bevy_ecs 0.18.1

Bevy Engine's entity component system
Documentation 
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use bevy_utils::prelude::DebugName;

use crate::{
    archetype::ArchetypeId,
    entity::{Entity, EntityNotSpawnedError},
};

/// An error that occurs when retrieving a specific [`Entity`]'s query result from [`Query`](crate::system::Query) or [`QueryState`](crate::query::QueryState).
// TODO: return the type_name as part of this error
#[derive(thiserror::Error, Clone, Copy, Debug, PartialEq, Eq)]
pub enum QueryEntityError {
    /// The given [`Entity`]'s components do not match the query.
    ///
    /// Either it does not have a requested component, or it has a component which the query filters out.
    #[error("The query does not match entity {0}")]
    QueryDoesNotMatch(Entity, ArchetypeId),
    /// The given [`Entity`] is not spawned.
    #[error("{0}")]
    NotSpawned(#[from] EntityNotSpawnedError),
    /// The [`Entity`] was requested mutably more than once.
    ///
    /// See [`Query::get_many_mut`](crate::system::Query::get_many_mut) for an example.
    #[error("The entity with ID {0} was requested mutably more than once")]
    AliasedMutability(Entity),
}

/// An error that occurs when evaluating a [`Query`](crate::system::Query) or [`QueryState`](crate::query::QueryState) as a single expected result via
/// [`single`](crate::system::Query::single) or [`single_mut`](crate::system::Query::single_mut).
#[derive(Debug, thiserror::Error)]
pub enum QuerySingleError {
    /// No entity fits the query.
    #[error("No entities fit the query {0}")]
    NoEntities(DebugName),
    /// Multiple entities fit the query.
    #[error("Multiple entities fit the query {0}")]
    MultipleEntities(DebugName),
}

#[cfg(test)]
mod test {
    use crate::{prelude::World, query::QueryEntityError};
    use bevy_ecs_macros::Component;

    #[test]
    fn query_does_not_match() {
        let mut world = World::new();

        #[derive(Component)]
        struct Present1;
        #[derive(Component)]
        struct Present2;
        #[derive(Component, Debug, PartialEq)]
        struct NotPresent;

        let entity = world.spawn((Present1, Present2));

        let (entity, archetype_id) = (entity.id(), entity.archetype().id());

        let result = world.query::<&NotPresent>().get(&world, entity);

        assert_eq!(
            result,
            Err(QueryEntityError::QueryDoesNotMatch(entity, archetype_id))
        );
    }
}