add support to tag validateFn

README.md

@@ -262,6 +262,8 @@ validate := validator.New(validator.WithRequiredStructEnabled())
 | excluded_without | Excluded Without |
 | excluded_without_all | Excluded Without All |
 | unique | Unique |
+| validateFn | Verify if the method `Validate() error` does not return an error |
+
 
 #### Aliases:
 | Tag | Description |

baked_in.go

@@ -244,6 +244,7 @@ var (
 		"cron":                          isCron,
 		"spicedb":                       isSpiceDB,
 		"ein":                           isEIN,
+		"validateFn":                    isValidateFn,
 	}
 )
 
@@ -3046,3 +3047,29 @@ func isEIN(fl FieldLevel) bool {
 
 	return einRegex().MatchString(field.String())
 }
+
+func isValidateFn(fl FieldLevel) bool {
+	if instance, ok := tryConvertFieldTo[interface{ Validate() error }](fl.Field()); ok {
+		return instance.Validate() == nil
+	}
+
+	return false
+}
+
+func tryConvertFieldTo[V any](field reflect.Value) (v V, ok bool) {
+	v, ok = convertFieldTo[V](field)
+	if !ok && field.CanAddr() {
+		v, ok = convertFieldTo[V](field.Addr())
+	}
+
+	return v, ok
+}
+
+func convertFieldTo[V any](field reflect.Value) (v V, ok bool) {
+	if v, ok = field.Interface().(V); ok {
+		return v, ok
+	}
+
+	var zero V
+	return zero, false
+}

doc.go

@@ -756,6 +756,13 @@ in a field of the struct specified via a parameter.
 	// For slices of struct:
 	Usage: unique=field
 
+# ValidateFn
+
+This validates that an object respects the interface `Validate() error` and
+the method `Validate` does not return an error.
+
+	Usage: validateFn
+
 # Alpha Only
 
 This validates that a string value contains ASCII alpha characters only

validator_test.go

@@ -7,6 +7,7 @@
 	"database/sql/driver"
 	"encoding/base64"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"image"
 	"image/jpeg"
@@ -14148,3 +14149,78 @@
 		Equal(t, len(errs), tc.errorNum)
 	}
 }
+
+type NotRed struct {
+	Color string
+}
+
+func (r *NotRed) Validate() error {
+	if r != nil && r.Color == "red" {
+		return errors.New("should not be red")
+	}
+
+	return nil
+}
+
+func TestIsValid(t *testing.T) {
+	t.Run("using pointer", func(t *testing.T) {
+		validate := New()
+
+		type Test struct {
+			String string
+			Inner  *NotRed `validate:"validateFn"`
+		}
+
+		var tt Test
+
+		errs := validate.Struct(tt)
+		NotEqual(t, errs, nil)
+
+		fe := errs.(ValidationErrors)[0]
+		Equal(t, fe.Field(), "Inner")
+		Equal(t, fe.Namespace(), "Test.Inner")
+		Equal(t, fe.Tag(), "validateFn")
+
+		tt.Inner = &NotRed{Color: "blue"}
+		errs = validate.Struct(tt)
+		Equal(t, errs, nil)
+
+		tt.Inner = &NotRed{Color: "red"}
+		errs = validate.Struct(tt)
+		NotEqual(t, errs, nil)
+
+		fe = errs.(ValidationErrors)[0]
+		Equal(t, fe.Field(), "Inner")
+		Equal(t, fe.Namespace(), "Test.Inner")
+		Equal(t, fe.Tag(), "validateFn")
+
+	})
+
+	t.Run("using struct", func(t *testing.T) {
+		validate := New()
+
+		type Test2 struct {
+			String string
+			Inner  NotRed `validate:"validateFn"`
+		}
+
+		var tt2 Test2
+
+		errs := validate.Struct(&tt2)
+		Equal(t, errs, nil)
+
+		tt2.Inner = NotRed{Color: "blue"}
+
+		errs = validate.Struct(&tt2)
+		Equal(t, errs, nil)
+
+		tt2.Inner = NotRed{Color: "red"}
+		errs = validate.Struct(&tt2)
+		NotEqual(t, errs, nil)
+
+		fe := errs.(ValidationErrors)[0]
+		Equal(t, fe.Field(), "Inner")
+		Equal(t, fe.Namespace(), "Test2.Inner")
+		Equal(t, fe.Tag(), "validateFn")
+	})
+}