Why `Token::Enum` + smth and `Token::...Variant` is exist?

[Mingun]

I found, that there are two ways to express how the enum should be serialized:

#[derive(Deserialize, Serialize, Debug, PartialEq)]
enum Enum {
  Unit,
}

assert_tokens(
  &Enum::Unit,
  &[
    Token::Enum { name: "Enum" },
    Token::Str("Unit"),
    Token::Unit,
  ],
);

assert_tokens(
  &Enum::Unit,
  &[
    Token::UnitVariant { name: "Enum", variant: "Unit" },
  ],
);

Surprisely, both of that asserts are passed, although is is obvious that serialization can give only one of that results. After examining code I've found this very confusing check:
https://github.com/serde-rs/serde/blob/0c6a2bbf794abe966a4763f5b7ff23acb535eb7f/serde_test/src/ser.rs#L184-L190
That means that tester will change it's behavior depending on the testing data, that is not expected from the testing tool.

So the questions are:

• why both variants are exist?
• what one should use?
• isn't it necessary to keep only one of them?

[dtolnay]

I think this came from the Serializer vs Deserializer API having slightly divergent representation of the Serde data model out of necessity. In particular, enums during deserialization are treated using deserialize_enum and visit_enum (this is analogous to Token::Enum), while during serialization there is no such thing as "enum", only serialize_{…}_variant (this is analogous to Token::…Variant).

When one is testing serialization only, or deserialization only, one or the other way of expressing the enum using Token may seem more appropriate. I wouldn't say it's necessary for serde_test to have both.

I'll go ahead and close this because the current situation seems fine to me, but we should feel free to scrutinize it again for 2.x or if someone is building a different serde testing library.

[Mingun]

Seems reasonable. I think, that this subtle difference with explanation should be documented on Token::Enum and have are references to that documentation in Token::...Variant also would be good. Unfortunately, I'm not sure that I can write good documentation because of my language. So if you could add a few lines, it would be great!