Add lint: bevy::main_return_without_appexit

[BD103]

This is the first lint! Closes #53. Please see the module documentation or this link for information about what the lint does.

Design decisions

• Lint modules are there documentation, and as such are public.

  

• lints/mod.rs has the LINTS static for register_passes() function, so you don't have to remember to touch BevyLintCallback each time you add a lint.

• For now, lints are going to be manually added as modules, to LINTS, and to register_passes(). I tried using a macro, but it was too complicated and not worth it.

• Lints should be declared with the builtin declare_tool_lint! and declare_lint_pass! macros.

Current drawbacks

• This lint does not detect if you emit AppExit through some other means, such as:

  fn main() {
    let app_exit = App::new().run();
    println!("{app_exit:?}");
  }

  I found this too complicated for a first lint, so I'm going to create a follow-up issue to tackle this.

• Lints currently do not support be silenced / changing levels. #[allow(bevy::main_return_without_appexit)], -A bevy::main_return_without_appexit, and adding main_return_without_appexit = "allow" to Cargo.toml all fail. This is another follow-up issue.

• I have not created the lint groups yet, so this is not within the bevy::style category yet. This is the final follow-up that I know of.

Testing!

Now for the fun part! Because there are no UI tests (yet), manually paste the following in bevy_lint/examples/lint_test.rs:

use bevy::prelude::*;

fn main() {
    App::new().run();
}

Then, run the following:

cd bevy_lint
cargo build
cargo run -- --example lint_test

The outputted warning should look like this:

warning: an entrypoint that calls `App::run()` does not return `AppExit`
 --> bevy_lint/examples/lint_test.rs:4:16
  |
3 | fn main() {
  |          - help: try: `-> AppExit`
4 |     App::new().run();
  |                ^^^^^
  |
  = note: `App::run()` returns `AppExit`, which can be used to determine whether the app exited successfully or not
  = note: `#[warn(bevy::main_return_without_appexit)]` on by default

[bas-ie]

Confirmed insofar it works, and goes away when I fix the problem 🙂

[janhohenheim]

Great stuff, tested it and it works very reliably. Left some minor suggestions, nothing blocking.