const

Declares a binding that cannot be reassigned: a local, a module constant, or a class field that is set only by new.

A const local is shallow: the variable cannot be reassigned, but the contents of a Vec, class instance or record it holds can still change (the compiler warns about that). On a class field, const forbids any write after construction, even from the class’s own methods.

A module-level const whose value is known at compile time (const LIMIT = 100;) is inlined where it is read. Any other value (an array such as const TABLE = [1, 2, 3];, or a function call) is computed once, before main, and the name still cannot be reassigned. static const is the old spelling of this: it warns (E0129), and coil fmt rewrites it to const.

A const set to a readonly array literal is a lookup table: a read at a constant index folds to the element, and a function-local const t = readonly [1, 2, 3]; is built on its first run and reused.

const VERSION = "1.0";

class Point {
    pub const x: int,
    pub const y: int,
}

fn main() {
    const limit = 100;
    let p = new Point(1, 2);
    let total = limit + p.x + p.y;
}

See also#

• Vec
• fn: Declares a function, a method, or (written in an expression) an anonymous function.
• let: Declares a mutable local variable.
• static: Declares a module-level or class-level slot shared by the whole program, or a method without self.
• readonly: A read-only view of a value: nothing reachable through it can be changed.
• if: Runs a block when a bool condition is true.
• else: Introduces the branch of an if that runs when its condition is false (or its pattern does not match).
• while: Repeats a block while a bool condition is true.