Support for fallback enums

[valkum]

While implementing an extension for instant-epp I would like to parse

<changePoll:caseId name="other" type="custom">urs123</changePoll:caseId>

into the following ergonomic Rust representation

#[derive(Debug, FromXml, ToXml)]
#[xml(rename = "caseId", ns(XMLNS))]
pub struct CaseIdentifier<'a> {
    #[xml(attribute, rename = "type")]
    pub id_type: CaseIdentifierType,
    #[xml(direct)]
    pub id: Cow<'a, str>,
}

#[derive(Debug, Copy, Clone, PartialEq, Eq)]
pub enum CaseIdentifierType<'a> {
    Udrp,
    Urs,
    Custom(&'a str),
}

It would be nice if CaseIdentifierType can take a #[from_attribute("name")] or similar to generate code that lets CaseIdentifierType consume an attribute node of the parent element node.

[djc]

Does serde have anything like this? It seems rather specific.

[valkum]

Serde supports #[serde(untagged)] on an enum Variant since v1.0.164, but I think this does not really cover cases like this where you have an explicit attribute that stores the fallback value.
I haven't worked with much XML schemas, so I am not sure if this is a common practice with XML.