Const ArcSwapOption doesn't update values

[Bauxitedev]

Hello, I am writing a program that uses a global ArcSwapOption<String>, but it seems the values I put into it aren't updated.

I call store to put something into it, but calling load afterwards still returns None.

A quick reproduction example: (you can run it directly with cargo script)

// cargo-deps: arc-swap="=1.5.0"
extern crate arc_swap;

use arc_swap::*;
use std::sync::Arc;

pub const SWAP: ArcSwapOption<String> = ArcSwapOption::const_empty();

fn main() {
    SWAP.store(Some(Arc::new("test".to_owned())));
    std::thread::sleep_ms(1000);
    dbg!(SWAP.load());
    dbg!(SWAP.load_full());
}

The program outputs...

[arc_swap_test.rs:12] SWAP.load() = None
[arc_swap_test.rs:13] SWAP.load_full() = None

...even though I clearly put the string test into the ArcSwapOption.

Am I doing something wrong here? Or is this a bug?

[Michal 'vorner' Vaner]

This is actually a weirdness of how const works, const is a compile time „bit pattern“ that can be copied any time it is used even though the type is not Copy. Therefore, it creates a new ArcSwapOption at the first load, stores it there and throws it away. Then it takes another (empty, original) copy of the const to load from. I'm not sure if I can prevent this trap from inside the crate in some way, I think (I'm not looking for it right now) it's even mentioned in the documentation somewhere to watch out for it.

Anyway, the solution is to use static SWAP: ... instead of const SWAP: ...

[Bauxitedev]

Ah I see, static fixed it. Thanks for the quick response.