Mutable Static Variables

It is safe to read an immutable static variable:

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

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

However, since data races can occur, it is unsafe to read and write mutable static variables:

static mut COUNTER: u32 = 0;

fn add_to_counter(inc: u32) {
    unsafe { COUNTER += inc; }  // Potential data race!
}

fn main() {
    add_to_counter(42);

    unsafe { println!("COUNTER: {}", COUNTER); }  // Potential data race!
}

Using a mutable static is generally a bad idea, but there are some cases where it might make sense in low-level no_std code, such as implementing a heap allocator or working with some C APIs.

no_std is used to prevent Rust from loading the standard library. This is used for bare metal development for example. no_std in the Embedded Rust Book