main.c

#include <stdio.h>

extern int add(int, int);

int
main(int argc, char* argv[])
{
  printf("2 + 3 = %d\n", add(2, 3));
  return 0;
}

Makefile

main: libsample.a
	gcc -lpthread -ldl main.c libsample.a -o main

libsample.a: sample.rs
	rustc --crate-type=staticlib sample.rs

clean:
	rm -f main libsample.a

sample.rs

#[no_mangle]
pub extern "C" fn add(x: isize, y: isize) -> isize {
    x + y
}