Is there a way to declare a vector of enums types inside a module?

October 15, 2022

I wish to add a static variable to a module so that I can access the variable by doing module_name::variable_name. However, I want this variable to be a vector of enum types.

Here is what I mean:

pub mod cards{
    #[derive(Debug)]
    pub enum SUIT{
        CLUB, DIAMOND, HEART, SPADE,
    }
    
    pub const all_cards: Vec<SUIT> = vec![SUIT::CLUB, SUIT::DIAMOND, SUIT::HEART, SUIT::SPADE];
}

fn main(){
    println!("{:?}", cards::all_cards);
}

But doing the above gives me two errors:

error[E0010]: allocations are not allowed in constants

and

error[E0015]: cannot call non-const fn `slice::<impl [SUIT]>::into_vec::<std::alloc::Global>` in constants

Is there a way to declare a vector of enums types inside a module?

>Solution :

In this example you can instead use an Array because of fixed size of ALL_CARDS. Playground link

use crate::cards::ALL_CARDS;

pub mod cards {
    #[derive(Debug)]
    pub enum SUIT {
        CLUB,
        DIAMOND,
        HEART,
        SPADE,
    }

    pub const ALL_CARDS: [SUIT; 4] = [SUIT::CLUB, SUIT::DIAMOND, SUIT::HEART, SUIT::SPADE];
}

fn main() {
    println!("{:?}", ALL_CARDS);
}

Vector requires a heap allocation and as error message says "allocation are not allowed in constants". So when you need a global static data which needs to be initialized on heap once during lifetime of a program: have a look at https://crates.io/crates/once_cell or https://crates.io/crates/lazy_static

Note I’m using uppercase for constant value as per Rust naming convention