Gave bevy::app::App the must_use attribute

[shoobooshooboo]

Objective

Attempts #5386

Solution

Added the must_use attribute to bevy::app::App

Testing

I tested it by linking the library to another cargo package and calling App::new() without using the return value.

(this is my first pr, so i just chose something very easy.)

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[mockersf]

This doesn't fix #5386

If you add plugins but still don't call .run(), it won't issue a warning

[shoobooshooboo]

thanks for the feedback! I'll look into this now

[mockersf]

The base issue is not really trivial in my opinion. This PR is good and could be merged as is, but it shouldn't say it fixes the issue as that will close it.

You can spend some time to investigate in this PR if you want, or you can edit the description and we can merge this now while you investigate with another PR

[shoobooshooboo]

So I found the obvious solution of just marking all of the App functions that return &mut Self as must_use, but I was wondering if this is actually a good solution. The original purpose of giving App the must_use trait was for cases like the following:

App::new()
.add_plugins(DefaultPlugins)
.insert_resource(MyResource::Default())
.run();

The point is to give us a warning if we ever forget the last line. But this doesn't support the other, completely valid use which is more along the lines of

let app = App::new();
app.add_plugins(DefaultPlugins);
app.insert_resource(MyResource::Default());
app.run();

Because in this case, it'll warn us on both of those middle lines if we don't put let _ = before them.
I have an idea for how I could let the user know they forgot to run the app, but it would involve a runtime check so it'd have to be a panic! instead of a warning. My idea also accounts for allowing an app to not run if you so choose. Should I draft up this idea or is a panic! too strict?

[alice-i-cecile]

Hmm, yeah, I don't think we should break the multi-line pattern.

I'm a bit skeptical of the panic idea: I think that it's a lot of complexity and risk to solve a beginner footgun.

Simply marking App as must_use is better than nothing though, so I feel like we should merge this PR.

[mockersf]

no panic for me: the idea is to warn the user in their IDE, they'll notice soon enough that nothing happens when they run it

[shoobooshooboo]

alright, sounds good. I'll edit the description to remove the fix. Thanks for the good feedback!