Rust FFI no stdlib

lib.rs

#![feature(lang_items)]
#![no_std]

use core::panic::PanicInfo;

#[no_mangle]
pub extern "C" fn factorial(x: u32) -> u32 {
    (1..).take_while(|&i| i <= x).product()
}

#[lang = "eh_personality"] extern fn rust_eh_personality() {}

#[panic_handler] fn panic(_info: &PanicInfo) -> ! {
    loop {}
}