`rustc` allows enum struct variants to be matched as if they were tuple variants

[pnkfelix]

struct variants have been unfeature gated, but there are corner cases that we did not intend.

Here is (playpen compatible) code that demonstrates this:
link: http://is.gd/uUVO80

 #![feature(struct_variant)]

 fn main() {
    enum Foo {
        FooB { x: i32, y: i32 }
    }

    let f = FooB { x: 3, y: 4 };
    match f {
        // The pattern for the arm below is problematic.
        FooB(a, b) => println!("{} {}", a, b),
    }
}

[pnkfelix]

Nomating for P-backcompat-lang, 1.0