allow(dead_code) does not work for static externs

[Wilfred]

The following Rust code:

extern "C" {
    #[allow(dead_code)]
    static Qt: u64;
}

fn main() {
    println!("hello world");
}

produces the warning:

warning: foreign static item is never used: `Qt`, #[warn(dead_code)] on by default

in spite of the allow(dead_code) annotation.