E0109 is out of date

[QuineDot]

Code

fn main() {
    let _ = Option::<_>::None::<()>;
    type Alias<T> = Option<T>;
    let _ = Alias::None::<()>;
}
// Followed by running `rustc --explain E0109`

Current output

[Bottom of `rustc --explain E0109`]

Note that generic arguments for `enum` variant constructors go after the variant, not after the `enum`. For example, you would write `Option::None::<u32>`, rather than `Option::<u32>::None`.

Desired output

Note that generic arguments for `enum` variant constructors can go after the variant *or* after the `enum`, but not both.  For example, you can write `Option::None::<u32>` or `Option::<u32>::None`, but not `Option::<u32>::None::<u32>`.

Additionally, generic arguments for `type` aliases of `enum`s go after the `enum`, not after the variant.  For example, if `Alias<T>` is a `type` alias of `Option<T>`, you would write `Alias::<u32>::None`, rather than `Alias::None::<u32>`.

Rationale and extra context

Generic arguments can go after the enum now. Moreover, for type aliases of enums, generic arguments must go after the enum and not after the variant.

See this stabilization report for details. In brief,

• Option::<T>::None has worked since Rust 1.33
• Alias::<T>::None has worked since Rust 1.37, but
• Alias::None::<T> doesn't work so far (Rust 1.73)

Other cases

No response

Anything else?

@rustbot labels +A-error-codes

[adityakode]

Can I work on this ?

[gurry]

Yes, why not? Just claim the issue as explained here and start working on it.

[QuineDot]

I forgot I filed this but just ran into it again while checking something else out, heh.

pub enum Foo<'a, T = String> {
    Bar(&'a T),
    Baz,
}

fn main() {
    // These two work
    let foo = Foo::Baz::<'_, String>;
    let foo = Foo::<'_, String>::Baz;
    // This fails
    let foo = Foo::<'_>::Baz::<String>;
}

Playground. The inline error is actually incorrect too.

error[E0109]: type arguments are not allowed on unit variant `Baz`
  --> src/main.rs:11:32
   |
11 |     let foo = Foo::<'_>::Baz::<String>;
   |                          ---   ^^^^^^ type argument not allowed
   |                          |
   |                          not allowed on unit variant `Baz`