Linter: unused_appexit

[BD103]

Closes #87.

This is a more generic form of main_return_without_appexit that checks if the AppExit returned from App::run() is handled at all, no matter the context. Please check out the module documentation for unused_appexit.rs for more info!

[musjj]

Actually, is there a reason this is a custom lint? Why not just add a #[must_use] attribute upstream?

[BD103]

Actually, is there a reason this is a custom lint? Why not just add a #[must_use] attribute upstream?

The reason we use a lint is because you cannot opt-out of #[must_use]. In the end, handling the AppExit isn't all that important in the grand scheme of things. That's why the lint is pedantic, and off by default. This lint is a "nice-to-have", but not a must.