In the v1 implementation, unknown MessageSet items are stored in a
message's unknown fields section in non-MessageSet format. For example,
consider a MessageSet containing an item with type_id T and value V.
If the type_id is not resolvable, the item will be placed in the unknown
fields as a bytes-valued field with number T and contents V. This
conversion is then reversed when marshaling a MessageSet containing
unknown fields.
Preserve this behavior in v2.
One consequence of this change is that actual unknown fields in a
MessageSet (any field other than 1) are now discarded. This matches
the previous behavior.
Change-Id: I3d913613f84e0ae82481078dbc91cb25628651cc
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/205697
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
MessageSets are a deprecated proto1 feature, long since superseded by
extensions. Add disabled-by-default support behind flags.Proto1Legacy.
Change-Id: I7d3ace07f3b0efd59673034f3dc633b908345a88
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/185538
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
