`#[cfg]` on struct field permits empty unusable struct

[klutzy]

struct S {
    #[cfg(untrue)]
    a: int,
}

fn main() {
    let s = S;
}

<anon>:7:13: 7:14 error: `S` is a structure name, but this expression uses it like a function name
<anon>:7     let s = S;
                     ^
<anon>:7:13: 7:14 note: Did you mean to write: `S { /* fields */ }`?
<anon>:7     let s = S;
                     ^

Here S { } is suggested but it is also not allowed:

<anon>:7:14: 7:15 error: structure literal must either have at least one field or use functional structure update syntax
<anon>:7     let s = S{};
                      ^

[klutzy]

Triage: still an issue. Sub-bug of #24266.

[petrochenkov]

Note, that after #28336 empty tuple structs are still disallowed, but obtainable with #[cfg].

struct TS (
    #[cfg(untrue)]
    int,
);

IMO, amending rust-lang/rfcs#218 to allow empty tuple structs would be the best solution.

[pnkfelix]

(note that rust-lang/rfcs#218 did include discussion of the generalization to empty tuple structs... in short, I do not think you can directly generalize the RFC's strategy to tuple structs, though there may be some more limited generalization available.)

(I personally think the better approach here would be to explicitly disallow using #[cfg] attributes with tuple structs. I.e. If you need that kind of flexibility, use a record struct.)