Combine "transparent" and "skip_serializing_if" tags

[dspicher]

I would like to have a transparent struct with a single field, whose serialization is skipped if some condition is fulfilled:

#[derive(serde::Serialize)]
#[serde(transparent)]
struct Foo {
    #[serde(skip_serializing_if = "String::is_empty")]
    inner: String,
}

However, if the transparent tag is included, the inner tag is ignored, i.e. this compiles:

#[derive(serde::Serialize)]
#[serde(transparent)]
struct Foo {
    #[serde(skip_serializing_if = "String::i_am_ignored")]
    inner: String,
}

Is this expected behaviour?

[dtolnay]

What would you want the serialized representation to be if skipped?

[dtolnay]

By which I mean, in the case of something like:

use serde::Serialize;

#[derive(Serialize)]
#[serde(transparent)]
struct Foo {
    #[serde(skip_serializing_if = "String::is_empty")]
    inner: String,
}

fn main() {
    let empty_foo = Foo { inner: String::new() };
    println!("{}", serde_json::to_string(&empty_foo).unwrap());
}

[dspicher]

Well, is there something like an empty serialization?

[dspicher]

To clarify the intent, I would like to use this on an optional field, not on the top-level serialization object.

[toxeus]

Related: #65 (comment)

[dtolnay]

There isn't something like an empty serialization. When a Serialize impl runs, the only way it can succeed is by serializing something.

The thing whose field this is can decide not to serialize that field, though.