Don't panic with 'omitempty' and uncomparable type

Fixes #360

encode.go

@@ -261,7 +261,7 @@ func (enc *Encoder) eElement(rv reflect.Value) {
 			enc.eElement(reflect.ValueOf(v))
 			return
 		}
-		encPanic(errors.New(fmt.Sprintf("Unable to convert \"%s\" to neither int64 nor float64", n)))
+		encPanic(fmt.Errorf("unable to convert %q to int64 or float64", n))
 	}
 
 	switch rv.Kind() {
@@ -504,7 +504,8 @@ func (enc *Encoder) eStruct(key Key, rv reflect.Value, inline bool) {
 			if opts.name != "" {
 				keyName = opts.name
 			}
-			if opts.omitempty && isEmpty(fieldVal) {
+
+			if opts.omitempty && enc.isEmpty(fieldVal) {
 				continue
 			}
 			if opts.omitzero && isZero(fieldVal) {
@@ -648,11 +649,14 @@ func isZero(rv reflect.Value) bool {
 	return false
 }
 
-func isEmpty(rv reflect.Value) bool {
+func (enc *Encoder) isEmpty(rv reflect.Value) bool {
 	switch rv.Kind() {
 	case reflect.Array, reflect.Slice, reflect.Map, reflect.String:
 		return rv.Len() == 0
 	case reflect.Struct:
+		if !rv.Type().Comparable() {
+			encPanic(fmt.Errorf("type %q cannot be used with omitempty as it's uncomparable", rv.Type()))
+		}
 		return reflect.Zero(rv.Type()).Interface() == rv.Interface()
 	case reflect.Bool:
 		return !rv.Bool()

encode_test.go

@@ -212,6 +212,38 @@ time = 1985-06-18T15:16:17Z
 		v, expected, nil)
 }
 
+func TestEncodeWithOmitEmptyError(t *testing.T) {
+	type nest struct {
+		Field []string `toml:"Field,omitempty"`
+	}
+
+	tests := []struct {
+		in      interface{}
+		wantErr string
+	}{
+		{ // Make sure it doesn't panic on uncomparable types; #360
+			struct {
+				Values nest `toml:"values,omitempty"`
+				Empty  nest `toml:"empty,omitempty"`
+			}{Values: nest{[]string{"XXX"}}},
+			"cannot be used with omitempty as it's uncomparable",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run("", func(t *testing.T) {
+			buf := new(bytes.Buffer)
+			err := NewEncoder(buf).Encode(tt.in)
+			if !errorContains(err, tt.wantErr) {
+				t.Fatalf("wrong error: %v", err)
+			}
+			if buf.String() != "" {
+				t.Errorf("output not empty:\n%s", buf)
+			}
+		})
+	}
+}
+
 func TestEncodeWithOmitZero(t *testing.T) {
 	type simple struct {
 		Number   int     `toml:"number,omitzero"`