feat: introduce binding.FieldError type to tell the erroneous field

Signed-off-by: Maxime Soulé <btik-git@scoubidou.com>

binding/form_mapping.go

@@ -26,6 +26,29 @@ var (
 	ErrConvertToMapString = errors.New("can not convert to map of strings")
 )
 
+type FieldError struct {
+	Field string
+	Err   error
+}
+
+func NewFieldError(field string, err error) error {
+	if err == nil {
+		return nil
+	}
+	return &FieldError{
+		Field: field,
+		Err:   err,
+	}
+}
+
+func (e *FieldError) Error() string {
+	return fmt.Sprintf("field %q error: %s", e.Field, e.Err)
+}
+
+func (e *FieldError) Unwrap() error {
+	return e.Err
+}
+
 func mapURI(ptr any, m map[string][]string) error {
 	return mapFormByTag(ptr, m, "uri")
 }
@@ -161,7 +184,11 @@ func tryToSetValue(value reflect.Value, field reflect.StructField, setter setter
 		}
 	}
 
-	return setter.TrySet(value, field, tagValue, setOpt)
+	ok, err := setter.TrySet(value, field, tagValue, setOpt)
+	if err != nil {
+		return ok, NewFieldError(tagValue, err)
+	}
+	return ok, nil
 }
 
 func setByForm(value reflect.Value, field reflect.StructField, form map[string][]string, tagValue string, opt setOptions) (isSet bool, err error) {

binding/form_mapping_test.go

@@ -5,6 +5,7 @@
 package binding
 
 import (
+	"errors"
 	"reflect"
 	"testing"
 	"time"
@@ -124,7 +125,7 @@ func TestMappingUnknownFieldType(t *testing.T) {
 
 	err := mappingByPtr(&s, formSource{"U": {"unknown"}}, "form")
 	assert.Error(t, err)
-	assert.Equal(t, errUnknownType, err)
+	assert.Equal(t, &FieldError{"U", errUnknownType}, err)
 }
 
 func TestMappingURI(t *testing.T) {
@@ -288,3 +289,20 @@ func TestMappingIgnoredCircularRef(t *testing.T) {
 	err := mappingByPtr(&s, formSource{}, "form")
 	assert.NoError(t, err)
 }
+
+func TestNewField(t *testing.T) {
+	assert.NoError(t, NewFieldError("foo", nil))
+
+	orig := errors.New("bad value")
+	err := NewFieldError("foo", orig)
+	if assert.Error(t, err) {
+		assert.Equal(t, `field "foo" error: bad value`, err.Error())
+
+		if fieldErr, ok := err.(*FieldError); assert.True(t, ok) { //nolint: errorlint
+			assert.Equal(t, fieldErr.Field, "foo")
+			assert.Equal(t, fieldErr.Err, orig)
+		}
+
+		assert.Equal(t, orig, errors.Unwrap(err))
+	}
+}