Undefined reference when using rust library in C++

I'm trying to use a Rust code in C. I used no mangle for Rust and extern "C" for C++ code that is going to be defined in Rust. However I keep getting undefined references whem trying to link my Rust library into my C++ code

interface.rs

use std::os::raw::{c_int};
type OnDataCallback = unsafe extern "C" fn(data: *mut u8, len: usize) -> c_int;

static mut onDataCallback_: Option<OnDataCallback> = None;

#[no_mangle]
pub extern "C" fn registerOnDataCallback(cb: Option<OnDataCallback>) -> c_int
{
    unsafe{onDataCallback_ = cb;}
    return 0;
}

#[no_mangle]
pub extern "C" fn doSomething()
{

    let mut s = String::from("hello world\0");
    unsafe {
        unsafe{onDataCallback_.unwrap()(s.as_mut_ptr() , 100)};
    }
}

interface.cpp

#include <iostream>

typedef int (*onDataCallback_)(uint8_t *data, size_t len);

extern "C" int registerOnDataCallback(onDataCallback_ cb);

extern "C" void doSomething();

extern "C" int onDataCallback(uint8_t *data, size_t len) {
    std::cout << "called onData with size " + len << std::endl;
}

int main() {
    registerOnDataCallback(&onDataCallback);
    doSomething();
    getchar();
    return 0;
}

How I'm compiling:

rustc --crate-type=staticlib interface.rs
g++ -o rust_c -L. -linterface interface.cpp

Error:

/tmp/ccgjsC7i.o: In function `main':
interface.cpp:(.text+0x51): undefined reference to `registerOnDataCallback'
interface.cpp:(.text+0x56): undefined reference to `doSomething'
collect2: error: ld returned 1 exit status

I've followed 2 different tutorials. I also tried with Cargo:

[package]
name = "smoltcp_c_interface"
version = "0.1.0"

[lib]
name = "smoltcp_c_interface"
path = "interface.rs"
crate-type = ["staticlib"]

[dependencies]
libc = "0.2.0"

then did cargo build and tried to link with the g++ in the same way as above. Same undefined references.