Reporter should be more aggressive about pruning output

[dsnet]

The following snippet hangs:

mx := &descriptorpb.FileDescriptorProto{Name: proto.String("descriptor.proto")}
my := &descriptorpb.FileDescriptorProto{Name: proto.String("descriptor.proto")}

x := map[*descriptorpb.FileDescriptorProto]int{mx: 1}
y := map[*descriptorpb.FileDescriptorProto]int{my: 1}

cmp.Diff(x, y)

These two objects are inequal since map keys use == and not a recursive application of cmp.Equal. It hangs because cmp.Diff is trying to format the raw data structure for a descriptorpb.FileDescriptorProto. Since descriptorpb.FileDescriptorProto has a pointer to the protobuf type information, which itself closes over the transitive set of reachable protobuf types, this is a massive data structure. The reporter logic should be more aggressive about pruning the formatted value.

[dsnet]

The issue is that formatMapKey does not impose any verbosity limits:

go-cmp/cmp/report_reflect.go

Lines 347 to 356 in ade6b74

	func formatMapKey(v reflect.Value, disambiguate bool, ptrs *pointerReferences) string {
	var opts formatOptions
	opts.DiffMode = diffIdentical
	opts.TypeMode = elideType
	opts.PrintAddresses = disambiguate
	opts.AvoidStringer = disambiguate
	opts.QualifiedNames = disambiguate
	s := opts.FormatValue(v, reflect.Map, ptrs).String()
	return strings.TrimSpace(s)
	}

The fix is to simply impose a sensible verbosity limit.