`#[derive(uniffi::Record)]` should support `#[uniffi::skip]` attribute to skip fields

[oriongonza]

Right now the only way to pass the struct through uniffi with different fields is to create a "close but not quite" struct with the missing fields.

Serde has lots of custom attributes to control deserialization, uniffi could support a few of those. #[uniffi::skip] is almost trivial to implement.

This doesn't make much sense for Object or Enum, though.

[bendk]

How would this work when the record was passed back from the foreign language -> Rust? At that point, we'd need to fill in some value for the field. This is kind of similar to the situation we found ourselves in with "flat" errors.

[oriongonza]

Its Default value sounds like the reasonable approach.

You might say "but it doesn't look like a good idea to do that, from a type safety POV", but that's up to the library users to decide.
In my opinion this should be used for Rust -> FFI, not backwards but that's an app level decision.

[bendk]

Its Default value sounds like the reasonable approach.
In my opinion this should be used for Rust -> FFI, not backwards

Those 2 approaches are not exactly compatible though. If we go with the default approach, that means we need to generate a code to lift the types, using field_name: Default::default() for any missing fields. This means that things will fail to compile if the field doesn't implement default -- even if you only pass the value one way across the FFI.

To me, the second option seems more useful -- I want to define a struct that is only passed from Rust -> the foreign language and I want to skip certain fields.

The first option would also be possible, but it seems kind of confusing given that we already have a #[uniffi::default] attribute that works kind-of like that but it's subtly different.