# Objective
System param validation warnings should be configurable and default to
"warn once" (per system).
Fixes: #15391
## Solution
`SystemMeta` is given a new `ParamWarnPolicy` field.
The policy decides whether warnings will be emitted by each system param
when it fails validation.
The policy is updated by the system after param validation fails.
Example warning:
```
2024-09-30T18:10:04.740749Z WARN bevy_ecs::system::function_system: System fallible_params::do_nothing_fail_validation will not run because it requested inaccessible system parameter Single<(), (With<Player>, With<Enemy>)>
```
Currently, only the first invalid parameter is displayed.
Warnings can be disabled on function systems using
`.param_never_warn()`.
(there is also `.with_param_warn_policy(policy)`)
## Testing
Ran `fallible_params` example.
---------
Co-authored-by: SpecificProtagonist <vincentjunge@posteo.net>
# Objective
Add a `Populated` system parameter that acts like `Query`, but prevents
system from running if there are no matching entities.
Fixes: #15302
## Solution
Implement the system param which newtypes the `Query`.
The only change is new validation, which fails if query is empty.
The new system param is used in `fallible_params` example.
## Testing
Ran `fallible_params` example.
---------
Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
# Objective
Add the following system params:
- `QuerySingle<D, F>` - Valid if only one matching entity exists,
- `Option<QuerySingle<D, F>>` - Valid if zero or one matching entity
exists.
As @chescock pointed out, we don't need `Mut` variants.
Fixes: #15264
## Solution
Implement the type and both variants of system params.
Also implement `ReadOnlySystemParam` for readonly queries.
Added a new ECS example `fallible_params` which showcases `SingleQuery`
usage.
In the future we might want to add `NonEmptyQuery`,
`NonEmptyEventReader` and `Res` to it (or maybe just stop at mentioning
it).
## Testing
Tested with the example.
There is a lot of warning spam so we might want to implement #15391.
