Present Servo BGRA hardware surfaces

third_party/gpui/src/app/entity_map.rs

@@ -0,0 +1,889 @@
+use crate::{App, AppContext, GpuiBorrow, VisualContext, Window, seal::Sealed};
+use anyhow::{Context as _, Result};
+use collections::FxHashSet;
+use derive_more::{Deref, DerefMut};
+use parking_lot::{RwLock, RwLockUpgradableReadGuard};
+use slotmap::{KeyData, SecondaryMap, SlotMap};
+use std::{
+    any::{Any, TypeId, type_name},
+    cell::RefCell,
+    cmp::Ordering,
+    fmt::{self, Display},
+    hash::{Hash, Hasher},
+    marker::PhantomData,
+    mem,
+    num::NonZeroU64,
+    sync::{
+        Arc, Weak,
+        atomic::{AtomicU64, AtomicUsize, Ordering::SeqCst},
+    },
+    thread::panicking,
+};
+
+use super::Context;
+use crate::util::atomic_incr_if_not_zero;
+#[cfg(any(test, feature = "leak-detection"))]
+use collections::HashMap;
+
+slotmap::new_key_type! {
+    /// A unique identifier for a entity across the application.
+    pub struct EntityId;
+}
+
+impl From<u64> for EntityId {
+    fn from(value: u64) -> Self {
+        Self(KeyData::from_ffi(value))
+    }
+}
+
+impl EntityId {
+    /// Converts this entity id to a [NonZeroU64]
+    pub fn as_non_zero_u64(self) -> NonZeroU64 {
+        NonZeroU64::new(self.0.as_ffi()).unwrap()
+    }
+
+    /// Converts this entity id to a [u64]
+    pub fn as_u64(self) -> u64 {
+        self.0.as_ffi()
+    }
+}
+
+impl Display for EntityId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.as_u64())
+    }
+}
+
+pub(crate) struct EntityMap {
+    entities: SecondaryMap<EntityId, Box<dyn Any>>,
+    pub accessed_entities: RefCell<FxHashSet<EntityId>>,
+    ref_counts: Arc<RwLock<EntityRefCounts>>,
+}
+
+struct EntityRefCounts {
+    counts: SlotMap<EntityId, AtomicUsize>,
+    dropped_entity_ids: Vec<EntityId>,
+    #[cfg(any(test, feature = "leak-detection"))]
+    leak_detector: LeakDetector,
+}
+
+impl EntityMap {
+    pub fn new() -> Self {
+        Self {
+            entities: SecondaryMap::new(),
+            accessed_entities: RefCell::new(FxHashSet::default()),
+            ref_counts: Arc::new(RwLock::new(EntityRefCounts {
+                counts: SlotMap::with_key(),
+                dropped_entity_ids: Vec::new(),
+                #[cfg(any(test, feature = "leak-detection"))]
+                leak_detector: LeakDetector {
+                    next_handle_id: 0,
+                    entity_handles: HashMap::default(),
+                },
+            })),
+        }
+    }
+
+    /// Reserve a slot for an entity, which you can subsequently use with `insert`.
+    pub fn reserve<T: 'static>(&self) -> Slot<T> {
+        let id = self.ref_counts.write().counts.insert(1.into());
+        Slot(Entity::new(id, Arc::downgrade(&self.ref_counts)))
+    }
+
+    /// Insert an entity into a slot obtained by calling `reserve`.
+    pub fn insert<T>(&mut self, slot: Slot<T>, entity: T) -> Entity<T>
+    where
+        T: 'static,
+    {
+        let mut accessed_entities = self.accessed_entities.borrow_mut();
+        accessed_entities.insert(slot.entity_id);
+
+        let handle = slot.0;
+        self.entities.insert(handle.entity_id, Box::new(entity));
+        handle
+    }
+
+    /// Move an entity to the stack.
+    #[track_caller]
+    pub fn lease<T>(&mut self, pointer: &Entity<T>) -> Lease<T> {
+        self.assert_valid_context(pointer);
+        let mut accessed_entities = self.accessed_entities.borrow_mut();
+        accessed_entities.insert(pointer.entity_id);
+
+        let entity = Some(
+            self.entities
+                .remove(pointer.entity_id)
+                .unwrap_or_else(|| double_lease_panic::<T>("update")),
+        );
+        Lease {
+            entity,
+            id: pointer.entity_id,
+            entity_type: PhantomData,
+        }
+    }
+
+    /// Returns an entity after moving it to the stack.
