Assignee expressions can be enums

See also #2208 for more on assignee expressions.

The following code compiles:

enum Never {}

enum Foo {
    Variant,
}

enum Bar {
    One,
    Two(Never),
}

fn main() {
    Foo::Variant = Foo::Variant;
    Bar::One = Bar::One;
}

However, the reference doesn't say that enums can be used in assignee expressions. It only mentions that structs can be used there.

But based on how the compiler behaves, it seems that enums can be used in assignee expressions, as long as it's "irrefutable".

Start with the assignee expressions section of the Rust Reference linked in the issue, and read issue #2208 for related context. Compare the documented struct rule with the enum examples shown here, then update the reference so the supported enum case and its irrefutability condition are accurately described.