go-cmp should ignore generated proto fields

[aysylu]

XXX_cachesize auto-generated proto fields are not currently ignored by go-cmp. Example from Grafeas:

storage_test.go:287: GetOccurrence returned diff (want -> got):
            {*grafeas_go_proto.Occurrence}.Resource.XXX_sizecache:
            	-: 0
            	+: 16
            {*grafeas_go_proto.Occurrence}.Details.Vulnerability.PackageIssue[0].AffectedLocation.Version.XXX_sizecache:
            	-: 0
            	+: 16
            {*grafeas_go_proto.Occurrence}.Details.Vulnerability.PackageIssue[0].AffectedLocation.XXX_sizecache:
            	-: 0
            	+: 53
            {*grafeas_go_proto.Occurrence}.Details.Vulnerability.PackageIssue[0].FixedLocation.Version.XXX_sizecache:
            	-: 0
            	+: 16
            {*grafeas_go_proto.Occurrence}.Details.Vulnerability.PackageIssue[0].FixedLocation.XXX_sizecache:
            	-: 0
            	+: 53
            {*grafeas_go_proto.Occurrence}.Details.Vulnerability.PackageIssue[0].XXX_sizecache:
            	-: 0
            	+: 116
            {*grafeas_go_proto.Occurrence}.Details.Vulnerability.XXX_sizecache:
            	-: 0
            	+: 125
            {*grafeas_go_proto.Occurrence}.XXX_sizecache:
            	-: 0
            	+: 307

[dsnet]

It doesn't scale if cmp is taught the special case for every possible Go type that exists out there.

What you want is golang/protobuf#762

[aysylu]

Thanks for referencing the relevant issue! Just to clarify: XXX_cachesize isn't about Go types, but about proto fields. The Grafeas type here is just used for example, it's unrelated to the Grafeas itself.

[dsnet]

My point is that the presence of XXX_cachesize is an artifact of protobuf message implementations. cmp should not have any knowledge about these special cases.