feat: ignore change in array-encoded to struct

decode.go

@@ -16,6 +16,7 @@ import (
 const (
 	looseInterfaceDecodingFlag uint32 = 1 << iota
 	disallowUnknownFieldsFlag
+	ignoreMismatchedArrayFieldsFlag
 )
 
 const (
@@ -135,6 +136,16 @@ func (d *Decoder) UseLooseInterfaceDecoding(on bool) {
 	}
 }
 
+// IgnoreMismatchedArrayFields causes the Decoder to ignore mismatch fields number in the
+// array and struct.
+func (d *Decoder) IgnoreMismatchedArrayFields(on bool) {
+	if on {
+		d.flags |= ignoreMismatchedArrayFieldsFlag
+	} else {
+		d.flags &= ^ignoreMismatchedArrayFieldsFlag
+	}
+}
+
 // SetCustomStructTag causes the decoder to use the supplied tag as a fallback option
 // if there is no msgpack tag.
 func (d *Decoder) SetCustomStructTag(tag string) {

decode_map.go

@@ -292,10 +292,14 @@ func decodeStructValue(d *Decoder, v reflect.Value) error {
 		v.Set(reflect.Zero(v.Type()))
 		return nil
 	}
-
 	fields := structs.Fields(v.Type(), d.structTag)
 	if n != len(fields.List) {
-		return errArrayStruct
+		if d.flags&ignoreMismatchedArrayFieldsFlag == 0 {
+			return errArrayStruct
+		}
+		if n < len(fields.List) {
+			fields.List = fields.List[:n]
+		}
 	}
 
 	for _, f := range fields.List {

example_test.go

@@ -141,6 +141,70 @@ func ExampleEncoder_UseArrayEncodedStructs() {
 	// Output: [foo bar]
 }
 
+func ExampleDecoder_IgnoreMismatchedArrayFields() {
+	type Item struct {
+		Foo string
+		Bar string
+	}
+	type ItemLong struct {
+		Foo string
+		Bar string
+		Baz string
+	}
+
+	var buf bytes.Buffer
+
+	enc := msgpack.NewEncoder(&buf)
+	enc.UseArrayEncodedStructs(true)
+
+	err := enc.Encode(&Item{Foo: "foo", Bar: "bar"})
+	if err != nil {
+		panic(err)
+	}
+
+	dec := msgpack.NewDecoder(&buf)
+	dec.IgnoreMismatchedArrayFields(true)
+	var itemLong ItemLong
+	err = dec.Decode(&itemLong)
+	if err != nil {
+		panic(err)
+	}
+	fmt.Println(itemLong)
+	// Output: {foo bar }
+}
+
+func ExampleDecoder_IgnoreMismatchedArrayFields2() {
+	type Item struct {
+		Foo string
+		Bar string
+	}
+	type ItemLong struct {
+		Foo string
+		Bar string
+		Baz string
+	}
+
+	var buf bytes.Buffer
+
+	enc := msgpack.NewEncoder(&buf)
+	enc.UseArrayEncodedStructs(true)
+
+	err := enc.Encode(&ItemLong{Foo: "foo", Bar: "bar", Baz: "baz"})
+	if err != nil {
+		panic(err)
+	}
+
+	dec := msgpack.NewDecoder(&buf)
+	dec.IgnoreMismatchedArrayFields(true)
+	var item Item
+	err = dec.Decode(&item)
+	if err != nil {
+		panic(err)
+	}
+	fmt.Println(item)
+	// Output: {foo bar}
+}
+
 func ExampleMarshal_asArray() {
 	type Item struct {
 		_msgpack struct{} `msgpack:",as_array"`