foo.cpp

#include <cstdio>
class Foo
{
    int x;
public:
    Foo(int x);
    ~Foo();
    void greet();
};
Foo::Foo(int x) : x(x)
{
}
Foo::~Foo()
{
}
void Foo::greet()
{
    std::printf("Hello world! x=%d\n", x);
}
Foo *newFoo(int x)
{
    return new Foo(x);
}
void deleteFoo(Foo *p)
{
    delete p;
}

test.rs

#![feature(link_args)]

extern crate libc;
use libc::types::os::arch::c95::{c_int};

enum Foo {}

#[link_args = "foo.o"]
extern {
    fn _Z6newFooi(x: c_int) -> *mut Foo;
    fn _Z9deleteFooP3Foo(p: *mut Foo);
    fn _ZN3Foo5greetEv(this: *mut Foo);
}

fn main() {
    unsafe {
        let foo = _Z6newFooi(42);
        _ZN3Foo5greetEv(foo);
        _Z9deleteFooP3Foo(foo);
    }
}