+    pub fn end_lease<T>(&mut self, mut lease: Lease<T>) {
+        self.entities.insert(lease.id, lease.entity.take().unwrap());
+    }
+
+    pub fn read<T: 'static>(&self, entity: &Entity<T>) -> &T {
+        self.assert_valid_context(entity);
+        let mut accessed_entities = self.accessed_entities.borrow_mut();
+        accessed_entities.insert(entity.entity_id);
+
+        self.entities
+            .get(entity.entity_id)
+            .and_then(|entity| entity.downcast_ref())
+            .unwrap_or_else(|| double_lease_panic::<T>("read"))
+    }
+
+    fn assert_valid_context(&self, entity: &AnyEntity) {
+        debug_assert!(
+            Weak::ptr_eq(&entity.entity_map, &Arc::downgrade(&self.ref_counts)),
+            "used a entity with the wrong context"
+        );
+    }
+
+    pub fn extend_accessed(&mut self, entities: &FxHashSet<EntityId>) {
+        self.accessed_entities
+            .borrow_mut()
+            .extend(entities.iter().copied());
+    }
+
+    pub fn clear_accessed(&mut self) {
+        self.accessed_entities.borrow_mut().clear();
+    }
+
+    pub fn take_dropped(&mut self) -> Vec<(EntityId, Box<dyn Any>)> {
+        let mut ref_counts = self.ref_counts.write();
+        let dropped_entity_ids = mem::take(&mut ref_counts.dropped_entity_ids);
+        let mut accessed_entities = self.accessed_entities.borrow_mut();
+
+        dropped_entity_ids
+            .into_iter()
+            .filter_map(|entity_id| {
+                let count = ref_counts.counts.remove(entity_id).unwrap();
+                debug_assert_eq!(
+                    count.load(SeqCst),
+                    0,
+                    "dropped an entity that was referenced"
+                );
+                accessed_entities.remove(&entity_id);
+                // If the EntityId was allocated with `Context::reserve`,
+                // the entity may not have been inserted.
+                Some((entity_id, self.entities.remove(entity_id)?))
+            })
+            .collect()
+    }
+}
+
+#[track_caller]
+fn double_lease_panic<T>(operation: &str) -> ! {
+    panic!(
+        "cannot {operation} {} while it is already being updated",
+        std::any::type_name::<T>()
+    )
+}
+
+pub(crate) struct Lease<T> {
+    entity: Option<Box<dyn Any>>,
+    pub id: EntityId,
+    entity_type: PhantomData<T>,
+}
+
+impl<T: 'static> core::ops::Deref for Lease<T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        self.entity.as_ref().unwrap().downcast_ref().unwrap()
+    }
+}
+
+impl<T: 'static> core::ops::DerefMut for Lease<T> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        self.entity.as_mut().unwrap().downcast_mut().unwrap()
+    }
+}
+
+impl<T> Drop for Lease<T> {
+    fn drop(&mut self) {
+        if self.entity.is_some() && !panicking() {
+            panic!("Leases must be ended with EntityMap::end_lease")
+        }
+    }
+}
+
+#[derive(Deref, DerefMut)]
+pub(crate) struct Slot<T>(Entity<T>);
+
+/// A dynamically typed reference to a entity, which can be downcast into a `Entity<T>`.
+pub struct AnyEntity {
+    pub(crate) entity_id: EntityId,
+    pub(crate) entity_type: TypeId,
+    entity_map: Weak<RwLock<EntityRefCounts>>,
+    #[cfg(any(test, feature = "leak-detection"))]
+    handle_id: HandleId,
+}
+
+impl AnyEntity {
+    fn new(id: EntityId, entity_type: TypeId, entity_map: Weak<RwLock<EntityRefCounts>>) -> Self {
+        Self {
+            entity_id: id,
+            entity_type,
+            #[cfg(any(test, feature = "leak-detection"))]
+            handle_id: entity_map
+                .clone()
+                .upgrade()
+                .unwrap()
+                .write()
+                .leak_detector
+                .handle_created(id),
+            entity_map,
+        }
+    }
+
+    /// Returns the id associated with this entity.
