Handle skip_serializing_if

[Manishearth]

#[derive(serde::Serialize, serde::Deserialize, Debug, Clone)]
struct Foo {
    #[serde(skip_serializing_if = "test_fn")]
    foo: Option<u8>,
}

fn test_fn(x: &Option<u8>) -> bool { x.is_none() }

The above code will panic because of serde-rs/serde#1732. I do have a proposal to fix it on the serde side (serde-rs/serde#2012), but I haven't gotten confirmation they want it yet.

A way to fix this on the bincode side would be to start calling serialize_len for structs as well. This would be a breaking change on the format; I'm not sure if that's possible in bincode.

It would also impact the size of every bincode'd struct, which might not be desired.

[VictorKoenders]

I don't think we'd want this as a default feature, as it will be a breaking change in the binary format of existing data.

However we could add this to the Options trait (e.g. .allow_skipping_empty_fields()), so people can opt-in to this functionality.

[Manishearth]

Oh that would be neat.

[TedDriggs]

This would definitely have helped me recently; I have a large struct that was originally made for JSON, so I use skip_serializing_if liberally in it. To add bincode support, I had to copy/paste the struct without those attributes, and then added some unit tests that will (hopefully) fail if the two get out of sync.

Could bincode's default behavior detect when someone has used this and recommend they enable the option? If, say, the struct len was greater than the number of fields actually seen, error out with a message "If you are using skip_serializing_if, enable allow_skipping_empty_fields in your Options"?

[ZoeyR]

I don't think its possible to handle skip_serializing_if in bincode. Imagine a situation like this:

#[derive(serde::Serialize, serde::Deserialize, Debug, Clone)]
struct Foo {
    #[serde(skip_serializing_if = "test_fn")]
    foo: Option<u8>,
    #[serde(skip_serializing_if = "test_fn")]
    bar: Option<u8>,
}

fn test_fn(x: &Option<u8>) -> bool { x.is_none() }

If only one of the fields gets skipped, on deserialize we have no way to know which field was skipped.