Added zapcore.TimeEncoderOfLayout (#629)

I added `zapcore.TimeEncoderOfFormat` to make it easy to build `TimeEncoder` of custom format.

You can use it in code:

```go
config := zap.NewDevelopmentConfig()
config.EncoderConfig.EncodeTime = zapcore.TimeEncoderOfLayout("06/01/02 03:04pm")
```

or, with JSON:

```json
{
  "encoderConfig": {
    "timeEncoder": {
       "layout": "06/01/02 03:04pm"
     }
  }
}
```

or, with YAML:
```yaml
encoderConfig:
  timeEncoder:
    layout: '06/01/02 03:04pm'
```

go.mod

@@ -8,5 +8,6 @@ require (
 	go.uber.org/atomic v1.6.0
 	go.uber.org/multierr v1.5.0
 	golang.org/x/lint v0.0.0-20190930215403-16217165b5de
+	gopkg.in/yaml.v2 v2.2.2
 	honnef.co/go/tools v0.0.1-2019.2.3
 )

zapcore/encoder.go

@@ -21,6 +21,7 @@
 package zapcore
 
 import (
+	"encoding/json"
 	"time"
 
 	"go.uber.org/zap/buffer"
@@ -151,6 +152,14 @@ func RFC3339NanoTimeEncoder(t time.Time, enc PrimitiveArrayEncoder) {
 	encodeTimeLayout(t, time.RFC3339Nano, enc)
 }
 
+// TimeEncoderOfLayout returns TimeEncoder which serializes a time.Time using
+// given layout.
+func TimeEncoderOfLayout(layout string) TimeEncoder {
+	return func(t time.Time, enc PrimitiveArrayEncoder) {
+		encodeTimeLayout(t, layout, enc)
+	}
+}
+
 // UnmarshalText unmarshals text to a TimeEncoder.
 // "rfc3339nano" and "RFC3339Nano" are unmarshaled to RFC3339NanoTimeEncoder.
 // "rfc3339" and "RFC3339" are unmarshaled to RFC3339TimeEncoder.
@@ -176,6 +185,35 @@ func (e *TimeEncoder) UnmarshalText(text []byte) error {
 	return nil
 }
 
+// UnmarshalYAML unmarshals YAML to a TimeEncoder.
+// If value is an object with a "layout" field, it will be unmarshaled to  TimeEncoder with given layout.
+//     timeEncoder:
+//       layout: 06/01/02 03:04pm
+// If value is string, it uses UnmarshalText.
+//     timeEncoder: iso8601
+func (e *TimeEncoder) UnmarshalYAML(unmarshal func(interface{}) error) error {
+	var o struct {
+		Layout string `json:"layout" yaml:"layout"`
+	}
+	if err := unmarshal(&o); err == nil {
+		*e = TimeEncoderOfLayout(o.Layout)
+		return nil
+	}
+
+	var s string
+	if err := unmarshal(&s); err != nil {
+		return err
+	}
+	return e.UnmarshalText([]byte(s))
+}
+
+// UnmarshalJSON unmarshals JSON to a TimeEncoder as same way UnmarshalYAML does.
+func (e *TimeEncoder) UnmarshalJSON(data []byte) error {
+	return e.UnmarshalYAML(func(v interface{}) error {
+		return json.Unmarshal(data, v)
+	})
+}
+
 // A DurationEncoder serializes a time.Duration to a primitive type.
 type DurationEncoder func(time.Duration, PrimitiveArrayEncoder)
 

zapcore/encoder_test.go

@@ -21,12 +21,14 @@
 package zapcore_test
 
 import (
+	"encoding/json"
 	"strings"
 	"testing"
 	"time"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+	"gopkg.in/yaml.v2"
 
