[nightly regression] E0639 + adding #[non_exhaustive] to io::ErrorKind is a breaking change

[durka]

In stable and beta, you can write ErrorKind::Other {}, but in nightly this triggers E0639: cannot create non-exhaustive variant using struct expression. (As an aside, that error message should be rephrased and examples added to --explain.)

This is either a bug with #[non_exhaustive], or it means that the attribute can't be applied to public enums without creating a breaking change.

[durka]

I noticed this due to Travis tests of this code here. This is obviously(?) not a serious crate, but illustrates that this restriction is problematic for macros. That line expands to

    Err(_) => Err(io::Error::new(io::ErrorKind::Other{}, "invalid UTF-8"))

[Havvy]

This is a bug. Adding #[non_exhaustive] to an enum should not prevent the creation of enum variants, although adding the attribute to an enum variant explicitly allows for that. Especially since ErrorKind::Other without the {} is still valid.

[Havvy]

Minimal example.

Defining crate:

#[non_exhaustive]
pub enum NeEnum {
    Variant
}

Using crate:

fn make_ne_enum() -> NeEnum {
    // works
    NeEnum::Variant
}

fn make_ne_enum_2() -> NeEnum {
    // fails
    NeEnum::Variant{}
}

[arielb1]

I have a fix

[pnkfelix]

visiting for triage. P-high. (Fixed by #53721 in any case)