[Bug] Incompatiblity issue with enum variant externally tagged

[loganmzz]

Serde relies by default to externally tagged for enum variants. For example:

#[derive(Debug,Deserialize,PartialEq,Serialize)]
enum StringOrNumber {
    String(String),
    Number(u64),
}

#[test]
fn to_json() {
    assert_eq!(
        r#"
{
  "Number": 42
}
"#.trim(),
        serde_json::to_string_pretty(&StringOrNumber::Number(42)).unwrap(),
    );
}

#[test]
fn from_json() {
    assert_eq!(
        StringOrNumber::Number(42),
        serde_json::from_str(r#"
{
  "Number": 42
}
"#).unwrap(),
    );
}

Playground: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=c2ca0468b627ea6491eab5c122273ca3

---

However, you can't parse it from YAML-impl while:

• JSON input is compatible with YAML
• The same model/mapping/framework is in use.

Demonstration:

#[derive(Debug,Deserialize,PartialEq,Serialize)]
enum StringOrNumber {
    String(String),
    Number(u64),
}

#[test]
fn to_yaml() {
    assert_eq!(
        r#"
Number: 42
"#.trim_start(),
        serde_yaml::to_string(&StringOrNumber::Number(42)).unwrap(),
    );
}

#[test]
fn from_json() {
    assert_eq!(
        StringOrNumber::Number(42),
        serde_yaml::from_str(r#"
{
  "Number": 42
}
"#).unwrap(),
    );
}

#[test]
fn from_yaml() {
    assert_eq!(
        StringOrNumber::Number(42),
        serde_yaml::from_str(r#"
Number: 42
"#).unwrap(),
    );
}

Playground: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=e23c1abf4cb53373edb08862b69d7d4e

---

I have also initialized a crate to reference any other issues: https://github.com/loganmzz/serde-yaml-ng-compatibility-issues