Don't panic when a given field value is nil

This commit ensures that zap.Stringer, zap.Object and zap.Any don't
cause a panic when the given field value is nil.

zapcore/field.go

@@ -109,9 +109,17 @@ func (f Field) AddTo(enc ObjectEncoder) {
 
 	switch f.Type {
 	case ArrayMarshalerType:
-		err = enc.AddArray(f.Key, f.Interface.(ArrayMarshaler))
+		if am, ok := f.Interface.(ArrayMarshaler); ok && am != nil && !isNil(am) {
+			err = enc.AddArray(f.Key, am)
+		} else {
+			enc.AddString(f.Key, "nil")
+		}
 	case ObjectMarshalerType:
-		err = enc.AddObject(f.Key, f.Interface.(ObjectMarshaler))
+		if om, ok := f.Interface.(ObjectMarshaler); ok && om != nil && !isNil(om) {
+			err = enc.AddObject(f.Key, om)
+		} else {
+			enc.AddString(f.Key, "nil")
+		}
 	case BinaryType:
 		enc.AddBinary(f.Key, f.Interface.([]byte))
 	case BoolType:
@@ -160,7 +168,11 @@ func (f Field) AddTo(enc ObjectEncoder) {
 	case NamespaceType:
 		enc.OpenNamespace(f.Key)
 	case StringerType:
-		enc.AddString(f.Key, f.Interface.(fmt.Stringer).String())
+		if s, ok := f.Interface.(fmt.Stringer); ok && s != nil && !isNil(s) {
+			enc.AddString(f.Key, s.String())
+		} else {
+			enc.AddString(f.Key, "nil")
+		}
 	case ErrorType:
 		encodeError(f.Key, f.Interface.(error), enc)
 	case SkipType:
@@ -199,3 +211,7 @@ func addFields(enc ObjectEncoder, fields []Field) {
 		fields[i].AddTo(enc)
 	}
 }
+
+func isNil(ti interface{}) bool {
+	return reflect.ValueOf(ti) == reflect.Zero(reflect.TypeOf(ti))
+}

zapcore/field_test.go

@@ -24,6 +24,7 @@ import (
 	"errors"
 	"fmt"
 	"math"
+	"net/url"
 	"testing"
 	"time"
 
@@ -229,3 +230,20 @@ func TestEquals(t *testing.T) {
 		assert.Equal(t, tt.want, tt.b.Equals(tt.a), "b.Equals(a) a: %#v b: %#v", tt.a, tt.b)
 	}
 }
+
+func TestFieldNilValue(t *testing.T) {
+	tests := []struct {
+		t FieldType
+		iface interface{}
+	}{
+		{StringerType, (*url.URL)(nil)},
+		{ArrayMarshalerType, (ArrayMarshaler)(nil)},
+		{ObjectMarshalerType, (ObjectMarshaler)(nil)},
+	}
+
+	for _, tt := range tests {
+		f := Field{Key: "k", Interface: tt.iface, Type: tt.t}
+		enc := NewMapObjectEncoder()
+		assert.NotPanics(t, func() { f.AddTo(enc) }, "Unexpected panic when adding fields with nil value.")
+	}
+}