Mutable Static Variables

It is safe to read an immutable static variable:

// Copyright 2023 Google LLC
// SPDX-License-Identifier: Apache-2.0

static HELLO_WORLD: &str = "Hello, world!";

fn main() {
    println!("HELLO_WORLD: {HELLO_WORLD}");
}

However, mutable static variables are unsafe to read and write because multiple threads could do so concurrently without synchronization, constituting a data race.

Using mutable statics soundly requires reasoning about concurrency without the compiler’s help:

// Copyright 2023 Google LLC
// SPDX-License-Identifier: Apache-2.0

static mut COUNTER: u32 = 0;

fn add_to_counter(inc: u32) {
    // SAFETY: There are no other threads which could be accessing `COUNTER`.
    unsafe {
        COUNTER += inc;
    }
}

fn main() {
    add_to_counter(42);

    // SAFETY: There are no other threads which could be accessing `COUNTER`.
    unsafe {
        dbg!(COUNTER);
    }
}