constants
Rust has two different types of constants which can be declared in any scope including global. Both require explicit type annotation:
const
: An unchangeable value (the common case).static
: A possibly mutable variable with'static
lifetime. The static lifetime is inferred and does not have to be specified. Accessing or modifying a mutable static variable isunsafe
.
// Globals are declared outside all other scopes. static LANGUAGE: &str = "Rust"; const THRESHOLD: i32 = 10; fn is_big(n: i32) -> bool { // Access constant in some function n > THRESHOLD } fn main() { let n = 16; // Access constant in the main thread println!("This is {}", LANGUAGE); println!("The threshold is {}", THRESHOLD); println!("{} is {}", n, if is_big(n) { "big" } else { "small" }); // Error! Cannot modify a `const`. THRESHOLD = 5; // FIXME ^ Comment out this line }