Allow empty functional updating of structs

[tikue]

struct Foo {
    bar: int,
    baz: int
}

let foo = Foo {
    bar: 0,
    baz: 1
};

let foo_clone = Foo { ..foo }; // error

As @metajack pointed out, this would be useful to avoid special-casing in macros and the like.

[jeremyletang]

triage bump, maybe we can close this issue, it seems that no one is interested about this?

[lilyball]

At the very least the error message is quite misleading:

Untitled.rs:12:25: 12:26 error: expected `;` but found `{`
Untitled.rs:12     let foo_clone = Foo { ..foo }; // error
                                       ^