+    pub fn entity_id(&self) -> EntityId {
+        self.entity_id
+    }
+
+    /// Returns the [TypeId] associated with this entity.
+    pub fn entity_type(&self) -> TypeId {
+        self.entity_type
+    }
+
+    /// Converts this entity handle into a weak variant, which does not prevent it from being released.
+    pub fn downgrade(&self) -> AnyWeakEntity {
+        AnyWeakEntity {
+            entity_id: self.entity_id,
+            entity_type: self.entity_type,
+            entity_ref_counts: self.entity_map.clone(),
+        }
+    }
+
+    /// Converts this entity handle into a strongly-typed entity handle of the given type.
+    /// If this entity handle is not of the specified type, returns itself as an error variant.
+    pub fn downcast<T: 'static>(self) -> Result<Entity<T>, AnyEntity> {
+        if TypeId::of::<T>() == self.entity_type {
+            Ok(Entity {
+                any_entity: self,
+                entity_type: PhantomData,
+            })
+        } else {
+            Err(self)
+        }
+    }
+}
+
+impl Clone for AnyEntity {
+    fn clone(&self) -> Self {
+        if let Some(entity_map) = self.entity_map.upgrade() {
+            let entity_map = entity_map.read();
+            let count = entity_map
+                .counts
+                .get(self.entity_id)
+                .expect("detected over-release of a entity");
+            let prev_count = count.fetch_add(1, SeqCst);
+            assert_ne!(prev_count, 0, "Detected over-release of a entity.");
+        }
+
+        Self {
+            entity_id: self.entity_id,
+            entity_type: self.entity_type,
+            entity_map: self.entity_map.clone(),
+            #[cfg(any(test, feature = "leak-detection"))]
+            handle_id: self
+                .entity_map
+                .upgrade()
+                .unwrap()
+                .write()
+                .leak_detector
+                .handle_created(self.entity_id),
+        }
+    }
+}
+
+impl Drop for AnyEntity {
+    fn drop(&mut self) {
+        if let Some(entity_map) = self.entity_map.upgrade() {
+            let entity_map = entity_map.upgradable_read();
+            let count = entity_map
+                .counts
+                .get(self.entity_id)
+                .expect("detected over-release of a handle.");
+            let prev_count = count.fetch_sub(1, SeqCst);
+            assert_ne!(prev_count, 0, "Detected over-release of a entity.");
+            if prev_count == 1 {
+                // We were the last reference to this entity, so we can remove it.
+                let mut entity_map = RwLockUpgradableReadGuard::upgrade(entity_map);
+                entity_map.dropped_entity_ids.push(self.entity_id);
+            }
+        }
+
+        #[cfg(any(test, feature = "leak-detection"))]
+        if let Some(entity_map) = self.entity_map.upgrade() {
+            entity_map
+                .write()
+                .leak_detector
+                .handle_released(self.entity_id, self.handle_id)
+        }
+    }
+}
+
+impl<T> From<Entity<T>> for AnyEntity {
+    fn from(entity: Entity<T>) -> Self {
+        entity.any_entity
+    }
+}
+
+impl Hash for AnyEntity {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.entity_id.hash(state);
+    }
+}
+
+impl PartialEq for AnyEntity {
+    fn eq(&self, other: &Self) -> bool {
+        self.entity_id == other.entity_id
+    }
+}
+
+impl Eq for AnyEntity {}
+
+impl Ord for AnyEntity {
+    fn cmp(&self, other: &Self) -> Ordering {
+        self.entity_id.cmp(&other.entity_id)
+    }
+}
+
+impl PartialOrd for AnyEntity {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl std::fmt::Debug for AnyEntity {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("AnyEntity")
+            .field("entity_id", &self.entity_id.as_u64())
+            .finish()
+    }
+}
+
+/// A strong, well-typed reference to a struct which is managed
+/// by GPUI
+#[derive(Deref, DerefMut)]
+pub struct Entity<T> {
+    #[deref]
+    #[deref_mut]
+    pub(crate) any_entity: AnyEntity,
+    pub(crate) entity_type: PhantomData<fn(T) -> T>,
+}
+
+impl<T> Sealed for Entity<T> {}
+
+impl<T: 'static> Entity<T> {
+    fn new(id: EntityId, entity_map: Weak<RwLock<EntityRefCounts>>) -> Self
+    where
+        T: 'static,
+    {
+        Self {
+            any_entity: AnyEntity::new(id, TypeId::of::<T>(), entity_map),
+            entity_type: PhantomData,
+        }
+    }
+
+    /// Get the entity ID associated with this entity
+    pub fn entity_id(&self) -> EntityId {
+        self.any_entity.entity_id
+    }
+
+    /// Downgrade this entity pointer to a non-retaining weak pointer
+    pub fn downgrade(&self) -> WeakEntity<T> {
+        WeakEntity {
+            any_entity: self.any_entity.downgrade(),
+            entity_type: self.entity_type,
+        }
+    }
+
+    /// Convert this into a dynamically typed entity.
