Hash stability guarantees (#11690)

# Objective

We currently over/underpromise hash stability:
- `HashMap`/`HashSet` use `BuildHasherDefault<AHasher>` instead of
`RandomState`. As a result, the hash is stable within the same run.
- [aHash isn't stable between devices (and
versions)](https://github.com/tkaitchuck/ahash?tab=readme-ov-file#goals-and-non-goals),
yet it's used for `StableHashMap`/`StableHashSet`
- the specialized hashmaps are stable

Interestingly, `StableHashMap`/`StableHashSet` aren't used by Bevy
itself (anymore).

## Solution
Add/fix documentation

## Alternatives
For `StableHashMap`/`StableHashSet`:
- remove them
- revive #7107

---

## Changelog
- added iteration stability guarantees for different hashmaps

crates/bevy_reflect/src/utility.rs

@@ -1,7 +1,7 @@
 //! Helpers for working with Bevy reflection.
 
 use crate::TypeInfo;
-use bevy_utils::{FixedState, StableHashMap};
+use bevy_utils::{FixedState, NoOpTypeIdHash, TypeIdMap};
 use std::{
     any::{Any, TypeId},
     hash::BuildHasher,
@@ -195,7 +195,7 @@ impl<T: TypedProperty> NonGenericTypeCell<T> {
 /// ```
 /// [`impl_type_path`]: crate::impl_type_path
 /// [`TypePath`]: crate::TypePath
-pub struct GenericTypeCell<T: TypedProperty>(RwLock<StableHashMap<TypeId, &'static T::Stored>>);
+pub struct GenericTypeCell<T: TypedProperty>(RwLock<TypeIdMap<&'static T::Stored>>);
 
 /// See [`GenericTypeCell`].
 pub type GenericTypeInfoCell = GenericTypeCell<TypeInfo>;
@@ -205,9 +205,7 @@ pub type GenericTypePathCell = GenericTypeCell<TypePathComponent>;
 impl<T: TypedProperty> GenericTypeCell<T> {
     /// Initialize a [`GenericTypeCell`] for generic types.
     pub const fn new() -> Self {
-        // Use `bevy_utils::StableHashMap` over `bevy_utils::HashMap`
-        // because `BuildHasherDefault` is unfortunately not const.
-        Self(RwLock::new(StableHashMap::with_hasher(FixedState)))
+        Self(RwLock::new(TypeIdMap::with_hasher(NoOpTypeIdHash)))
     }
 
     /// Returns a reference to the [`TypedProperty`] stored in the cell.

crates/bevy_utils/src/lib.rs

@@ -87,13 +87,17 @@ impl BuildHasher for FixedState {
 /// speed keyed hashing algorithm intended for use in in-memory hashmaps.
 ///
 /// aHash is designed for performance and is NOT cryptographically secure.
+///
+/// Within the same execution of the program iteration order of different
+/// `HashMap`s only depends on the order of insertions and deletions,
+/// but it will not be stable between multiple executions of the program.
 pub type HashMap<K, V> = hashbrown::HashMap<K, V, BuildHasherDefault<AHasher>>;
 
 /// A stable hash map implementing aHash, a high speed keyed hashing algorithm
 /// intended for use in in-memory hashmaps.
 ///
-/// Unlike [`HashMap`] this has an iteration order that only depends on the order
-/// of insertions and deletions and not a random source.
+/// Unlike [`HashMap`] the iteration order stability extends between executions
+/// using the same Bevy version on the same device.
 ///
 /// aHash is designed for performance and is NOT cryptographically secure.
 pub type StableHashMap<K, V> = hashbrown::HashMap<K, V, FixedState>;
@@ -102,13 +106,17 @@ pub type StableHashMap<K, V> = hashbrown::HashMap<K, V, FixedState>;
 /// speed keyed hashing algorithm intended for use in in-memory hashmaps.
 ///
 /// aHash is designed for performance and is NOT cryptographically secure.
+///
+/// Within the same execution of the program iteration order of different
+/// `HashSet`s only depends on the order of insertions and deletions,
+/// but it will not be stable between multiple executions of the program.
 pub type HashSet<K> = hashbrown::HashSet<K, BuildHasherDefault<AHasher>>;
 
 /// A stable hash set implementing aHash, a high speed keyed hashing algorithm
 /// intended for use in in-memory hashmaps.
 ///
-/// Unlike [`HashSet`] this has an iteration order that only depends on the order
-/// of insertions and deletions and not a random source.
+/// Unlike [`HashMap`] the iteration order stability extends between executions
+/// using the same Bevy version on the same device.
 ///
 /// aHash is designed for performance and is NOT cryptographically secure.
 pub type StableHashSet<K> = hashbrown::HashSet<K, FixedState>;
@@ -224,6 +232,7 @@ impl Hasher for PassHasher {
 }
 
 /// A [`HashMap`] pre-configured to use [`Hashed`] keys and [`PassHash`] passthrough hashing.
+/// Iteration order only depends on the order of insertions and deletions.
 pub type PreHashMap<K, V> = hashbrown::HashMap<Hashed<K>, V, PassHash>;
 
 /// Extension methods intended to add functionality to [`PreHashMap`].
@@ -322,17 +331,30 @@ impl Hasher for EntityHasher {
 }
 
 /// A [`HashMap`] pre-configured to use [`EntityHash`] hashing.
+/// Iteration order only depends on the order of insertions and deletions.
 pub type EntityHashMap<K, V> = hashbrown::HashMap<K, V, EntityHash>;
 
 /// A [`HashSet`] pre-configured to use [`EntityHash`] hashing.
+/// Iteration order only depends on the order of insertions and deletions.
 pub type EntityHashSet<T> = hashbrown::HashSet<T, EntityHash>;
 
 /// A specialized hashmap type with Key of [`TypeId`]
-pub type TypeIdMap<V> =
-    hashbrown::HashMap<TypeId, V, std::hash::BuildHasherDefault<NoOpTypeIdHasher>>;
+/// Iteration order only depends on the order of insertions and deletions.
+pub type TypeIdMap<V> = hashbrown::HashMap<TypeId, V, NoOpTypeIdHash>;
 
-#[doc(hidden)]
+/// [`BuildHasher`] for [`TypeId`]s.
 #[derive(Default)]
+pub struct NoOpTypeIdHash;
+
+impl BuildHasher for NoOpTypeIdHash {
+    type Hasher = NoOpTypeIdHasher;
+
+    fn build_hasher(&self) -> Self::Hasher {
+        NoOpTypeIdHasher(0)
+    }
+}
+
+#[doc(hidden)]
 pub struct NoOpTypeIdHasher(u64);
 
 // TypeId already contains a high-quality hash, so skip re-hashing that hash.
@@ -469,4 +491,18 @@ mod tests {
 
         std::hash::Hash::hash(&TypeId::of::<()>(), &mut Hasher);
     }
+
+    #[test]
+    fn stable_hash_within_same_program_execution() {
+        let mut map_1 = HashMap::new();
+        let mut map_2 = HashMap::new();
+        for i in 1..10 {
+            map_1.insert(i, i);
+            map_2.insert(i, i);
+        }
+        assert_eq!(
+            map_1.iter().collect::<Vec<_>>(),
+            map_2.iter().collect::<Vec<_>>()
+        );
+    }
 }