Consider Visit::visit_key + Visit::visit_value about valuable

[carllerche]

wdyt @taiki-e ?

[carllerche]

@KodrAus can you provide an example of a case where the key & value are not available at the same time or when we would need this separation to bridge an API?

[KodrAus]

@carllerche Something I’ve had is when lazily parsing a value while only keeping a single token around at a time. That’s bespoke enough to reasonably suggest “not fit for purpose” for, but is something I’ve run into.

For bridging, we’ve currently got valuable -> serde implemented, but for the other way around, in order to generically bridge a SerializeMap into a Visit we can’t rely on serialize_entry, because its contract states you need to support keys and values independently

[taiki-e]

The initial idea of valuable-serde had ToSerde (Serializable) and FromSerde, and IIRC this was needed when implementing FromSerde.

In the end, only ToSerde (Serializable) was implemented, so it was deferred. However, I think this is one of the features needed to support "what serde supports", so I have no objection to adding this.