Mutexed singleton example in Rust

singleton.rs

use lazy_static::lazy_static;
use std::sync::{RwLock, Mutex};
use std::cell::Cell;

lazy_static! {
    static ref SINGLETON: Mutex<Option<Singleton>> = Mutex::new(None);
}

#[derive(Debug, PartialEq)]
struct Singleton {
    state: String,
}

impl Singleton {
    pub fn initialize(state: String) {
        let mut st = SINGLETON.lock().unwrap();
        if st.is_none() {
            *st = Some(Singleton { state })
        } else {
            panic!("Singleton is already initialized");
        }
    }

    pub fn get() -> &'static Mutex<Option<Self>> {
        if SINGLETON.lock().unwrap().is_some() {
            &SINGLETON
        } else {
            panic!("Singleton must be initialized before use");
        }
    }
}

#[cfg(test)]
mod tests {
    use super::{Singleton, SINGLETON};
    use std::sync::Mutex;
    use lazy_static::lazy_static;
    use std::panic;
    use std::sync::atomic::{AtomicUsize, Ordering};

    lazy_static! {
        // Force tests to run one-at-a-time
        static ref TEST_CTR: AtomicUsize = AtomicUsize::new(0);
    }

    fn run_test(order: usize, body: impl Fn() + panic::RefUnwindSafe) {
        // Wait until the order is correct
        while TEST_CTR.load(Ordering::Acquire) != order {};
        // Remove value from singleton, make it uninit
        SINGLETON.lock().unwrap().take();
        // Catch panic so we could increment the atomic before crashing test if panic occured
        let result = panic::catch_unwind(|| body());
        // Let the next test run
        TEST_CTR.store(order + 1, Ordering::Release);
        // Possibly crash
        result.unwrap();
    }

    #[test]
    #[should_panic]
    fn use_uninitialized() {
        run_test(3, || {
            Singleton::get();
        });
    }

    #[test]
    #[should_panic]
    fn double_initialize() {
        run_test(2,|| {
            Singleton::initialize(String::from("hello"));
            Singleton::initialize(String::from("world"));
        });
    }

    #[test]
    fn initialize() {
        run_test(1, || {
            Singleton::initialize(String::from("hello, world!"));
            assert_eq!(
                *SINGLETON.lock().unwrap(),
                Some(Singleton { state: String::from("hello, world!") })
            )
        });
    }

    #[test]
    fn initialize_and_get() {
        run_test(0, || {
            Singleton::initialize(String::from("hello, world!"));
            let instance = Singleton::get().lock().unwrap();
            assert_eq!(
                *instance,
                Some(Singleton { state: String::from("hello, world!") })
            );
        });
    }
}