The safety invariant of `SystemParam::validate_param` is not documented.

[spectria-limina]

Bevy version

0.15

What you did

I want to write my own SystemParam that can valid validation and therefore delay system running until validation passes.

What went wrong

validate_param takes an UnsafeWorldCell as a parameter, so I can't access the world unless my access is proven safe.

Expected behaviour: The documentation clearly states the conditions required for access to the UnsafeWorldCell to be safe.

Actual behaviour: There is no documentation on what would make that access safe. (There is only documentation on what makes calling validate_param unsafe, which is different. The safety invariants of validate_param are the caller's responsibility.)

[hymm]

I believe we need to say that init_state also needs to register any data accessed by get_state or validate_param here: https://docs.rs/bevy/latest/bevy/ecs/system/trait.SystemParam.html#safety