Add support for pkg/errors

Users who take the time (and performance hit) of using Dave Cheney's errors
package should get rich, annotated errors and stacktraces in their logs.
However, that package is only one of a number of similar packages hasn't yet
committed to a stable API, so we don't want to take a direct dependency on it.

To support these sorts of packages, serialize errors that implement
`fmt.Formatter` differently from those that don't.

zapcore/field.go

@@ -151,7 +151,14 @@ func (f Field) AddTo(enc ObjectEncoder) {
 	case StringerType:
 		enc.AddString(f.Key, f.Interface.(fmt.Stringer).String())
 	case ErrorType:
-		enc.AddString(f.Key, f.Interface.(error).Error())
+		val := f.Interface.(error)
+		if fancy, ok := val.(fmt.Formatter); ok {
+			// This is a rich error type, like those produced by
+			// github.com/pkg/errors.
+			enc.AddString(f.Key, fmt.Sprintf("%+v", fancy))
+		} else {
+			enc.AddString(f.Key, val.Error())
+		}
 	case SkipType:
 		break
 	default:

zapcore/field_test.go

@@ -27,6 +27,7 @@ import (
 	"testing"
 	"time"
 
+	richErrors "github.com/pkg/errors"
 	"github.com/stretchr/testify/assert"
 
 	. "go.uber.org/zap/zapcore"
@@ -127,3 +128,19 @@ func TestFields(t *testing.T) {
 		assert.Equal(t, tt.want, enc.Fields["k"], "Unexpected output from field %+v.", f)
 	}
 }
+
+func TestRichErrorSupport(t *testing.T) {
+	f := Field{
+		Type:      ErrorType,
+		Interface: richErrors.WithMessage(richErrors.New("egad"), "failed"),
+		Key:       "k",
+	}
+	enc := NewMapObjectEncoder()
+	f.AddTo(enc)
+	serialized := enc.Fields["k"]
+	// Don't assert the exact format used by a third-party package, but ensure
+	// that some critical elements are present.
+	assert.Regexp(t, `egad`, serialized, "Expected original error message to be present.")
+	assert.Regexp(t, `failed`, serialized, "Expected error annotation to be present.")
+	assert.Regexp(t, `TestRichErrorSupport`, serialized, "Expected calling function to be present in stacktrace.")
+}