Constants

A constant is a named, compile-time value.

const WORLD_COUNT: u8 = 10
const DEFAULT_NAME: str = "flower"

const NAME: Type = value

• NAME — a plain identifier (not scoped).
• Type — any type; in practice a primitive.
• value — one of:
  • a literal: an integer or a "quoted string" (plain, no interpolation)
  • an identifier — another constant, by name
  • a ::-path — u32::MIN, pkg::mod::DEFAULT
  • an f-string — f"page {N} of {total}", with {path} placeholders and {{ / }} escapes (same interpolation as an error's message)

Everything except a plain literal is recorded as text and not resolved to a value yet — including the {N} in an f-string.

A constant can carry a docstring:

/// Largest number of worlds a provider may report.
const WORLD_COUNT: u8 = 10

Using one

A constant is referenced by name as a struct field default:

struct Person {
    name: str = DEFAULT_NAME
}

Array sizes currently take an integer literal only (Article[10]), not a constant name.