewProductionConfig().EncoderConfig
        cfg.EncodeTime = zapcore.RFC3339TimeEncoder
        logger := New(
                zapcore.NewCore(
                        zapcore.NewJSONEncoder(cfg),
                        &ztest.Discarder{},
                        DebugLevel,
                ))
        b.ResetTimer()

        for i := 0; i < b.N; i++ {
                logger.Info("test")
        }
}
```

Results:
```
> benchcmp -best old new
benchmark                   old ns/op     new ns/op     delta
BenchmarkTimeEncoder-12     695           571           -17.84%

benchmark                   old allocs     new allocs     delta
BenchmarkTimeEncoder-12     1              0              -100.00%

benchmark                   old bytes     new bytes     delta
BenchmarkTimeEncoder-12     32            0             -100.00%
```

buffer/buffer.go

@@ -23,7 +23,10 @@
 // package's zero-allocation formatters.
 package buffer // import "go.uber.org/zap/buffer"
 
-import "strconv"
+import (
+	"strconv"
+	"time"
+)
 
 const _size = 1024 // by default, create 1 KiB buffers
 
@@ -49,6 +52,11 @@ func (b *Buffer) AppendInt(i int64) {
 	b.bs = strconv.AppendInt(b.bs, i, 10)
 }
 
+// AppendTime appends the time formatted using the specified layout.
+func (b *Buffer) AppendTime(t time.Time, layout string) {
+	b.bs = t.AppendFormat(b.bs, layout)
+}
+
 // AppendUint appends an unsigned integer to the underlying buffer (assuming
 // base 10).
 func (b *Buffer) AppendUint(i uint64) {

buffer/buffer_test.go

@@ -24,6 +24,7 @@ import (
 	"bytes"
 	"strings"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -46,6 +47,7 @@ func TestBufferWrites(t *testing.T) {
 		// Intenationally introduce some floating-point error.
 		{"AppendFloat32", func() { buf.AppendFloat(float64(float32(3.14)), 32) }, "3.14"},
 		{"AppendWrite", func() { buf.Write([]byte("foo")) }, "foo"},
+		{"AppendTime", func() { buf.AppendTime(time.Date(2000, 1, 2, 3, 4, 5, 6, time.UTC), time.RFC3339) }, "2000-01-02T03:04:05Z"},
 	}
 
 	for _, tt := range tests {

zapcore/encoder.go

@@ -112,21 +112,43 @@ func EpochNanosTimeEncoder(t time.Time, enc PrimitiveArrayEncoder) {
 	enc.AppendInt64(t.UnixNano())
 }
 
+func encodeTimeLayout(t time.Time, layout string, enc PrimitiveArrayEncoder) {
+	type appendTimeEncoder interface {
+		AppendTimeLayout(time.Time, string)
+	}
+
+	if enc, ok := enc.(appendTimeEncoder); ok {
+		enc.AppendTimeLayout(t, layout)
+		return
+	}
+
+	enc.AppendString(t.Format(layout))
+}
+
 // ISO8601TimeEncoder serializes a time.Time to an ISO8601-formatted string
 // with millisecond precision.
+//
+// If enc supports AppendTimeLayout(t time.Time,layout string), it's used
+// instead of appending a pre-formatted string value.
 func ISO8601TimeEncoder(t time.Time, enc PrimitiveArrayEncoder) {
-	enc.AppendString(t.Format("2006-01-02T15:04:05.000Z0700"))
+	encodeTimeLayout(t, "2006-01-02T15:04:05.000Z0700", enc)
 }
 
 // RFC3339TimeEncoder serializes a time.Time to an RFC3339-formatted string.
+//
+// If enc supports AppendTimeLayout(t time.Time,layout string), it's used
+// instead of appending a pre-formatted string value.
 func RFC3339TimeEncoder(t time.Time, enc PrimitiveArrayEncoder) {
-	enc.AppendString(t.Format(time.RFC3339))
+	encodeTimeLayout(t, time.RFC3339, enc)
 }
 
 // RFC3339NanoTimeEncoder serializes a time.Time to an RFC3339-formatted string
 // with nanosecond precision.
+//
+// If enc supports AppendTimeLayout(t time.Time,layout string), it's used
+// instead of appending a pre-formatted string value.