+    pub fn into_any(self) -> AnyEntity {
+        self.any_entity
+    }
+
+    /// Grab a reference to this entity from the context.
+    pub fn read<'a>(&self, cx: &'a App) -> &'a T {
+        cx.entities.read(self)
+    }
+
+    /// Read the entity referenced by this handle with the given function.
+    pub fn read_with<R, C: AppContext>(
+        &self,
+        cx: &C,
+        f: impl FnOnce(&T, &App) -> R,
+    ) -> C::Result<R> {
+        cx.read_entity(self, f)
+    }
+
+    /// Updates the entity referenced by this handle with the given function.
+    pub fn update<R, C: AppContext>(
+        &self,
+        cx: &mut C,
+        update: impl FnOnce(&mut T, &mut Context<T>) -> R,
+    ) -> C::Result<R> {
+        cx.update_entity(self, update)
+    }
+
+    /// Updates the entity referenced by this handle with the given function.
+    pub fn as_mut<'a, C: AppContext>(&self, cx: &'a mut C) -> C::Result<GpuiBorrow<'a, T>> {
+        cx.as_mut(self)
+    }
+
+    /// Updates the entity referenced by this handle with the given function.
+    pub fn write<C: AppContext>(&self, cx: &mut C, value: T) -> C::Result<()> {
+        self.update(cx, |entity, cx| {
+            *entity = value;
+            cx.notify();
+        })
+    }
+
+    /// Updates the entity referenced by this handle with the given function if
+    /// the referenced entity still exists, within a visual context that has a window.
+    /// Returns an error if the entity has been released.
+    pub fn update_in<R, C: VisualContext>(
+        &self,
+        cx: &mut C,
+        update: impl FnOnce(&mut T, &mut Window, &mut Context<T>) -> R,
+    ) -> C::Result<R> {
+        cx.update_window_entity(self, update)
+    }
+}
+
+impl<T> Clone for Entity<T> {
+    fn clone(&self) -> Self {
+        Self {
+            any_entity: self.any_entity.clone(),
+            entity_type: self.entity_type,
+        }
+    }
+}
+
+impl<T> std::fmt::Debug for Entity<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Entity")
+            .field("entity_id", &self.any_entity.entity_id)
+            .field("entity_type", &type_name::<T>())
+            .finish()
+    }
+}
+
+impl<T> Hash for Entity<T> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.any_entity.hash(state);
+    }
+}
+
+impl<T> PartialEq for Entity<T> {
+    fn eq(&self, other: &Self) -> bool {
+        self.any_entity == other.any_entity
+    }
+}
+
+impl<T> Eq for Entity<T> {}
+
+impl<T> PartialEq<WeakEntity<T>> for Entity<T> {
+    fn eq(&self, other: &WeakEntity<T>) -> bool {
+        self.any_entity.entity_id() == other.entity_id()
+    }
+}
+
+impl<T: 'static> Ord for Entity<T> {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        self.entity_id().cmp(&other.entity_id())
+    }
+}
+
+impl<T: 'static> PartialOrd for Entity<T> {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+/// A type erased, weak reference to a entity.
