Generation of const pointers

[metaspace]

Given the following C heder:

const int* foo = ((int*) 3ULL);

bindgen will generate the following Rust code:

/* automatically generated by rust-bindgen 0.71.1 */

unsafe extern "C" {
    pub static mut foo: *const ::std::os::raw::c_int;
}

The output I would expect (and need) to be generated is the following:

const FOO: *const c_int = 3usize as *const c_int;

Questions

• Is there any way to configure bindgen to provide the output above?
• Is the generation of current output a feature or a bug?
  • If it is a feature, what is the rationale behind it?

For context, we are hitting this in the Linux kernel when generating bindings for these kind of pointers.

[tamird]

I think the output is correct? The const in the C code constifies the pointee. In order to constify the pointer itself, it would have to be const int* const foo.

But even if the pointer itself were constified we'd have to produce an extern declaration because the C compiler is going to actually produce the symbol foo (right?).

A related question might be: how can this C code be rewritten to produce a compile-time constant in Rust via bindgen?

[ojeda]

I agree with Tamir -- the output you suggest is not correct.

What I think you are looking for is constexpr, but we don't have support for it in the Linux kernel yet.

A (trivial enough) #define would also give you a const.

[metaspace]

Thanks for pointing out the error. I think I got Rust/C const mixed up.

The concrete C code I have in the kernel is

#define XAS_RESTART	((struct xa_node *)3UL)

And the code I think I need in Rust is

const XAS_RESTART: *const = 3usize as *const bindings::xa_node;

I am uncertain about how to achieve this.

[tamird]

Thanks for pointing out the error. I think I got Rust/C const mixed up.

The concrete C code I have in the kernel is

#define XAS_RESTART ((struct xa_node *)3UL)

what does bindgen generate?

[metaspace]

Thanks for pointing out the error. I think I got Rust/C const mixed up.
The concrete C code I have in the kernel is
#define XAS_RESTART ((struct xa_node *)3UL)

what does bindgen generate?

Nothing. What we usually do when we have this situation is define a constant:

// C code has this
#define FOO_3 ((int)3ULL)

// We write this
const int foo = FOO_3;

And then we get this

/* automatically generated by rust-bindgen 0.71.1 */

pub const foo: ::std::os::raw::c_int = 3;

Which is what led me to define the pointer as I did in the first post.

[metaspace]

I guess the culprit is that

const int * const foo = ((int*) 3ULL);

is a memory location in C, but the thing I expect to have generated in rust

const FOO: *const c_int = 3usize as *const c_int;

is a substitution in Rust. So it is closer to a C define than a C constant?