chore: refactor mapping errors (#3843)

core/mapping/unmarshaler.go

@@ -250,8 +250,9 @@ func (u *Unmarshaler) fillSliceFromString(fieldType reflect.Type, value reflect.
 func (u *Unmarshaler) fillSliceValue(slice reflect.Value, index int,
 	baseKind reflect.Kind, value any, fullName string) error {
 	if value == nil {
-		return errUnsupportedType
+		return errNilSliceElement
 	}
+
 	ithVal := slice.Index(index)
 	switch v := value.(type) {
 	case fmt.Stringer:

core/mapping/unmarshaler_test.go

@@ -1340,7 +1340,7 @@ func TestUnmarshalNullableSlice(t *testing.T) {
 	}
 
 	ast := assert.New(t)
-	ast.Equal(UnmarshalKey(m, &v), errUnsupportedType)
+	ast.Equal(UnmarshalKey(m, &v), errNilSliceElement)
 }
 
 func TestUnmarshalIntSlice(t *testing.T) {

core/mapping/utils.go

@@ -36,6 +36,7 @@ const (
 var (
 	errUnsupportedType  = errors.New("unsupported type on setting field value")
 	errNumberRange      = errors.New("wrong number range setting")
+	errNilSliceElement  = errors.New("null element for slice")
 	optionsCache        = make(map[string]optionsCacheValue)
 	cacheLock           sync.RWMutex
 	structRequiredCache = make(map[reflect.Type]requiredCacheValue)