Summary

Just like structs, variants can come in three forms - unit-like, tuple-like, or struct-like:

enum Foo {
    Foo,
    Bar(int, String),
    Baz { a: int, b: String }
}

The last form is currently feature gated. This RFC proposes to remove that gate before 1.0.

Motivation

Tuple variants with multiple fields can become difficult to work with, especially when the types of the fields don't make it obvious what each one is. It is not an uncommon sight in the compiler to see inline comments used to help identify the various variants of an enum, such as this snippet from rustc::middle::def:

pub enum Def {
    // ...
    DefVariant(ast::DefId /* enum */, ast::DefId /* variant */, bool /* is_structure */),
    DefTy(ast::DefId, bool /* is_enum */),
    // ...
}

If these were changed to struct variants, this ad-hoc documentation would move into the names of the fields themselves. These names are visible in rustdoc, so a developer doesn't have to go source diving to figure out what's going on. In addition, the fields of struct variants can have documentation attached.

pub enum Def {
    // ...
    DefVariant {
        enum_did: ast::DefId,
        variant_did: ast::DefId,
        /// Identifies the variant as tuple-like or struct-like
        is_structure: bool,
    },
    DefTy {
        did: ast::DefId,
        is_enum: bool,
    },
    // ...
}

As the number of fields in a variant increases, it becomes increasingly crucial to use struct variants. For example, consider this snippet from rust-postgres:

enum FrontendMessage<'a> {
    // ...
    Bind {
        pub portal: &'a str,
        pub statement: &'a str,
        pub formats: &'a [i16],
        pub values: &'a [Option<Vec<u8>>],
        pub result_formats: &'a [i16]
    },
    // ...
}

If we convert Bind to a tuple variant:

enum FrontendMessage<'a> {
    // ...
    Bind(&'a str, &'a str, &'a [i16], &'a [Option<Vec<u8>>], &'a [i16]),
    // ...
}

we run into both the documentation issues discussed above, as well as ergonomic issues. If code only cares about the values and formats fields, working with a struct variant is nicer:

match msg {
    // you can reorder too!
    Bind { values, formats, .. } => ...
    // ...
}

versus

match msg {
    Bind(_, _, formats, values, _) => ...
    // ...
}

This feature gate was originally put in place because there were many serious bugs in the compiler's support for struct variants. This is not the case today. The issue tracker does not appear have any open correctness issues related to struct variants and many libraries, including rustc itself, have been using them without trouble for a while.

Detailed design

Change the Status of the struct_variant feature from Active to Accepted.

The fields of struct variants use the same style of privacy as normal struct fields - they're private unless tagged pub. This is inconsistent with tuple variants, where the fields have inherited visibility. Struct variant fields will be changed to have inhereted privacy, and pub will no longer be allowed.

Drawbacks

Adding formal support for a feature increases the maintenance burden of rustc.

Alternatives

If struct variants remain feature gated at 1.0, libraries that want to ensure that they will continue working into the future will be forced to avoid struct variants since there are no guarantees about backwards compatibility of feature-gated parts of the language.

Unresolved questions

N/A

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