Bevy linter

[alice-i-cecile]

What problem does this solve or what need does it fill?

A linter could be quite helpful to catch and help correct obvious mistakes in ways the compiler can't, or warn about bad practices in Bevy code.

What solution would you like?

A command line tool is introduced to lint Bevy apps. As some initial ideas, this could catch:

• malformed systems (see Better error messages for IntoSystem and IntoExclusive System #1519)
• unit structs in the first type parameter of Query (these should just use With)
• using Not<With<T>> or Not<Without<T>> or the like

What alternative(s) have you considered?

Some of this can be implemented either in compiler errors or run-time errors / warnings. The former has serious technical limitations (see #1519), and the latter is noisy and hard to configure.

Clippy doesn't appear to support custom lints, but it would be quite nice to integrate with it if it did.

Additional context

This is not super high priority, but serves as a nice place to track features that we could put in it. This would be part of #436, but should be able to stand alone.

[jakobhellermann]

As suggested in the discord, this can also be done as a procedural macro.
I started implementing such a macro in https://github.com./jakobhellermann/bevycheck.

[alice-i-cecile]

Dylint is a new crate that dynamically loads custom lints and then runs them with a cargo command. This seems like the perfect tool for us to use here.

[alice-i-cecile]

Decrees for bevy engine dev:

I like sorting by "commonness" and "type". Standard derives like Debug / Clone first, custom/most descriptive last (ex: StageLabel). Related derives like Copy/Clone, Eq/PartialEq should live together

[alice-i-cecile]

An existing project for defining custom lints in Rust that could use help getting off the ground: https://github.com./rust-linting

[alice-i-cecile]

A dylint-based prototype has been made by @MinerSebas: https://github.com./MinerSebas/bevy_lint

[alice-i-cecile]

I've written up a few of our use cases for this in the new rust-linting repository as they're looking for user stories!

1. User story: redundant query parameters in Bevy rust-marker/design#18
2. User story: invalid system parameter in Bevy system rust-marker/design#19
3. User story: manual implementation of semi-sealed trait rust-marker/design#20