+#[derive(Clone)]
+pub struct AnyWeakEntity {
+    pub(crate) entity_id: EntityId,
+    entity_type: TypeId,
+    entity_ref_counts: Weak<RwLock<EntityRefCounts>>,
+}
+
+impl AnyWeakEntity {
+    /// Get the entity ID associated with this weak reference.
+    pub fn entity_id(&self) -> EntityId {
+        self.entity_id
+    }
+
+    /// Check if this weak handle can be upgraded, or if the entity has already been dropped
+    pub fn is_upgradable(&self) -> bool {
+        let ref_count = self
+            .entity_ref_counts
+            .upgrade()
+            .and_then(|ref_counts| Some(ref_counts.read().counts.get(self.entity_id)?.load(SeqCst)))
+            .unwrap_or(0);
+        ref_count > 0
+    }
+
+    /// Upgrade this weak entity reference to a strong reference.
+    pub fn upgrade(&self) -> Option<AnyEntity> {
+        let ref_counts = &self.entity_ref_counts.upgrade()?;
+        let ref_counts = ref_counts.read();
+        let ref_count = ref_counts.counts.get(self.entity_id)?;
+
+        if atomic_incr_if_not_zero(ref_count) == 0 {
+            // entity_id is in dropped_entity_ids
+            return None;
+        }
+        drop(ref_counts);
+
+        Some(AnyEntity {
+            entity_id: self.entity_id,
+            entity_type: self.entity_type,
+            entity_map: self.entity_ref_counts.clone(),
+            #[cfg(any(test, feature = "leak-detection"))]
+            handle_id: self
+                .entity_ref_counts
+                .upgrade()
+                .unwrap()
+                .write()
+                .leak_detector
+                .handle_created(self.entity_id),
+        })
+    }
+
+    /// Assert that entity referenced by this weak handle has been released.
+    #[cfg(any(test, feature = "leak-detection"))]
+    pub fn assert_released(&self) {
+        self.entity_ref_counts
+            .upgrade()
+            .unwrap()
+            .write()
+            .leak_detector
+            .assert_released(self.entity_id);
+
+        if self
+            .entity_ref_counts
+            .upgrade()
+            .and_then(|ref_counts| Some(ref_counts.read().counts.get(self.entity_id)?.load(SeqCst)))
+            .is_some()
+        {
+            panic!(
+                "entity was recently dropped but resources are retained until the end of the effect cycle."
+            )
+        }
+    }
+
+    /// Creates a weak entity that can never be upgraded.
+    pub fn new_invalid() -> Self {
+        /// To hold the invariant that all ids are unique, and considering that slotmap
+        /// increases their IDs from `0`, we can decrease ours from `u64::MAX` so these
+        /// two will never conflict (u64 is way too large).
+        static UNIQUE_NON_CONFLICTING_ID_GENERATOR: AtomicU64 = AtomicU64::new(u64::MAX);
+        let entity_id = UNIQUE_NON_CONFLICTING_ID_GENERATOR.fetch_sub(1, SeqCst);
+
+        Self {
+            // Safety:
+            //   Docs say this is safe but can be unspecified if slotmap changes the representation
+            //   after `1.0.7`, that said, providing a valid entity_id here is not necessary as long
+            //   as we guarantee that `entity_id` is never used if `entity_ref_counts` equals
+            //   to `Weak::new()` (that is, it's unable to upgrade), that is the invariant that
+            //   actually needs to be hold true.
+            //
+            //   And there is no sane reason to read an entity slot if `entity_ref_counts` can't be
+            //   read in the first place, so we're good!
+            entity_id: entity_id.into(),
+            entity_type: TypeId::of::<()>(),
+            entity_ref_counts: Weak::new(),
+        }
+    }
+}
+
+impl std::fmt::Debug for AnyWeakEntity {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct(type_name::<Self>())
+            .field("entity_id", &self.entity_id)
+            .field("entity_type", &self.entity_type)
+            .finish()
+    }
+}
+
+impl<T> From<WeakEntity<T>> for AnyWeakEntity {
+    fn from(entity: WeakEntity<T>) -> Self {
+        entity.any_entity
+    }
+}
+
+impl Hash for AnyWeakEntity {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.entity_id.hash(state);
+    }
+}
+
+impl PartialEq for AnyWeakEntity {
+    fn eq(&self, other: &Self) -> bool {
+        self.entity_id == other.entity_id
+    }
+}
+
+impl Eq for AnyWeakEntity {}
+
+impl Ord for AnyWeakEntity {
+    fn cmp(&self, other: &Self) -> Ordering {
+        self.entity_id.cmp(&other.entity_id)
+    }
+}
+
+impl PartialOrd for AnyWeakEntity {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+/// A weak reference to a entity of the given type.
