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0026-remove-priv.md

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Summary

This RFC is a proposal to remove the usage of the keyword priv from the Rust language.

Motivation

By removing priv entirely from the language, it significantly simplifies the privacy semantics as well as the ability to explain it to newcomers. The one remaining case, private enum variants, can be rewritten as such:

// pub enum Foo {
//     Bar,
//     priv Baz,
// }

pub enum Foo {
    Bar,
    Baz(BazInner)
}

pub struct BazInner(());

// pub enum Foo2 {
//     priv Bar2,
//     priv Baz2,
// }

pub struct Foo2 {
    variant: FooVariant
}

enum FooVariant {
    Bar2,
    Baz2,
}

Private enum variants are a rarely used feature of the language, and are generally not regarded as a strong enough feature to justify the priv keyword entirely.

Detailed design

There remains only one use case of the priv visibility qualifier in the Rust language, which is to make enum variants private. For example, it is possible today to write a type such as:

pub enum Foo {
    Bar,
    priv Baz
}

In this example, the variant Bar is public, while the variant Baz is private. This RFC would remove this ability to have private enum variants.

In addition to disallowing the priv keyword on enum variants, this RFC would also forbid visibility qualifiers in front of enum variants entirely, as they no longer serve any purpose.

Status of the identifier priv

This RFC would demote the identifier priv from being a keyword to being a reserved keyword (in case we find a use for it in the future).

Alternatives

  • Allow private enum variants, as-is today.
  • Add a new keyword for enum which means "my variants are all private" with controls to make variants public.

Unresolved questions

  • Is the assertion that private enum variants are rarely used true? Are there legitimate use cases for keeping the priv keyword?