This is C and I tossed it together VERY quickly but

        $ cat hello.rs
        #[no_mangle]
        pub extern fn plus_one(x: isize) -> isize {
            x + 1
        }
        $ cat hello.c
        #include<stdio.h>

        extern int plus_one(int);

        int main() {
                int result = plus_one(5);

                printf("result is %d\n", result);

                return 0;
        }
        $ rustc hello.rs --crate-type=dylib
        $ gcc hello.c -lhello -L .
        $ LD_LIBRARY_PATH=. ./a.out
        result is 6