+#[derive(Deref, DerefMut)]
+pub struct WeakEntity<T> {
+    #[deref]
+    #[deref_mut]
+    any_entity: AnyWeakEntity,
+    entity_type: PhantomData<fn(T) -> T>,
+}
+
+impl<T> std::fmt::Debug for WeakEntity<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct(type_name::<Self>())
+            .field("entity_id", &self.any_entity.entity_id)
+            .field("entity_type", &type_name::<T>())
+            .finish()
+    }
+}
+
+impl<T> Clone for WeakEntity<T> {
+    fn clone(&self) -> Self {
+        Self {
+            any_entity: self.any_entity.clone(),
+            entity_type: self.entity_type,
+        }
+    }
+}
+
+impl<T: 'static> WeakEntity<T> {
+    /// Upgrade this weak entity reference into a strong entity reference
+    pub fn upgrade(&self) -> Option<Entity<T>> {
+        Some(Entity {
+            any_entity: self.any_entity.upgrade()?,
+            entity_type: self.entity_type,
+        })
+    }
+
+    /// Updates the entity referenced by this handle with the given function if
+    /// the referenced entity still exists. Returns an error if the entity has
+    /// been released.
+    pub fn update<C, R>(
+        &self,
+        cx: &mut C,
+        update: impl FnOnce(&mut T, &mut Context<T>) -> R,
+    ) -> Result<R>
+    where
+        C: AppContext,
+        Result<C::Result<R>>: crate::Flatten<R>,
+    {
+        crate::Flatten::flatten(
+            self.upgrade()
+                .context("entity released")
+                .map(|this| cx.update_entity(&this, update)),
+        )
+    }
+
+    /// Updates the entity referenced by this handle with the given function if
+    /// the referenced entity still exists, within a visual context that has a window.
+    /// Returns an error if the entity has been released.
+    pub fn update_in<C, R>(
+        &self,
+        cx: &mut C,
+        update: impl FnOnce(&mut T, &mut Window, &mut Context<T>) -> R,
+    ) -> Result<R>
+    where
+        C: VisualContext,
+        Result<C::Result<R>>: crate::Flatten<R>,
+    {
+        let window = cx.window_handle();
+        let this = self.upgrade().context("entity released")?;
+
+        crate::Flatten::flatten(window.update(cx, |_, window, cx| {
+            this.update(cx, |entity, cx| update(entity, window, cx))
+        }))
+    }
+
+    /// Reads the entity referenced by this handle with the given function if
+    /// the referenced entity still exists. Returns an error if the entity has
+    /// been released.
+    pub fn read_with<C, R>(&self, cx: &C, read: impl FnOnce(&T, &App) -> R) -> Result<R>
+    where
+        C: AppContext,
+        Result<C::Result<R>>: crate::Flatten<R>,
+    {
+        crate::Flatten::flatten(
+            self.upgrade()
+                .context("entity released")
+                .map(|this| cx.read_entity(&this, read)),
+        )
+    }
+
+    /// Create a new weak entity that can never be upgraded.
