Const array from range

Question

How can I generate a long const array with values taken from a range? Bonus: 1) working with no_std, 2) without using any crates

What I would like to do:

struct A {
    a: i32,
    b: B
}

enum B {
    One,
    Two
}

const AS: [A; 1024] = [A{a: 1, b: B::One}, A{a: 2, b: B::One}, ..., A{a: 1024, b: B::One}]

The furthest I got so far is:

macro_rules! generate_list {
    ($($a:expr)+, $variant:expr) =>  {
        [ $( A { a: $a, b: $variant } )* ]
    }
}
const AS: [A; 1024] = generate_list!(1..1025i32, B::One);

At least one problem with this seems to be that the macro is evaluated before the range expression is expanded to a list of literals.

I reopened because a) I never meant to dupe-hammer. b) I think there are better answers possible in 2021 with const fn — Denys Séguret, May 14 '21 at 19:23
@DenysSéguret you can't use for loop in const fn so I don't think so — Stargateur, May 14 '21 at 20:43

Jason · Accepted Answer · 2021-05-14T23:02:06.737

I'm not sure if its idiomatic, but perhaps the following might work for your use case:

#[derive(Debug, Copy, Clone)]
pub struct A {
    a: i32,
    b: B,
}

#[derive(Debug, Copy, Clone)]
pub enum B {
    One,
    Two,
}

const fn gen_array<const N: usize>(offset: i32, b: B) -> [A; N] {
    let mut res = [A { a: offset, b }; N];
    
    let mut i = 0;
    while i < N as i32 {
        res[i as usize].a = offset + i;
        i += 1;
    }
    
    res
}

const AS: [A; 42] = gen_array(0, B::One);

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

Playground

I used a while loop, as a for loop is currently not allowed within a const context as E0744 indicates.

The above would generate 42 values for AS with an offset of 0:

[
    A {
        a: 0,
        b: One,
    },
    A {
        a: 1,
        b: One,
    },
    ...
    A {
        a: 41,
        b: One,
    },
]

The Godbolt Compiler Explorer shows that it generates the same instructions as writing it out manually.

1

so we can use while loop – Stargateur May 14 '21 at 21:25

Const array from range

1 Answers1