How do I create a global, mutable singleton?

Question

What is the best way to create and use a struct with only one instantiation in the system? Yes, this is necessary, it is the OpenGL subsystem, and making multiple copies of

Answer 1

Use SpinLock for global access.

#[derive(Default)]
struct ThreadRegistry {
    pub enabled_for_new_threads: bool,
    threads: Option>,
}

impl ThreadRegistry {
    fn threads(&mut self) -> &mut HashMap {
        self.threads.get_or_insert_with(HashMap::new)
    }
}

static THREAD_REGISTRY: SpinLock = SpinLock::new(Default::default());

fn func_1() {
    let thread_registry = THREAD_REGISTRY.lock();  // Immutable access
    if thread_registry.enabled_for_new_threads {
    }
}

fn func_2() {
    let mut thread_registry = THREAD_REGISTRY.lock();  // Mutable access
    thread_registry.threads().insert(
        // ...
    );
}

If you want mutable state(NOT Singleton), see What Not to Do in Rust for more descriptions.

Hope it's helpful.