Constant value not known at compile time

Built from the repository at commit a8614dc.

A constant's value is made of literals, operators, the conversions that cannot fail, i64(x) of a u8 or a char and f64(n), and constants declared above it. Arithmetic and comparisons on f64 fold too, to what the program would compute at run time, nan and the infinities included. A conversion that can fail, u8(x), char(x) or i64(x) of an f64, is E0100. A call, a variable, an array, an if or a block is not known until the program runs. Interpolation does not fold either, even of constants: join string constants with +. A number cannot become a string in a constant.

Why

The compiler computes each constant's value before the program runs. Every part of the value must be known then, on both sides of && and || too: a part that run time would skip must still be one the compiler can read. Compute the value from literals and earlier constants, or use a let in a function.

Broken

fn limit() -> i64 {
    100
}

const LIMIT = limit()

fn main() {
    print(LIMIT)
}

What sisu says

error[E0086]: a constant's value must be known at compile time
 --> E0086.sisu:5:15
  |
5 | const LIMIT = limit()
  |               ^^^^^^^
  |
  = help: use literals, operators and earlier constants

sisu: 1 error
sisu: for more on a code, run `sisu explain <code>`: E0086

Fixed

const LIMIT = 100

fn main() {
    print(LIMIT)
}

Related

• E0087: constant that uses itself or a constant below it.
• E0100: conversion that can fail, in a constant.