在全局静态变量中存储对外部 C 函数的静态函数引用：不匹配的类型：预期的 fn 指针，找到了 fn 项

Question

I need a global static variable, that references a extern "C" function.

I get the following compiler error:

error[E0308]: mismatched types
  --> src/main.rs:17:28
   |
17 | static BAR: Bar = Bar::new(&foobar);
   |                            ^^^^^^^ expected fn pointer, found fn item
   |
   = note: expected reference `&'static unsafe extern "C" fn() -> !`
              found reference `&unsafe extern "C" fn() -> ! {foobar}`

My code down below or on Rust playground

extern "C" {
    fn foobar() -> !;
}

struct Bar {
    foo: &'static unsafe extern "C" fn() -> !
}

impl Bar {
    const fn new(foo: &'static unsafe extern "C" fn() -> !) -> Self {
        Self {
            foo
        }
    }
}

static BAR: Bar = Bar::new(&foobar);

fn main() {

}

How can I solve this?

Answer 1

The fn type is already a pointer (called a "function pointer"), so you don't need to put it behind a reference:

struct Bar {
    foo: unsafe extern "C" fn() -> !,
}

It can be created like this:

impl Bar {
    const fn new(foo: unsafe extern "C" fn() -> !) -> Self {
        Self {
            foo
        }
    }
}

static BAR: Bar = Bar::new(foobar);

When you try to compile this, Rust tells you that function pointers in const contexts are unstable. By using the Nightly channel and enabling the const_fn_fn_ptr_basics feature, it works:

Playground

(This is likely to change in the future, don't hesitate to update this answer when needed)