If any messages do not have a subfilter then don't introspect them.

copy.go

@@ -113,6 +113,12 @@ func structToStruct(filter FieldFilter, src, dst *reflect.Value, userOptions *op
 				return errors.Errorf("dst type is %s, expected: %s ", dst.Type(), "*any.Any")
 			}
 
+			// If subfilter is empty then copy the entire any without any unmarshalling.
+			if filter.IsEmpty() && !userOptions.UnmarshalAllAny {
+				dst.Set(*src)
+				break
+			}
+
 			srcProto, err := srcAny.UnmarshalNew()
 			if err != nil {
 				return errors.WithStack(err)
@@ -250,6 +256,12 @@ type options struct {
 	// It is called before copying the data from source to destination allowing custom processing.
 	// If the visitor function returns true the visited field is skipped.
 	MapVisitor mapVisitor
+
+	// UnmarshalAllAny is used to indicate unmarshal all any fields. Default to true to keep backward compatibility.
+	//
+	// If an any field is encountered and this flag is not set, it will only Unmarshal it if there is a subfilter for that field.
+	// If set it will always Unmarshal all any fields
+	UnmarshalAllAny bool
 }
 
 // mapVisitor is called for every filtered field in structToMap.
@@ -293,9 +305,18 @@ func WithMapVisitor(visitor mapVisitor) Option {
 	}
 }
 
+func WithUnmarshalAllAny(unmarshal bool) Option {
+	return func(o *options) {
+		o.UnmarshalAllAny = unmarshal
+	}
+}
+
 func newDefaultOptions() *options {
 	// set default CopyListSize is func which return src.Len()
-	return &options{CopyListSize: func(src *reflect.Value) int { return src.Len() }}
+	return &options{
+		CopyListSize:    func(src *reflect.Value) int { return src.Len() },
+		UnmarshalAllAny: true,
+	}
 }
 
 // fieldName gets the field name according to the field's tag, or gets StructField.Name default when the field's tag is empty.
@@ -334,7 +355,7 @@ func structToMap(filter FieldFilter, src, dst reflect.Value, userOptions *option
 		}
 		srcType := src.Type()
 		for i := 0; i < src.NumField(); i++ {
-			srcName  := fieldName(userOptions.SrcTag, srcType.Field(i))
+			srcName := fieldName(userOptions.SrcTag, srcType.Field(i))
 			if !isExported(srcType.Field(i)) {
 				// Unexported fields can not be copied.
 				continue

copy_proto_test.go

@@ -277,6 +277,43 @@ func TestStructToStruct_NonProtoFail(t *testing.T) {
 	assert.NotNil(t, err)
 }
 
+func TestStructToStruct_UnknownAny(t *testing.T) {
+	getUsers := func() (*testproto.User, *testproto.User) {
+		user1 := &testproto.User{
+			Details: []*anypb.Any{
+				{
+					TypeUrl: "example.com/example/UnknownType",
+					Value:   []byte("unknown"),
+				},
+			},
+		}
+		user2 := &testproto.User{}
+		return user1, user2
+	}
+
+	userWithUnknown, emptyUser := getUsers()
+
+	mask := fieldmask_utils.MaskFromString("Details")
+
+	err := fieldmask_utils.StructToStruct(mask, userWithUnknown, emptyUser, fieldmask_utils.WithUnmarshalAllAny(false))
+	assert.NoError(t, err)
+	assert.Equal(t, userWithUnknown.Details, emptyUser.Details)
+
+	userWithUnknown, emptyUser = getUsers()
+
+	err = fieldmask_utils.StructToStruct(mask, emptyUser, userWithUnknown, fieldmask_utils.WithUnmarshalAllAny(false))
+	assert.NoError(t, err)
+	assert.Equal(t, userWithUnknown.Details, emptyUser.Details)
+
+	userWithUnknown, emptyUser = getUsers()
+
+	err = fieldmask_utils.StructToStruct(mask, userWithUnknown, emptyUser)
+	assert.Equal(t, "proto:\u00a0not found", err.Error())
+
+	mask = fieldmask_utils.MaskFromString("Details{Id}")
+	err = fieldmask_utils.StructToStruct(mask, userWithUnknown, emptyUser, fieldmask_utils.WithUnmarshalAllAny(false))
+	assert.Equal(t, "proto:\u00a0not found", err.Error())
+}
 func TestStructToMap_Success(t *testing.T) {
 	userDst := make(map[string]interface{})
 	mask := fieldmask_utils.MaskFromString(