Oh clever. Yeah, this will catch the "I forget to call run" problem that beginners hit sometimes!

It won't help with that, sadly :(
I think this only adds a warning if you do something silly like:

App::new();

Not worth adding in my opinion. Unless, there's some other example in which this helps, @doonv?

Oh clever. Yeah, this will catch the "I forget to call run" problem that beginners hit sometimes!

It won't help with that, sadly :( I think this only adds a warning if you do something silly like:

App::new();

Not worth adding in my opinion. Unless, there's some other example in which this helps, @doonv?

Yeah unfortunately there's not much you can do about that apart from making App's methods also #[must_use], but that comes at the cost of not allowing non-chained method calls.

Making App #[must_use] still helps in some rare situations though, there's no reason not to add it.

Would be good to add must_use in App::update. that would say to the user that they must use either update or run (as run call update internally)
But this can be a bit tricky, maybe the Runner isn't sure that it'll call update 🤔

prior work #5395

bors r+

Oh clever. Yeah, this will catch the "I forget to call run" problem that beginners hit sometimes!

It won't help with that, sadly :( I think this only adds a warning if you do something silly like:

App::new();

Not worth adding in my opinion. Unless, there's some other example in which this helps, @doonv?

I'm pretty sure this is wrong. Since nearly all App functions take an owned App and returns an App, the returned App will also be must_use. So even if you do: App::new().add_system(Update, system1).add_system(Update, system2);, it will still complain. It will only be satisfied if you call .run() or store it in a variable.

I tested this out with a very simple example:

my-bevy/src/lib.rs

#[must_use]
pub struct MyBuilder;

impl MyBuilder {
    pub fn new() -> Self {
        Self
    }

    pub fn a(self) -> Self {
        self
    }
    pub fn b(self) -> Self {
        self
    }
    pub fn c(self) -> Self {
        self
    }
}

my-bin/src/main.rs

use my_bevy::MyBuilder;

fn main() {
    MyBuilder::new().a().b().c();
}

This still gives me warnings about my unused MyBuilder.

Edit: Therefore, App should also give the same warnings with this PR.

The methods on App take and return &mut Self, not Self.
App::new().add_system(Update, system); won't trigger the warning from #[must_use] because of that, I did test this before I commented before :)

Adding #[must_use] to a type also adds this warning to any API that returns the type downstream.
I'd still err on not adding it to avoid any potential bother that might cause or any confusion about why App would be marked as #[must_use].