Store static function reference to extern C function in global static var: mismatched types: expected fn pointer, found fn item

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?

Funnily enough, when you change it a bit and add an explicit constant in between, you don't get a type error anymore, but “error[E0658]: function pointers cannot appear in constant functions” — mcarton
– mcarton, Commented Jul 15, 2021 at 10:06
Also note that in Rust, fn() -> ! is a type useful by itself, you don't need explicit references to make "function pointers": play.rust-lang.org/… — mcarton
– mcarton, Commented Jul 15, 2021 at 10:09

Aloso · Accepted Answer · 2021-07-15 11:00:41Z

3

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)

answered Jul 15, 2021 at 11:00

Aloso

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