(Nested) enums provided with the #[serde(tag = "type")]-containing anonymus enum types fail during serialisation.

[gomolsoft]

If a toplevel enum contains a nested enum both are provided with the "#[serde(tag = "type")]" and the nested enum contains, among other things, an anonymous string, an error occurs during serialisation:

use serde::{Deserialize, Serialize};

#[derive(PartialEq, Debug, Clone, Serialize, Deserialize)]
#[serde(tag = "type")]
pub enum EnumNested {
    SimpleType, 
    WithString(String),
}

#[derive(PartialEq, Debug, Clone, Serialize, Deserialize)]
#[serde(tag = "type")]
pub enum EnumTopLevel {
    NestedEnumType {
      name: String,
      nested: Option<EnumNested>,
    },
}

#[cfg(test)]
mod tests {
    use super::*;
    #[test]
    fn test_struct_output_v1() {
        let enum_type = EnumTopLevel::NestedEnumType {
            name: "test..123".to_string(),
            nested: Some(EnumNested::WithString("some... ".to_string())),
         };
         
        let serialized = serde_json::to_string(&enum_type).unwrap(); 
    }
}

The following error occures:

Error("cannot serialize tagged newtype variant EnumNested::WithString containing a string"

-> let serialized = serde_json::to_string_pretty(&enum_type).unwrap(); 

running 1 test
test tests::test_struct_output_v1 ... FAILED

failures:

---- tests::test_struct_output_v1 stdout ----
thread 'tests::test_struct_output_v1' panicked at 'called Result::unwrap() on an Err value: Error("cannot serialize tagged newtype variant EnumNested::WithString containing a string", line: 0, column: 0)', src/lib.rs:34:71
note: run with RUST_BACKTRACE=1 environment variable to display a backtrace

failures:
tests::test_struct_output_v1

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

[dtolnay]

This is working correctly as far as I can tell. https://serde.rs/enum-representations.html#internally-tagged documents that tag = "type" is for struct variants, newtype variants containing structs or maps, and unit variants — which is not what WithString is.