+    pub fn new_invalid() -> Self {
+        Self {
+            any_entity: AnyWeakEntity::new_invalid(),
+            entity_type: PhantomData,
+        }
+    }
+}
+
+impl<T> Hash for WeakEntity<T> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.any_entity.hash(state);
+    }
+}
+
+impl<T> PartialEq for WeakEntity<T> {
+    fn eq(&self, other: &Self) -> bool {
+        self.any_entity == other.any_entity
+    }
+}
+
+impl<T> Eq for WeakEntity<T> {}
+
+impl<T> PartialEq<Entity<T>> for WeakEntity<T> {
+    fn eq(&self, other: &Entity<T>) -> bool {
+        self.entity_id() == other.any_entity.entity_id()
+    }
+}
+
+impl<T: 'static> Ord for WeakEntity<T> {
+    fn cmp(&self, other: &Self) -> Ordering {
+        self.entity_id().cmp(&other.entity_id())
+    }
+}
+
+impl<T: 'static> PartialOrd for WeakEntity<T> {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+#[cfg(any(test, feature = "leak-detection"))]
+static LEAK_BACKTRACE: std::sync::LazyLock<bool> =
+    std::sync::LazyLock::new(|| std::env::var("LEAK_BACKTRACE").is_ok_and(|b| !b.is_empty()));
+
+#[cfg(any(test, feature = "leak-detection"))]
+#[derive(Clone, Copy, Debug, Default, Hash, PartialEq, Eq)]
+pub(crate) struct HandleId {
+    id: u64, // id of the handle itself, not the pointed at object
+}
+
+#[cfg(any(test, feature = "leak-detection"))]
+pub(crate) struct LeakDetector {
+    next_handle_id: u64,
+    entity_handles: HashMap<EntityId, HashMap<HandleId, Option<backtrace::Backtrace>>>,
+}
+
+#[cfg(any(test, feature = "leak-detection"))]
+impl LeakDetector {
+    #[track_caller]
+    pub fn handle_created(&mut self, entity_id: EntityId) -> HandleId {
+        let id = util::post_inc(&mut self.next_handle_id);
+        let handle_id = HandleId { id };
+        let handles = self.entity_handles.entry(entity_id).or_default();
+        handles.insert(
+            handle_id,
+            LEAK_BACKTRACE.then(backtrace::Backtrace::new_unresolved),
+        );
+        handle_id
+    }
+
+    pub fn handle_released(&mut self, entity_id: EntityId, handle_id: HandleId) {
+        let handles = self.entity_handles.entry(entity_id).or_default();
+        handles.remove(&handle_id);
+    }
+
+    pub fn assert_released(&mut self, entity_id: EntityId) {
+        let handles = self.entity_handles.entry(entity_id).or_default();
+        if !handles.is_empty() {
+            for backtrace in handles.values_mut() {
+                if let Some(mut backtrace) = backtrace.take() {
+                    backtrace.resolve();
+                    eprintln!("Leaked handle: {:#?}", backtrace);
+                } else {
+                    eprintln!("Leaked handle: export LEAK_BACKTRACE to find allocation site");
+                }
+            }
+            panic!();
+        }
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use crate::EntityMap;
+
+    struct TestEntity {
+        pub i: i32,
+    }
+
+    #[test]
+    fn test_entity_map_slot_assignment_before_cleanup() {
+        // Tests that slots are not re-used before take_dropped.
+        let mut entity_map = EntityMap::new();
+
+        let slot = entity_map.reserve::<TestEntity>();
+        entity_map.insert(slot, TestEntity { i: 1 });
+
+        let slot = entity_map.reserve::<TestEntity>();
+        entity_map.insert(slot, TestEntity { i: 2 });
+
+        let dropped = entity_map.take_dropped();
+        assert_eq!(dropped.len(), 2);
+
+        assert_eq!(
+            dropped
+                .into_iter()
+                .map(|(_, entity)| entity.downcast::<TestEntity>().unwrap().i)
+                .collect::<Vec<i32>>(),
+            vec![1, 2],
+        );
+    }
+
+    #[test]
+    fn test_entity_map_weak_upgrade_before_cleanup() {
+        // Tests that weak handles are not upgraded before take_dropped
+        let mut entity_map = EntityMap::new();
+
+        let slot = entity_map.reserve::<TestEntity>();
+        let handle = entity_map.insert(slot, TestEntity { i: 1 });
+        let weak = handle.downgrade();
+        drop(handle);
+
+        let strong = weak.upgrade();
+        assert_eq!(strong, None);
+
+        let dropped = entity_map.take_dropped();
+        assert_eq!(dropped.len(), 1);
+
+        assert_eq!(
+            dropped
+                .into_iter()
+                .map(|(_, entity)| entity.downcast::<TestEntity>().unwrap().i)
+                .collect::<Vec<i32>>(),
+            vec![1],
+        );
+    }
+}