rust - ErrorKind::__Noneexhaustive 的目的是什么？

Question

std::io::ErrorKind 有一个变体 __Noneexhaustive。如果这个变体不存在，我不知道会出现什么问题。

这个变体的目的是什么？

Answer 1

它旨在允许 ErrorKind 枚举在未来扩展，通过强制稳定代码中的任何 match 语句具有包罗万象的 _ ARM 。

具体来说，the variant is marked unstable所以不能在稳定 channel 上引用，所以编译器拒绝像

这样的代码

fn foo(x: Error) {
    match x.kind() {
        ErrorKind::NotFound => {}
        ErrorKind::PermissionDenied => {}
        ErrorKind::ConnectionRefused => {}
        ErrorKind::ConnectionReset => {}
        ErrorKind::ConnectionAborted => {}
        ErrorKind::NotConnected => {}
        ErrorKind::AddrInUse => {}
        ErrorKind::AddrNotAvailable => {}
        ErrorKind::BrokenPipe => {}
        ErrorKind::AlreadyExists => {}
        ErrorKind::WouldBlock => {}
        ErrorKind::InvalidInput => {}
        ErrorKind::InvalidData => {}
        ErrorKind::TimedOut => {}
        ErrorKind::WriteZero => {}
        ErrorKind::Interrupted => {}
        ErrorKind::Other => {}
        ErrorKind::UnexpectedEof => {}
        ErrorKind::UnexpectedEOF => {}
        ErrorKind::__Nonexhaustive => {}
    }
}

<anon>:24:9: 24:35 error: use of unstable library feature 'io_error_internals': better expressed through extensible enums that this enum cannot be exhaustively matched against (see issue #0)
<anon>:24         ErrorKind::__Nonexhaustive => {}
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~

如果此代码在稳定版 Rust 上成功编译，则在未来的版本中向 ErrorKind 添加一个变体会破坏任何具有上述 match 的代码，并破坏稳定的代码不好。代码中断是因为 Rust 中的匹配必须是详尽无遗的，也就是说，它们必须以某种方式涵盖所有可能性，因此添加变体将意味着未涵盖可能性。

相反，程序员必须写:

fn foo(x: Error) {
    match x.kind() {
        ErrorKind::NotFound => {}
        ErrorKind::PermissionDenied => {}
        ErrorKind::ConnectionRefused => {}
        ErrorKind::ConnectionReset => {}
        ErrorKind::ConnectionAborted => {}
        ErrorKind::NotConnected => {}
        ErrorKind::AddrInUse => {}
        ErrorKind::AddrNotAvailable => {}
        ErrorKind::BrokenPipe => {}
        ErrorKind::AlreadyExists => {}
        ErrorKind::WouldBlock => {}
        ErrorKind::InvalidInput => {}
        ErrorKind::InvalidData => {}
        ErrorKind::TimedOut => {}
        ErrorKind::WriteZero => {}
        ErrorKind::Interrupted => {}
        ErrorKind::Other => {}
        ErrorKind::UnexpectedEof => {}
        ErrorKind::UnexpectedEOF => {}
        _ => {}
    }
}

这意味着将来添加到 ErrorKind 的任何变体(例如，新 IO 函数的新错误可能性)都将归入 _ 分支，因此现有的稳定代码将胜出'休息。