Add Has query parameter

[alice-i-cecile]

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

A common pattern is to use Option<&C> query parameters, then check if it's .is_some.

Unfortunately, this is slightly more verbose than needed (and the intent is less clear). More critically, it pointlessly locks the resource to a higher permissions level than needed.

What solution would you like?

Add a Has<C: Component> query parameter, which returns true if the entity has the particular component. This does not require any read permissions, as components cannot be changed during the execution of a parallel stage.

What alternative(s) have you considered?

Use the existing strategy.

Additional context

Idea blatantly borrowed from @Ralith's HECS, where it is called Satisfies, and can also be used to check whether an entity matches a given query. We don't need that particular bit of functionality: if anything, a simple Query::contains(entity) method would work well (but should be a seperate PR). See #3089 for that issue.

A Not query parameter, used for this and query filters, might be helpful for clarity but is not essential.

[BoxyUwU]

Option<With<T>> and then calling .is_some() is the same behaviour as Has<T> isnt it?

[alice-i-cecile]

Option<With> and then calling .is_some() is the same behaviour as Has isnt it?

Yes, but the intent to the reader is dramatically less clear. I don't think I've ever seen With<T> being used in a non-filtering position (and had to verify that this in fact works).

[MinerSebas]

Another "problem" with Option<With<T>> is, that the Some(false) will never occur, because either the Entity has it (Some(true)) or it doesn't (None). #2665 (comment)

[alice-i-cecile]

Closing this out; I don't think it's useful enough to be worth expanding the API.

If we can only have one, I think it needs to be With because of the symmetry with Without. Has and DoesNotHave is a no-go.

• cart in Add Has WorldQuery #3093.