non-reentrant function in Rust

non-reentrant-function.rs

use std::thread;
use std::time::Duration;

static mut SUM: i32 = 0;

fn calculate(first: i32, second: i32) -> i32 {
    unsafe {
        SUM = first + second;
    }
        thread::sleep(Duration::from_secs(3));
    unsafe {
        SUM
    }
}

fn main() {

    loop {
        println!("Result from main: {}\n", calculate(3, 5));
        println!("---\n");
    }

}