Cleanup the generated logic by having the implementation be backed
by protoimpl rather that directly generated.
Weak fields are a deprecated feature of protobufs and
have entirely be superceded by extensions.
Unfortunately, there are still some usages of it.
Change-Id: Ie1a4b7da253e2ccf5e56627775d9b2fb4090d59a
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/229717
Reviewed-by: Damien Neil <dneil@google.com>
Fix a reversed error check in impl.Export{}.WeakNil.
Check to see if we have a type for the weak field on marshal/size.
Treat a typed nil valued in XXX_Weak as not indicating presence for
the field.
Change-Id: Id667ac7eb4f53236be9e181017082bd8cd21d115
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/198717
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>