Always return ValidationErrors, expose failing index

This way callers can always expect ValidationErrors, and in case
of slice validation, they can also get indexes of the failing elements.

binding/default_validator.go

@@ -7,7 +7,6 @@ package binding
 import (
 	"fmt"
 	"reflect"
-	"strings"
 	"sync"
 
 	"github.com/go-playground/validator/v10"
@@ -18,34 +17,33 @@ type defaultValidator struct {
 	validate *validator.Validate
 }
 
-type sliceValidateError []error
+// SliceFieldError is returned for invalid slice or array elements.
+// It extends validator.FieldError with the index of the failing element.
+type SliceFieldError interface {
+	validator.FieldError
+	Index() int
+}
 
-// Error concatenates all error elements in sliceValidateError into a single string separated by \n.
-func (err sliceValidateError) Error() string {
-	n := len(err)
-	switch n {
-	case 0:
-		return ""
-	default:
-		var b strings.Builder
-		if err[0] != nil {
-			fmt.Fprintf(&b, "[%d]: %s", 0, err[0].Error())
-		}
-		if n > 1 {
-			for i := 1; i < n; i++ {
-				if err[i] != nil {
-					b.WriteString("\n")
-					fmt.Fprintf(&b, "[%d]: %s", i, err[i].Error())
-				}
-			}
-		}
-		return b.String()
-	}
+type sliceFieldError struct {
+	validator.FieldError
+	index int
+}
+
+func (fe sliceFieldError) Index() int {
+	return fe.index
+}
+
+func (fe sliceFieldError) Error() string {
+	return fmt.Sprintf("[%d]: %s", fe.index, fe.FieldError.Error())
+}
+
+func (fe sliceFieldError) Unwrap() error {
+	return fe.FieldError
 }
 
 var _ StructValidator = &defaultValidator{}
 
-// ValidateStruct receives any kind of type, but only performed struct or pointer to struct type.
+// ValidateStruct receives any kind of type, but validates only structs, pointers, slices, and arrays.
 func (v *defaultValidator) ValidateStruct(obj interface{}) error {
 	if obj == nil {
 		return nil
@@ -59,16 +57,18 @@ func (v *defaultValidator) ValidateStruct(obj interface{}) error {
 		return v.validateStruct(obj)
 	case reflect.Slice, reflect.Array:
 		count := value.Len()
-		validateRet := make(sliceValidateError, 0)
+		var errs validator.ValidationErrors
 		for i := 0; i < count; i++ {
 			if err := v.ValidateStruct(value.Index(i).Interface()); err != nil {
-				validateRet = append(validateRet, err)
+				for _, fieldError := range err.(validator.ValidationErrors) { // nolint: errorlint
+					errs = append(errs, sliceFieldError{fieldError, i})
 				}
 			}
-		if len(validateRet) == 0 {
+		}
+		if len(errs) > 0 {
+			return errs
+		}
 		return nil
-		}
-		return validateRet
 	default:
 		return nil
 	}

binding/default_validator_benchmark_test.go

@@ -1,20 +0,0 @@
-package binding
-
-import (
-	"errors"
-	"strconv"
-	"testing"
-)
-
-func BenchmarkSliceValidateError(b *testing.B) {
-	const size int = 100
-	for i := 0; i < b.N; i++ {
-		e := make(sliceValidateError, size)
-		for j := 0; j < size; j++ {
-			e[j] = errors.New(strconv.Itoa(j))
-		}
-		if len(e.Error()) == 0 {
-			b.Errorf("error")
-		}
-	}
-}

binding/default_validator_test.go

@@ -7,43 +7,27 @@ package binding
 import (
 	"errors"
 	"testing"
+
+	"github.com/go-playground/validator/v10"
+	"github.com/stretchr/testify/assert"
 )
 
-func TestSliceValidateError(t *testing.T) {
-	tests := []struct {
-		name string
-		err  sliceValidateError
-		want string
-	}{
-		{"has nil elements", sliceValidateError{errors.New("test error"), nil}, "[0]: test error"},
-		{"has zero elements", sliceValidateError{}, ""},
-		{"has one element", sliceValidateError{errors.New("test one error")}, "[0]: test one error"},
-		{"has two elements",
-			sliceValidateError{
-				errors.New("first error"),
-				errors.New("second error"),
-			},
-			"[0]: first error\n[1]: second error",
-		},
-		{"has many elements",
-			sliceValidateError{
-				errors.New("first error"),
-				errors.New("second error"),
-				nil,
-				nil,
-				nil,
-				errors.New("last error"),
-			},
-			"[0]: first error\n[1]: second error\n[5]: last error",
-		},
-	}
-	for _, tt := range tests {
-		t.Run(tt.name, func(t *testing.T) {
-			if got := tt.err.Error(); got != tt.want {
-				t.Errorf("sliceValidateError.Error() = %v, want %v", got, tt.want)
-			}
-		})
-	}
+func TestSliceFieldError(t *testing.T) {
+	var fe validator.FieldError = dummyFieldError{msg: "test error"}
+
+	var err SliceFieldError = sliceFieldError{fe, 10}
+	assert.Equal(t, 10, err.Index())
+	assert.Equal(t, "[10]: test error", err.Error())
+	assert.Equal(t, fe, errors.Unwrap(err))
+}
+
+type dummyFieldError struct {
+	validator.FieldError
+	msg string
+}
+
+func (fe dummyFieldError) Error() string {
+	return fe.msg
 }
 
 func TestDefaultValidator(t *testing.T) {