How can you make a safe static singleton in Rust?

Question

This is something of a controversial topic, so let me start by explaining my use case, and then talk about the actual problem.

I find that for a bunch of unsafe thin

Answer 1

See also lazy_static, which makes things a little more ergonomic. It does essentially the same thing as a static Once for each variable, but wraps it in a type that implements Deref so that you can access it like a normal reference.

Usage looks like this (from the documentation):

#[macro_use]
extern crate lazy_static;

use std::collections::HashMap;

lazy_static! {
    static ref HASHMAP: HashMap<u32, &'static str> = {
        let mut m = HashMap::new();
        m.insert(0, "foo");
        m.insert(1, "bar");
        m.insert(2, "baz");
        m
    };
    static ref COUNT: usize = HASHMAP.len();
    static ref NUMBER: u32 = times_two(21);
}

fn times_two(n: u32) -> u32 { n * 2 }

fn main() {
    println!("The map has {} entries.", *COUNT);
    println!("The entry for `0` is \"{}\".", HASHMAP.get(&0).unwrap());
    println!("A expensive calculation on a static results in: {}.", *NUMBER);
}

Note that autoderef means that you don't even have to use * whenever you call a method on your static variable. The variable will be initialized the first time it's Deref'd.

However, lazy_static variables are immutable (since they're behind a reference). If you want a mutable static, you'll need to use a Mutex:

lazy_static! {
    static ref VALUE: Mutex<u64>;
}

impl Drop for IsFoo {
    fn drop(&mut self) {
        let mut value = VALUE.lock().unwrap();
        *value += 1;
    }
}

#[test]
fn test_drops_actually_work() {
    // Have to drop the mutex guard to unlock, so we put it in its own scope
    {
        *VALUE.lock().unwrap() = 0;
    }
    {
        let c = CBox;
        c.set(IsFoo);
        c.set(IsFoo);
        c.poll(/*...*/);
    }
    assert!(*VALUE.lock().unwrap() == 2); // Assert that all expected drops were invoked
}

Answer 2

It looks like a use case for std::sync::Once:

use std::sync::{Once, ONCE_INIT};
static INIT: Once = ONCE_INIT;

Then in your tests call

INIT.doit(|| unsafe { init(); });

Once guarantees that your init will only be executed once, no matter how many times you call INIT.doit().