Query filters using T can Trigger conflict with Query<&mut T>

[PhaestusFox]

Bevy version

0.15

What you did

fn test(changed: Query<&Visibility, Changed>, mut transforms: Query<&mut Transform>) {
panic!()
}

What went wrong

Query filters trigger conflicting Query Error B0001

• what were you expecting?
  to be able to access components from one query while filtering for changes in another
• what actually happened?
  bevy panics saying I'm trying to access the same data mutably twice

Additional information

this might be working as intended. if so could we make that part of the error message since it suggests using Without to fix the conflict but that's not exactly helpful when it requires the component to have it changed

[hymm]

Is the filter supposed to say Changed<Transform> in your system?

[alice-i-cecile]

For Changed, this isn't a bug. Changed<T> needs to read the change ticks of T, which &mut T can mutate. This would create aliased mutability, and thus UB.

That said, we should be able to allow Added<T>, despite also accessing Ticks, since &mut T queries can't perform structural changes. I suspect that's currently banned, but I would appreciate a quick check,