可變的靜態變數

您可以放心讀取不可變的靜態變數：

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

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

不過，讀取並寫入可變的靜態變數並不安全，因為可能發生資料競爭：

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.