 	. "go.uber.org/zap/zapcore"
 )
@@ -523,29 +525,65 @@ func TestLevelEncoders(t *testing.T) {
 func TestTimeEncoders(t *testing.T) {
 	moment := time.Unix(100, 50005000).UTC()
 	tests := []struct {
-		name     string
+		yamlDoc  string
+		expected interface{} // output of serializing moment
+	}{
+		{"timeEncoder: iso8601", "1970-01-01T00:01:40.050Z"},
+		{"timeEncoder: ISO8601", "1970-01-01T00:01:40.050Z"},
+		{"timeEncoder: millis", 100050.005},
+		{"timeEncoder: nanos", int64(100050005000)},
+		{"timeEncoder: {layout: 06/01/02 03:04pm}", "70/01/01 12:01am"},
+		{"timeEncoder: ''", 100.050005},
+		{"timeEncoder: something-random", 100.050005},
+		{"timeEncoder: rfc3339", "1970-01-01T00:01:40Z"},
+		{"timeEncoder: RFC3339", "1970-01-01T00:01:40Z"},
+		{"timeEncoder: rfc3339nano", "1970-01-01T00:01:40.050005Z"},
+		{"timeEncoder: RFC3339Nano", "1970-01-01T00:01:40.050005Z"},
+	}
+
+	for _, tt := range tests {
+		cfg := EncoderConfig{}
+		require.NoError(t, yaml.Unmarshal([]byte(tt.yamlDoc), &cfg), "Unexpected error unmarshaling %q.", tt.yamlDoc)
+		require.NotNil(t, cfg.EncodeTime, "Unmashalled timeEncoder is nil for %q.", tt.yamlDoc)
+		assertAppended(
+			t,
+			tt.expected,
+			func(arr ArrayEncoder) { cfg.EncodeTime(moment, arr) },
+			"Unexpected output serializing %v with %q.", moment, tt.yamlDoc,
+		)
+	}
+}
+
+func TestTimeEncodersWrongYAML(t *testing.T) {
+	tests := []string{
+		"timeEncoder: [1, 2, 3]", // wrong type
+		"timeEncoder: {foo:bar",  // broken yaml
+	}
+	for _, tt := range tests {
+		cfg := EncoderConfig{}
+		assert.Error(t, yaml.Unmarshal([]byte(tt), &cfg), "Expected unmarshaling %q to become error, but not.", tt)
+	}
+}
+
+func TestTimeEncodersParseFromJSON(t *testing.T) {
+	moment := time.Unix(100, 50005000).UTC()
+	tests := []struct {
+		jsonDoc  string
 		expected interface{} // output of serializing moment
 	}{
-		{"iso8601", "1970-01-01T00:01:40.050Z"},
-		{"ISO8601", "1970-01-01T00:01:40.050Z"},
-		{"millis", 100050.005},
-		{"nanos", int64(100050005000)},
-		{"", 100.050005},
-		{"something-random", 100.050005},
-		{"rfc3339", "1970-01-01T00:01:40Z"},
-		{"RFC3339", "1970-01-01T00:01:40Z"},
-		{"rfc3339nano", "1970-01-01T00:01:40.050005Z"},
-		{"RFC3339Nano", "1970-01-01T00:01:40.050005Z"},
+		{`{"timeEncoder": "iso8601"}`, "1970-01-01T00:01:40.050Z"},
+		{`{"timeEncoder": {"layout": "06/01/02 03:04pm"}}`, "70/01/01 12:01am"},
 	}
 
 	for _, tt := range tests {
-		var te TimeEncoder
-		require.NoError(t, te.UnmarshalText([]byte(tt.name)), "Unexpected error unmarshaling %q.", tt.name)
+		cfg := EncoderConfig{}
+		require.NoError(t, json.Unmarshal([]byte(tt.jsonDoc), &cfg), "Unexpected error unmarshaling %q.", tt.jsonDoc)
+		require.NotNil(t, cfg.EncodeTime, "Unmashalled timeEncoder is nil for %q.", tt.jsonDoc)
 		assertAppended(
 			t,
 			tt.expected,
-			func(arr ArrayEncoder) { te(moment, arr) },
-			"Unexpected output serializing %v with %q.", moment, tt.name,
+			func(arr ArrayEncoder) { cfg.EncodeTime(moment, arr) },
+			"Unexpected output serializing %v with %q.", moment, tt.jsonDoc,
 		)
 	}
 }