Dependency injection of a SeedableRng in a Rust project with `rand::rngs::ThreadRng`

Question

I'm starting with a regular &mut ThreadRng in all my functions, but it limits dependency injection capacities. There are SeedableRng ofc, but how to use it in places where initially it was a ThreadRng ?

Coming from other languages I'd like to specify some interface in the fn declaration, but &mut dyn Rng doesn't work because Rng is not object safe.

What I ended up with is passing &mut StdRng everywhere and

let mut rng = match config.seed {
  Some(seed) => StdRng::seed_from_u64(seed),
  _ => StdRng::from_rng(rand::thread_rng()).unwrap_or(StdRng::from_entropy())
};

But it's awkward to init 1 Rng only to setup another Rng

I also searched the pattern StdRng::from_rng(rand::thread_rng()) and it's just 56 samples in github, from 141k of using rand::thread_rng()

So what's the best signature for a fn accepting Rng and the best way to init with an optional seed?

p.s. I don't care about wasm/etc runtimes, it's just a regular server-side app

Answer 1

The rand documentation has a bit about generic usage in it you can find the following:

Since Rng: RngCore and every RngCore implements Rng, it makes no difference whether we use R: Rng or R: RngCore

From that we can deduce, that we can just use the object safe RngCore in place of Rng in situations that require dynamic dispatch:

use rand::{Rng, RngCore};use rand::rngs::StdRng;
fn main() {
    let seed = None;
    let mut std_rng;
    let mut thread_rng;
    let rng: &mut dyn RngCore = match seed {
        Some(seed) => {
            std_rng = StdRng::seed_from_u64(seed);
            &mut std_rng
        }
        _ => {
            thread_rng = rand::thread_rng();
            &mut thread_rng
        }
    };
    
    random(rng);
}

fn random(rng: &mut dyn RngCore) {
    println!("{}", rng.gen_range(0..9));
}

Though the more idiomatic way would be to use static dispatch instead, and that does work with Rng:

fn random(mut rng: impl Rng) {
    println!("{}", rng.gen_range(0..9));
}