Keyword static

A static item is a value which is valid for the entire duration of your program (a 'static lifetime).

On the surface, static items seem very similar to consts: both contain a value, both require type annotations and both can only be initialized with constant functions and values. However, statics are notably different in that they represent a location in memory. That means that you can have references to static items and potentially even modify them, making them essentially global variables.

Static items do not call drop at the end of the program.

There are two types of static items: those declared in association with the mut keyword and those without.

Static items cannot be moved:

static VEC: Vec<u32> = vec![];

fn move_vec(v: Vec<u32>) -> Vec<u32> {
    v
}

// This line causes an error
move_vec(VEC);

Simple statics

Accessing non-mut static items is considered safe, but some restrictions apply. Most notably, the type of a static value needs to implement the Sync trait, ruling out interior mutability containers like RefCell. See the Reference for more information.

static FOO: [i32; 5] = [1, 2, 3, 4, 5];

let r1 = &FOO as *const _;
let r2 = &FOO as *const _;
// With a strictly read-only static, references will have the same address
assert_eq!(r1, r2);
// A static item can be used just like a variable in many cases
println!("{FOO:?}");

Mutable statics

If a static item is declared with the mut keyword, then it is allowed to be modified by the program. However, accessing mutable statics can cause undefined behavior in a number of ways, for example due to data races in a multithreaded context. As such, all accesses to mutable statics require an unsafe block.

When possible, it’s often better to use a non-mutable static with an interior mutable type such as Mutex, OnceLock, or an atomic.

Despite their unsafety, mutable statics are necessary in many contexts: they can be used to represent global state shared by the whole program or in extern blocks to bind to variables from C libraries.

In an extern block:

extern "C" {
    static mut ERROR_MESSAGE: *mut std::os::raw::c_char;
}

Mutable statics, just like simple statics, have some restrictions that apply to them. See the Reference for more information.