Can a Rust constant/static be exposed to C?

Question

Say I have a Rust API that contains a constant or a static, an i32 for example. I want to use this Rust API from C. From the C side, I want to use that constant as an array size. Am I correct that there is no way to do it? Is the best solution to redeclare the constant in my C header files that are providing the declarations for the rest of the Rust API?

Update: To be more specific, I am using a compiler which does not support variable-length arrays (Visual C++ 2005)

Answer 1

You most certainly can do it, at least inside functions:

cnst.rs:

#[no_mangle]
pub static X: i32 = 42;

cnstuse.c:

#include <stdint.h>
#include <stdio.h>

extern const int32_t X;

int main() {
    char data[X];
    printf("%lu\n", sizeof(data));
    return 0;
}

Compilation:

% rustc --crate-type=staticlib cnst.rs
note: link against the following native artifacts when linking against this static library
note: the order and any duplication can be significant on some platforms, and so may need to be preserved
note: library: System
note: library: pthread
note: library: c
note: library: m
% gcc -o cnstuse cnstuse.c libcnst.a
% ./cnstuse
42

Top-level array declarations, however, can't use global variables/constants for size, so this will only work inside functions.