document common enum fields

[jesse99]

Commit 5c9c9a6 talks about something called common enum fields and includes the following wacky rust example:

enum Foo {
    struct {
        x: int;
        y: int;
    }

    Bar(int),
    Baz(int)
}

Don't see this documented anywhere.

[jesse99]

Wanted to document this but I have no idea what it's for or how you'd go about initializing x and y. Fwiw I searched all of src using:
enum \s+ [a-zA-Z0-9]+ \s* { \s* struct \s* {
and the only place that syntax seems to be used is the one trivial example from the original commit.

[nikomatsakis]

I think we've decided not to support these.

[catamorphism]

Closing since we're not supporting them.