How to write a collection of static variable in Rust

Question

I want to collect the static variable like java

Class A {
  public static final String HELLO = "hello";
  public static final String WORLD = "world";
}

But it's not supported by enum or struct in Rust! Should I use something like static HashMap?

Answer 1

The most literal and straightforward conversion of this would be "associated constants":

struct A { /* .. */ }

impl A {
    const HELLO: &str = "hello";
    const WORLD: &str = "world";
}

which would be accessible via A::HELLO and A::WORLD.

---

However, if you don't intend on using A and just wanted it as a scoping mechanism, you should use a module:

mod constants {
    const HELLO: &str = "hello";
    const WORLD: &str = "world";
}

which would be accessible via constants::HELLO and constants::WORLD.

---

If you wanted this as a static hash map, that would look like this:

use std::collections::HashMap;
use once_cell::sync::Lazy; // 1.15.0

static A: Lazy<HashMap<&str, &str>> = Lazy::new(|| {
    let mut map = HashMap::new();
    map.insert("HELLO", "hello");
    map.insert("WORLD", "world");
    map
});

In Rust, static variables must be initialized with a const expression, but since inserting into a HashMap is not const, we can use Lazy from the once-cell crate to initialize it lazily. This will obviously be different to access since it is not a set of known definitions: A.get("HELLO").unwrap()