You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: decode.go
    path of file: ./repos/plik/vendor/google.golang.org/protobuf/proto
    
    the code of the file until where you have to start completion: // Copyright 2018 The Go Authors. All rights reserved.
// Use of this source code is gove
    
func (o UnmarshalOptions) unmarshalMessageSlow(b []byte, m protoreflect.Message) error {
	md := m.Descriptor()
	if messageset.IsMessageSet(md) {
		return o.unmarshalMessageSet(b, m)
	}
	fields := md.Fields()
	for len(b) > 0 {
		// Parse the tag (field number and wire type).
		num, wtyp, tagLen := protowire.ConsumeTag(b)
		if tagLen < 0 {
			return errDecode
		}
		if num > protowire.MaxValidNumber {
			return errDecode
		}

		// Find the field descriptor for this field number.
		fd := fields.ByNumber(num)
		if fd == nil && md.ExtensionRanges().Has(num) {
			extType, err := o.Resolver.FindExtensionByNumber(md.FullName(), num)
			if err != nil && err != protoregistry.NotFound {
				return errors.New("%v: unable to resolve extension %v: %v", md.FullName(), num, err)
			}
			if extType != nil {
				fd = extType.TypeDescriptor()
			}
		}
		var err error
		if fd == nil {
			err = errUnknown
		} else if flags.ProtoLegacy {
			if fd.IsWeak() && fd.Message().IsPlaceholder() {
				err = errUnknown // weak referent is not linked in
			}
		}

		// Parse the field value.
		var valLen int
		switch {
		case err != nil:
		case fd.IsList():
			valLen, err = o.unmarshalList(b[tagLen:], wtyp, m.Mutable(fd).List(), fd)
		case fd.IsMap():
			valLen, err = o.unmarshalMap(b[tagLen:], wtyp, m.Mutable(fd).Map(), fd)
		default:
			valLen, err = o.unmarshalSingular(b[tagLen:], wtyp, m, fd)
		}
		if err != nil {
			if err != errUnknown {
				return err
			}
			valLen = protowire.ConsumeFieldValue(num, wtyp, b[tagLen:])
			if valLen < 0 {
				return errDecode
			}
			if !o.DiscardUnknown {
				m.SetUnknown(append(m.GetUnknown(), b[:tagLen+valLen]...))
			}
		}
		b = b[tagLen+valLen:]
	}
	return nil
}