fn main() {
    println!("{}", unsafe { foo() });
}

#[link_name = "foobar"]
extern "C" {
    fn foo() -> u32;
}

This fails to link with undefined reference to `foo', but it didn't even try to find any foobar library.

With #[link(name = "foobar")], the error is the expected ld: cannot find -lfoobar.

There are a few places in libstd using #[link_name = "m"] extern {...}, so I wonder if those actually have any effect either.