Simple `upsert` implementation for the rust ECS hecs

upsert_for_hecs.rs

/// Trait to extend hecs to support upsert operations.
/// ```rust
/// use Upsert;
/// let mut world = hecs::World::new();
/// struct s(u32);
///
/// world.upsert(player_entity, |mut c| c.0 = 42, s(42))?;
/// ```
pub trait Upsert {
    fn upsert<F, T: 'static>(&mut self, entity: Entity, update: F, insert: T) -> Result<()>
    where
        T: Copy + Send + Sync,
        F: FnOnce(RefMut<T>);
}

impl Upsert for hecs::World {
    fn upsert<F, T: 'static>(&mut self, entity: Entity, update: F, insert: T) -> Result<()>
    where
        T: Copy + Send + Sync,
        F: FnOnce(RefMut<T>),
    {
        let present = if let Ok(c) = self.get_mut::<T>(entity) {
            update(c);
            true
        } else {
            false
        };

        if !present {
            self.insert(entity, (insert,))?;
        }

        Ok(())
    }
}