Idiomatic way to create a constant HashMap with values in Rust

Question

I want to know what is the idiomatic way to create a constant Hashmap with values in Rust?

I have an implementation like:

const LOOKUP_TABLE: HashMap<usize, f64> = HashMap::from([
    (1, 1.1),   // Values are randomly put
    (2, 2.2),
]);

and I get an error:

error[E0015]: cannot call non-const fn `<HashMap<usize, f64> as From<[(usize, f64); 12]>>::from` in constants
  --> 
   |
18 |   const LOOKUP_TABLE: HashMap<usize, f64> = HashMap::from([
   |  _________________________________________________^
19 | |     (1, 1.1),
20 | |     (2, 2.2),
31 | | ]);
   | |__^
   |
   = note: calls in constants are limited to constant functions, tuple structs and tuple variants

I usually use [`lazy_static`](https://docs.rs/lazy_static/latest/lazy_static/). — tadman, Apr 17 '23 at 23:37
....aaaand now I'm wishing Rust had more data literals in the syntax. — Jared Smith, Apr 18 '23 at 11:28
Related: https://stackoverflow.com/questions/27582739/how-do-i-create-a-hashmap-literal — E_net4, Apr 18 '23 at 13:44

score 2 · Answer 1 · answered Apr 18 '23 at 13:35

Rust's std::collections::HashMap cannot be used to create a const HashMap directly, because it requires dynamic memory allocation, which is not allowed for const values.

One possible way is to use the lazy_static crate, which allows you to initialize a static variable lazily at runtime. For example:

use std::collections::HashMap;
use lazy_static::lazy_static;

lazy_static! {
    static ref COUNTRIES: HashMap<usize, f64> = {
        let mut m = HashMap::new();
        m.insert(1, 1.1);
        m.insert(2, 2.2);
    };
}

Another possible way is to use the phf crate, which allows you to create compile-time static collections. For example:

use phf::{phf_map};

static COUNTRIES: phf::Map<usize, f64> = phf_map! {
    1 => 1.1,
    2 => 2.2,
};

Thanks, could you explain how the lazy static works? – Coder Apr 18 '23 at 18:50 — Coder, Apr 18 '23 at 18:50

Idiomatic way to create a constant HashMap with values in Rust

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