Struct Has 

pub struct Has<T>(PhantomData<fn() -> T>);

Yields whether the archetype carries T.

The answer is read from the archetype’s component set, so it reaches no value and never conflicts: (&mut T, Has<T>) is legal. Use it instead of Option<&T>::is_some() to ask about a component without borrowing it.

It narrows nothing: every archetype matches, and a row whose archetype lacks T yields false. That is what lets a query report presence for rows it was going to visit anyway, and what keeps query_one::<Has<T>> answering Ok(false) rather than an error for an entity that lacks T. To visit only the archetypes carrying T, pair it with crate::QueryBorrow::by: .by(|archetype: &Archetype| archetype.has::<T>()).

Tuple Fields

0: PhantomData<fn() -> T>

Trait Implementations

impl<T: Clone> Clone for Has<T>

fn clone(&self) -> Self

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<T: Copy> Copy for Has<T>

impl<T: Debug> Debug for Has<T>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<T: 'static> Query for Has<T>

type Item<'a> = bool

What one matched row yields.

type Plan<'w> = bool

What is resolved once per archetype and then read once per row.

fn prepare<'w>(builder: &mut PlanBuilder<'w>) -> Option<Self::Plan<'w>>

Resolve the plan for one archetype.

fn read<'a, 'w: 'a>(plan: &'a mut Self::Plan<'w>, _: usize) -> Self::Item<'a>

Read the row at row through the resolved plan.