skip_serializing_if is a footgun

[khuey]

skip_serializing_if only works if the serialization format is self-describing. It does not work with e.g. bincode. This is a footgun in a codebase that uses more than one serialization format at different points, because I can add skip_serializing_if to something that gets fed into the JSON serializer and it works fine, but when it gets fed into the bincode serializer it produces garbage that will fail to deserialize.

I think the solution here is for Serializers to declare whether or not they support skipping things and then make skip_serializing_if only have an effect for serializers that can tolerate it.

[dtolnay]

I think I would prefer not to build an additional toggle into the Serializer API for this.

This is a subissue of a limitation that data structures which need to follow a different codepath for serializing to different formats are not well supported by serde.

[sffc]

The lack of this feature causes problems when building serializable data structures to work with an arbitrary serializer. We have to feature-gate our code to disable skip_serializing_if on the "bincode" feature, which is really awkward.

[Manishearth]

What if skip_field were still passed the field so that the serializer can decide to serialize it anyway?

[Manishearth]

Filed #2012 for that idea

[khuey]

This continues to be a source of woe.