Static and Constant Variables

Static and constant variables are two different ways to create globally-scoped values that cannot be moved or reallocated during the execution of the program.


Constant variables are evaluated at compile time and their values are inlined wherever they are used:

const DIGEST_SIZE: usize = 3;
const ZERO: Option<u8> = Some(42);

fn compute_digest(text: &str) -> [u8; DIGEST_SIZE] {
    let mut digest = [ZERO.unwrap_or(0); DIGEST_SIZE];
    for (idx, &b) in text.as_bytes().iter().enumerate() {
        digest[idx % DIGEST_SIZE] = digest[idx % DIGEST_SIZE].wrapping_add(b);

fn main() {
    let digest = compute_digest("Hello");
    println!("digest: {digest:?}");

According to the Rust RFC Book these are inlined upon use.

Only functions marked const can be called at compile time to generate const values. const functions can however be called at runtime.


Static variables will live during the whole execution of the program, and therefore will not move:

static BANNER: &str = "Welcome to RustOS 3.14";

fn main() {

As noted in the Rust RFC Book, these are not inlined upon use and have an actual associated memory location. This is useful for unsafe and embedded code, and the variable lives through the entirety of the program execution. When a globally-scoped value does not have a reason to need object identity, const is generally preferred.

Because static variables are accessible from any thread, they must be Sync. Interior mutability is possible through a Mutex, atomic or similar. It is also possible to have mutable statics, but they require manual synchronisation so any access to them requires unsafe code. We will look at mutable statics in the chapter on Unsafe Rust.

  • Mention that const behaves semantically similar to C++’s constexpr.
  • static, on the other hand, is much more similar to a const or mutable global variable in C++.
  • static provides object identity: an address in memory and state as required by types with interior mutability such as Mutex<T>.
  • It isn’t super common that one would need a runtime evaluated constant, but it is helpful and safer than using a static.
  • thread_local data can be created with the macro std::thread_local.

Properties table:

Has an address in memoryYesNo (inlined)
Lives for the entire duration of the programYesNo
Can be mutableYes (unsafe)No
Evaluated at compile timeYes (initialised at compile time)Yes
Inlined wherever it is usedNoYes