reflect/protodesc: descriptors that use legacy features (message set wire format, weak imports and weak fields) cannot be round-tripped to protos and back

What version of protobuf and what language are you using? v1.25.0

What did you do? Example proto file tmp.proto

syntax = "proto2";

option go_package = "main";

message Foo {
  option message_set_wire_format = true;

  extensions 100 to max;
}

We can successfully generate Go code for this:

protoc --go_out=. tmp.proto

And the resulting code can successfully retrieve the descriptor:

package main

import "github.com/golang/protobuf/proto"
import "fmt"

func main() {
  r := proto.MessageReflect(&Foo{})
  fmt.Println(r.Descriptor().FullName())
}

However, if we try to convert the descriptor to a proto and then create a descriptor from that (which would also happen if, for example, we loaded the descriptor from a protoset produced by protoc or from a gRPC server via server reflection), we get an error:

package main

import "github.com/golang/protobuf/proto"
import "google.golang.org/protobuf/reflect/protodesc"
import "google.golang.org/protobuf/reflect/protoregistry"
import "fmt"

func main() {
  r := proto.MessageReflect(&Foo{})
  fmt.Println(r.Descriptor().FullName())
  pr := protodesc.ToFileDescriptorProto(r.Descriptor().ParentFile())
  var reg protoregistry.Files
  fd, err := protodesc.NewFile(pr, &reg)
  if err != nil {
    panic(err)
  }
  fmt.Println(fd.Path())
}

What did you expect to see? The second, longer program succeeds and prints "tmp.proto".

What did you see instead? Running the new program actually fails:

panic: proto: message "Foo" is a MessageSet, which is a legacy proto1 feature that is no longer supported

I can understand an error like that if we tried to use such a message, to serialize or de-serialize. But it seems really weird that we can't even create the descriptor, especially since we can successfully create the descriptor if it is packaged in the protoc-gen-go-generated code.