CODEWITHSUNDEEP
randomrustconstants
Ali Shah lakhani
Ali Shah lakhani

Reputation: 105

Rust throwing error 'attempt to use a non-constant value in a constant'

I'm a rust newbie and I wanted to understand why Rust is throwing me an error when I run the following code:

let rng = rand::thread_rng();
const NUMBER_TO_GUESS: u32 = rng.gen();

Error:

error[E0435]: attempt to use a non-constant value in a constant
  --> src/main.rs:12:34
   |
12 |     const NUMBER_TO_GUESS: u32 = rng.gen();
   |     ---------------------        ^^^ non-constant value
   |     |
   |     help: consider using `let` instead of `const`: `let NUMBER_TO_GUESS`

Upvotes: 3

Views: 5221

Answers (1)

Daniel T
Daniel T

Reputation: 1179

If you really want a const (generated during compile time and does not change each time the program is run), then cargo add const-random and use the following:

use const_random::const_random;

fn main() {
    const NUMBER_TO_GUESS: u32 = const_random!(u32);
    println!("Hello {}", NUMBER_TO_GUESS);
}

I see that the variable is named NUMBER_TO_GUESS. A guessing game should probably change the number on each run, so you would use the following:

use rand::Rng;

fn main() {
    // rng needs to be mut because it needs to be modified so that
    // rng.gen() returns different values each time
    let mut rng = rand::thread_rng();
    let number_to_guess: u32 = rng.gen();
    println!("Hello {}", number_to_guess);
}

I see in the comments you are from Javascript. Here is a table with some roughly corresponding syntax:

Typescript Rust
let x: Something = y; let mut x = y;
let x: Readonly<Something> = y; let mut x = &y;
const x: Something = y; let x = &mut y;
const x: Readonly<Something> = y; let x = y;
Closest is Webpack's EnvironmentPlugin/DefinePlugin with const x: string = process.env.SOMETHING; plus a minifier const x = "That thing";

In the Rust code you have, you see let rng before const NUMBER_TO_GUESS. The compiler does not see it this way. Ahead-of-time compiled languages always calculate the const one before other variables.

Upvotes: 5

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