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Per the Rust reference: https://doc.rust-lang.org/reference/items/enumerations.html
Meaning that all possible enum items (identifiers, structs or tuples) can have a discriminant, even in 1.49 - however, the feature was still experimental back then.
Still, we need to remove the EnumItemDiscriminant class and instead add an optional std::unique_ptr<Expr> to the base EnumItem class, for all enum items to use it if required.
Eventually we'll also need to fix the parser to handle this