Unused Component/Resource Warning

[Braymatter]

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

It would be very nice if we had some tool that would emit a warning whenever a component or a resource is not a part of any query in the application.

Sometimes this scenario can result in hard to track down bugs.

We have a DespawnAfterDuration component / system we use. After moving the parent plugin around to a subcrate, but not remounting the plugin to the app, the component never gets processed as the system/plugin aren't added to the app builder.

Often times when moving code around / refactoring we'll forget to add a plugin / system etc. and we run into these sorts of things. Also very common for me when adding entirely new plugins / systems.

What solution would you like?

Ideally a clippy lint / dead_code would be best I think.

What alternative(s) have you considered?

I've attempted to just be better, but it hasn't worked out yet.

[nicopap]

A lead would be to accumulate the FilteredAccess of all the schedules registered in the App, OR-ing the write, read-write, with, without fields, and check if there is any 0s in the resulting bitset.

[hymm]

A lead would be to accumulate the FilteredAccess of all the schedules registered in the App, OR-ing the write, read-write, with, without fields, and check if there is any 0s in the resulting bitset.

I don't think this will work. Components are registered when they're spawned or used in systems.

[hymm]

This is the whole life before main problem that needs support from rust to fix nicely.

The only way checking the schedule could work is if the user manually calls init_component on the world.

[tormeh]

Some related discussion:

• Feature request: custom library-specific lints rust-lang/rust-clippy#6872
• Bevy linter #1602