 func RFC3339NanoTimeEncoder(t time.Time, enc PrimitiveArrayEncoder) {
-	enc.AppendString(t.Format(time.RFC3339Nano))
+	encodeTimeLayout(t, time.RFC3339Nano, enc)
 }
 
 // UnmarshalText unmarshals text to a TimeEncoder.

zapcore/json_encoder.go

@@ -266,6 +266,12 @@ func (enc *jsonEncoder) AppendString(val string) {
 	enc.buf.AppendByte('"')
 }
 
+func (enc *jsonEncoder) AppendTimeLayout(time time.Time, layout string) {
+	enc.buf.AppendByte('"')
+	enc.buf.AppendTime(time, layout)
+	enc.buf.AppendByte('"')
+}
+
 func (enc *jsonEncoder) AppendTime(val time.Time) {
 	cur := enc.buf.Len()
 	enc.EncodeTime(val, enc)

zapcore/json_encoder_impl_test.go

@@ -36,6 +36,11 @@ import (
 	"go.uber.org/multierr"
 )
 
+var _defaultEncoderConfig = EncoderConfig{
+	EncodeTime:     EpochTimeEncoder,
+	EncodeDuration: SecondsDurationEncoder,
+}
+
 func TestJSONClone(t *testing.T) {
 	// The parent encoder is created with plenty of excess capacity.
 	parent := &jsonEncoder{buf: bufferpool.Get()}
@@ -224,7 +229,55 @@ func TestJSONEncoderObjectFields(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.desc, func(t *testing.T) {
-			assertOutput(t, tt.expected, tt.f)
+			assertOutput(t, _defaultEncoderConfig, tt.expected, tt.f)
+		})
+	}
+}
+
+func TestJSONEncoderTimeFormats(t *testing.T) {
+	date := time.Date(2000, time.January, 2, 3, 4, 5, 6, time.UTC)
+
+	f := func(e Encoder) {
+		e.AddTime("k", date)
+		e.AddArray("a", ArrayMarshalerFunc(func(enc ArrayEncoder) error {
+			enc.AppendTime(date)
+			return nil
+		}))
+	}
+	tests := []struct {
+		desc     string
+		cfg      EncoderConfig
+		expected string
+	}{
+		{
+			desc: "time.Time ISO8601",
+			cfg: EncoderConfig{
+				EncodeDuration: NanosDurationEncoder,
+				EncodeTime:     ISO8601TimeEncoder,
+			},
+			expected: `"k":"2000-01-02T03:04:05.000Z","a":["2000-01-02T03:04:05.000Z"]`,
+		},
+		{
+			desc: "time.Time RFC3339",
+			cfg: EncoderConfig{
+				EncodeDuration: NanosDurationEncoder,
+				EncodeTime:     RFC3339TimeEncoder,
+			},
+			expected: `"k":"2000-01-02T03:04:05Z","a":["2000-01-02T03:04:05Z"]`,
+		},
+		{
+			desc: "time.Time RFC3339Nano",
+			cfg: EncoderConfig{
+				EncodeDuration: NanosDurationEncoder,
+				EncodeTime:     RFC3339NanoTimeEncoder,
+			},
+			expected: `"k":"2000-01-02T03:04:05.000000006Z","a":["2000-01-02T03:04:05.000000006Z"]`,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.desc, func(t *testing.T) {
+			assertOutput(t, tt.cfg, tt.expected, f)
 		})
 	}
 }
@@ -324,7 +377,7 @@ func TestJSONEncoderArrays(t *testing.T) {
 					return nil
 				}))
 			}
-			assertOutput(t, `"array":`+tt.expected, func(enc Encoder) {
+			assertOutput(t, _defaultEncoderConfig, `"array":`+tt.expected, func(enc Encoder) {
 				err := f(enc)
 				assert.NoError(t, err, "Unexpected error adding array to JSON encoder.")
 			})
@@ -336,11 +389,8 @@ func assertJSON(t *testing.T, expected string, enc *jsonEncoder) {
 	assert.Equal(t, expected, enc.buf.String(), "Encoded JSON didn't match expectations.")
 }
 
-func assertOutput(t testing.TB, expected string, f func(Encoder)) {
-	enc := &jsonEncoder{buf: bufferpool.Get(), EncoderConfig: &EncoderConfig{
-		EncodeTime:     EpochTimeEncoder,
-		EncodeDuration: SecondsDurationEncoder,
-	}}
+func assertOutput(t testing.TB, cfg EncoderConfig, expected string, f func(Encoder)) {
+	enc := &jsonEncoder{buf: bufferpool.Get(), EncoderConfig: &cfg}
 	f(enc)
 	assert.Equal(t, expected, enc.buf.String(), "Unexpected encoder output after adding.")