Fields of type `Option<Vec<T>>` or `Option<HashMap<K, V>>` are not serialized if they contain an empty collection

[nayuta723]

There is a serialization inconsistency between oci-spec-go and oci-spec-rs regarding empty Maps and Vectors.

• oci-spec-go: Skips serialization for empty collections due to the omitempty tag.
• oci-spec-rs: Still serializes empty collections because it only uses #[serde(skip_serializing_if = "Option::is_none")], which doesn't account for empty states."

To address this, there are two potential solutions:

• Add a custom skip function: Introduce a helper function for skip_serializing_if that checks if the collection is either None or empty. This maintains the current structure but aligns the output with Go's omitempty.

fn is_option_map_empty(opt: &Option<HashMap<String, String>>) -> bool {
    opt.as_ref().map_or(true, |m| m.is_empty())
}

pub struct State {
    /// annotations are key values associated with the container.
    #[serde(default, skip_serializing_if = "is_option_map_empty")]
    annotations: Option<HashMap<String, String>>,
}

• Remove the Option wrapper: Drop the Option and use HashMap or Vec directly with #[serde(default, skip_serializing_if = "HashMap::is_empty")]. While this makes the code cleaner and more "Rust-idiomatic," it would be a breaking change for users of this crate.

pub struct State {
    /// annotations are key values associated with the container.
    #[serde(default, skip_serializing_if = "HashMap::is_empty")]
    annotations: HashMap<String, String>,
}

I believe using a helper function is the more reasonable approach since removing the Option would have a significant impact as a breaking change. However, if using Option for collections is considered unidiomatic in Rust, I’m also open to the latter option. Which approach would you prefer?

[utam0k]

Since Some(empty) and None have different meanings in Rust, I prefer keeping Option. For example, None means clearing the field, whereas Some(empty) might have another meaning (e.g., keeping the field as an empty value).

[nayuta723]

Thanks. I’ll proceed with the first approach.

[utam0k]

@saschagrunert WDYT?