I still need to create a simple reproduction for the issue, and not sure if this is covered by another issue report, but effectively...

#[derive(Deserialize)]
#[serde(untagged)]
enum E {
    Variant(
        #[serde(deserialize_with="...")]
        Ty
    ),
}

Now fails to compile with a type mismatch error where it used to work properly. Moving the attribute outside of the variant (#[serde(deserialize_with="...")] Variant(Ty)) seems to work on both and I believe should be equivalent and a valid workaround for now? Tested with serde_derive v1.0.29 (worked) and v1.0.57 (broken).