Show "<nil>" for nil Stringer. (#854)

In encodeStringer, instead of returning an error when a panic
occurs when calling String() on a nil pointer, use the string value
"<nil>" like the fmt package does.

It is not always possible to handle this case by fixing the
implementation of String to not panic. This requires implementing
String with a pointer receiver, which doesn't work if you need to
be able to call String on non-addressable values.

zapcore/field.go

@@ -208,7 +208,12 @@ func addFields(enc ObjectEncoder, fields []Field) {
 func encodeStringer(key string, stringer interface{}, enc ObjectEncoder) (err error) {
 	defer func() {
 		if v := recover(); v != nil {
-			err = fmt.Errorf("PANIC=%v", v)
+			val := reflect.ValueOf(stringer)
+			if val.Kind() == reflect.Ptr && val.IsNil() {
+				enc.AddString(key, "<nil>")
+			} else {
+				err = fmt.Errorf("PANIC=%v", v)
+			}
 		}
 	}()
 

zapcore/field_test.go

@@ -102,7 +102,6 @@ func TestFieldAddingError(t *testing.T) {
 		{t: StringerType, iface: &obj{1}, want: empty, err: "PANIC=panic with string"},
 		{t: StringerType, iface: &obj{2}, want: empty, err: "PANIC=panic with error"},
 		{t: StringerType, iface: &obj{3}, want: empty, err: "PANIC=<nil>"},
-		{t: StringerType, iface: (*url.URL)(nil), want: empty, err: "PANIC=runtime error: invalid memory address or nil pointer dereference"},
 	}
 	for _, tt := range tests {
 		f := Field{Key: "k", Interface: tt.iface, Type: tt.t}
@@ -149,6 +148,8 @@ func TestFields(t *testing.T) {
 		{t: StringerType, iface: &obj{}, want: "obj"},
 		{t: StringerType, iface: (*obj)(nil), want: "nil obj"},
 		{t: SkipType, want: interface{}(nil)},
+		{t: StringerType, iface: (*url.URL)(nil), want: "<nil>"},
+		{t: StringerType, iface: (*users)(nil), want: "<nil>"},
 	}
 
 	for _, tt := range tests {