Don't allow array types in extern "C" functions.

[pnkfelix]

(imported from improperly closed bug #16221)

You cannot pass an array to or return an array from a C function.
The signature f(char x[N]) is equivalent to f(char *x) for every N in C. Therefore it is meaningless to write this in Rust, (or rather, it does not mean what a C programmer and/or FFI generation tool likely thinks it means):

    extern "C" fn f(x: [i32, ..4]); // void f(int x[4])

Option for dealing with this:

• We can extend our existing lint to detect cases like this and warn about them.
• We could make the extern "C" fn form reject this signature, since the expected correspondence between Rust and C does not hold here.
• If the Rust FFI binding generation scripts/programs out there are generating this sort of code, they need to be fixed.

[pnkfelix]

triage: P-low

[steveklabnik]

Triage: array syntax has changed, here's the correct syntax:

extern {
    fn f(x: [i32; 4]);
}

fn main() {}

no warning or anything.

[steveklabnik]

Triage: no change

[estebank]

This would have to be implemented as a lint now due to the potential of breaking existing builds. It should be easy enough to implement.