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

[mahkoh]

You cannot pass an array to or return an array from a C function. f(char x[N]) is equivalent to f(char *x) for every N. On x86_64 linux, the following two extern "C" functions produce the same assembly:

extern "C" {
    fn f(x: [i32, ..4]); // void f(int x[4])
    fn g(x: i64, y: i64);
}

This will segfault.

[alexcrichton]

Nominating.

[pnkfelix]

There are a couple of things here

• 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]

Assigning P-low, not 1.0 milestone.