Another crazy idea I share with this website.

I was developing a game and an engine in Rust, so I was reading many articles, most of which criticize the ‘borrow checker’.

I know that Rust is a big agenda language, and the extreme ‘borrow checker’ shows that, but if it weren’t for the checker, Rust would be a straight-up better C++ for Game development, so I thought: “Why not just use unsafe?”, but the truth is: unsafe is not ergonomic, and so is Refcell<T> so after thinking for a bit, I came up with this pattern:

let mut enemies = if cfg!(debug_assertions) {
    // We use `expect()` in debug mode as a layer of safety in order
    // to detect any possibility of undefined bahavior.
    enemies.expect("*message*");
    } else {
    // SAFETY: The `if` statement (if self.body.overlaps...) must
    // run only once, and it is the only thing that can make
    // `self.enemies == None`.
    unsafe { enemies.unwrap_unchecked() }
};

You can also use the same pattern to create a RefCell<T> clone that only does its checks in ‘debug’ mode, but I didn’t test that; it’s too much of an investment until I get feedback for the idea.

This has several benefits:

1 - No performance drawbacks, the compiler optimizes away the if statement if opt-level is 1 or more. (source: Compiler Explorer)

2 - It’s as safe as expect() for all practical use cases, since you’ll run the game in debug mode 1000s of times, and you’ll know it doesn’t produce Undefined Behavior If it doesn’t crash.

You can also wrap it in a “safe” API for convenience:

// The 'U' stands for 'unsafe'.
pub trait UnwrapUExt {
    type Target;

    fn unwrap_u(self) -> Self::Target;
}

impl<T> UnwrapUExt for Option<T> {
    type Target = T;

    fn unwrap_u(self) -> Self::Target {
        if cfg!(debug_assertions) {
            self.unwrap()
        } else {
            unsafe { self.unwrap_unchecked() }
        }
    }
}

I imagine you can do many cool things with these probably-safe APIs, an example of which is macroquad’s possibly unsound usage of get_context() to acquire a static mut variable.

Game development is a risky business, and while borrow-checking by default is nice, just like immutability-by-default, we shouldn’t feel bad about disabling it, as forcing it upon ourselves is like forcing immutability, just like Haskell does, and while it has 100% side-effect safety, you don’t use much software that’s written in Haskell, do you?

Conclusion: we shouldn’t fear unsafe even when it’s probably unsafe, and we must remember that we’re programming a computer, a machine built upon chaotic mutable state, and that our languages are but an abstraction around assembly.

  • lad@programming.dev
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    2 months ago

    I see now, that you were misunderstood in some parts.

    even if I got reported a really weird bug related to UB, I should (I am not experienced enough to make a claim) be able to know it’s UB since the game’s gonna crash when I try to recreate the bug in Debug.

    This may be problematic for several reasons: it may be hard to reproduce, the more complicated the state, the harder; bug may rely on some race condition that may be much rarer in Debug because of speed difference; UB is notorious for causing things that should (seemingly) never happen, like returning from infinite loops, proving false statements true, and such, so it may be hard to understand what at all happened and why.

    Regarding optimisations, it might still be better to try to profile the code (I will be honest, I don’t do that until the moment when I can’t go further without optimisation, and I haven’t reached that with Rust) and see what are the real hot spots that require optimisations. I hope that someday you will be able to upgrade your machine, and hope that your game will be a good example of something that really runs anywhere

    • Doods@infosec.pubOP
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      2 months ago

      Oh no don’t get me wrong, a year back I upgraded to an I5-7500 prebuilt, and it’s a beast for all my tasks. (maybe compiling is quick because I split modules a little too much?)

      Your advice is good for not knowing what I’m making. If I was making something multi threaded with much state I would fear UB more.

      may be much rarer in Debug because of speed difference

      Thanks, then I will remember to recreate bugs with opt-level = 3.

      Wait no, this doesn’t make sense if I don’t have access to the user’s machine, maybe I should send him a log-heavy version of some sort? How should I even what I am supposed to log? I should think about this some more before release.