Use a fixed state hasher in bevy_reflect for deterministic Reflect::reflect_hash() across processes (#7583)

# Objective

- bevy_ggrs uses `reflect_hash` in order to produce checksums for its world snapshots. These checksums are sent between clients in order to detect desyncronization.
- However, since we currently use `async::AHasher` with the `std` feature, this means that hashes will always be different for different peers, even if the state is identical.
- This means bevy_ggrs needs a way to get a deterministic (fixed) hash.

## Solution

- ~~Add a feature to use `bevy_utils::FixedState` for the hasher used by bevy_reflect.~~
- Always use `bevy_utils::FixedState` for initializing the bevy_reflect hasher. 

---

## Changelog

- bevy_reflect now uses a fixed state for its hasher, which means the output of `Reflect::reflect_hash` is now deterministic across processes.

crates/bevy_reflect/bevy_reflect_derive/src/container_attributes.rs

@@ -5,7 +5,7 @@
 //! the derive helper attribute for `Reflect`, which looks like:
 //! `#[reflect(PartialEq, Default, ...)]` and `#[reflect_value(PartialEq, Default, ...)]`.
 
-use crate::fq_std::{FQAny, FQDefault, FQOption};
+use crate::fq_std::{FQAny, FQOption};
 use crate::utility;
 use proc_macro2::{Ident, Span};
 use quote::quote_spanned;
@@ -225,7 +225,7 @@ impl ReflectTraits {
             &TraitImpl::Implemented(span) => Some(quote_spanned! {span=>
                 fn reflect_hash(&self) -> #FQOption<u64> {
                     use ::core::hash::{Hash, Hasher};
-                    let mut hasher: #bevy_reflect_path::ReflectHasher = #FQDefault::default();
+                    let mut hasher = #bevy_reflect_path::utility::reflect_hasher();
                     Hash::hash(&#FQAny::type_id(self), &mut hasher);
                     Hash::hash(self, &mut hasher);
                     #FQOption::Some(Hasher::finish(&hasher))

crates/bevy_reflect/src/array.rs

@@ -1,6 +1,6 @@
 use crate::{
-    utility::NonGenericTypeInfoCell, DynamicInfo, Reflect, ReflectMut, ReflectOwned, ReflectRef,
-    TypeInfo, Typed,
+    utility::{reflect_hasher, NonGenericTypeInfoCell},
+    DynamicInfo, Reflect, ReflectMut, ReflectOwned, ReflectRef, TypeInfo, Typed,
 };
 use std::{
     any::{Any, TypeId},
@@ -340,7 +340,7 @@ impl<'a> ExactSizeIterator for ArrayIter<'a> {}
 /// Returns the `u64` hash of the given [array](Array).
 #[inline]
 pub fn array_hash<A: Array>(array: &A) -> Option<u64> {
-    let mut hasher = crate::ReflectHasher::default();
+    let mut hasher = reflect_hasher();
     std::any::Any::type_id(array).hash(&mut hasher);
     array.len().hash(&mut hasher);
     for value in array.iter() {

crates/bevy_reflect/src/enums/helpers.rs

@@ -1,11 +1,11 @@
-use crate::{Enum, Reflect, ReflectRef, VariantType};
+use crate::{utility::reflect_hasher, Enum, Reflect, ReflectRef, VariantType};
 use std::fmt::Debug;
 use std::hash::{Hash, Hasher};
 
 /// Returns the `u64` hash of the given [enum](Enum).
 #[inline]
 pub fn enum_hash<TEnum: Enum>(value: &TEnum) -> Option<u64> {
-    let mut hasher = crate::ReflectHasher::default();
+    let mut hasher = reflect_hasher();
     std::any::Any::type_id(value).hash(&mut hasher);
     value.variant_name().hash(&mut hasher);
     value.variant_type().hash(&mut hasher);

crates/bevy_reflect/src/impls/std.rs

@@ -8,7 +8,7 @@ use crate::{
     VariantInfo, VariantType,
 };
 
-use crate::utility::{GenericTypeInfoCell, NonGenericTypeInfoCell};
+use crate::utility::{reflect_hasher, GenericTypeInfoCell, NonGenericTypeInfoCell};
 use bevy_reflect_derive::{impl_from_reflect_value, impl_reflect_value};
 use bevy_utils::{Duration, Instant};
 use bevy_utils::{HashMap, HashSet};
@@ -993,7 +993,7 @@ impl Reflect for Cow<'static, str> {
     }
 
     fn reflect_hash(&self) -> Option<u64> {
-        let mut hasher = crate::ReflectHasher::default();
+        let mut hasher = reflect_hasher();
         Hash::hash(&std::any::Any::type_id(self), &mut hasher);
         Hash::hash(self, &mut hasher);
         Some(hasher.finish())
@@ -1101,7 +1101,7 @@ impl Reflect for &'static Path {
     }
 
     fn reflect_hash(&self) -> Option<u64> {
-        let mut hasher = crate::ReflectHasher::default();
+        let mut hasher = reflect_hasher();
         Hash::hash(&std::any::Any::type_id(self), &mut hasher);
         Hash::hash(self, &mut hasher);
         Some(hasher.finish())

crates/bevy_reflect/src/list.rs

@@ -2,7 +2,7 @@ use std::any::{Any, TypeId};
 use std::fmt::{Debug, Formatter};
 use std::hash::{Hash, Hasher};
 
-use crate::utility::NonGenericTypeInfoCell;
+use crate::utility::{reflect_hasher, NonGenericTypeInfoCell};
 use crate::{
     DynamicInfo, FromReflect, Reflect, ReflectMut, ReflectOwned, ReflectRef, TypeInfo, Typed,
 };
@@ -378,7 +378,7 @@ impl<'a> ExactSizeIterator for ListIter<'a> {}
 /// Returns the `u64` hash of the given [list](List).
 #[inline]
 pub fn list_hash<L: List>(list: &L) -> Option<u64> {
-    let mut hasher = crate::ReflectHasher::default();
+    let mut hasher = reflect_hasher();
     std::any::Any::type_id(list).hash(&mut hasher);
     list.len().hash(&mut hasher);
     for value in list.iter() {

crates/bevy_reflect/src/reflect.rs

@@ -9,7 +9,6 @@ use std::{
 };
 
 use crate::utility::NonGenericTypeInfoCell;
-pub use bevy_utils::AHasher as ReflectHasher;
 
 /// An immutable enumeration of "kinds" of reflected type.
 ///

crates/bevy_reflect/src/utility.rs

@@ -1,10 +1,13 @@
 //! Helpers for working with Bevy reflection.
 
 use crate::TypeInfo;
-use bevy_utils::HashMap;
+use bevy_utils::{FixedState, HashMap};
 use once_cell::race::OnceBox;
 use parking_lot::RwLock;
-use std::any::{Any, TypeId};
+use std::{
+    any::{Any, TypeId},
+    hash::BuildHasher,
+};
 
 /// A container for [`TypeInfo`] over non-generic types, allowing instances to be stored statically.
 ///
@@ -147,3 +150,15 @@ impl GenericTypeInfoCell {
         })
     }
 }
+
+/// Deterministic fixed state hasher to be used by implementors of [`Reflect::reflect_hash`].
+///
+/// Hashes should be deterministic across processes so hashes can be used as
+/// checksums for saved scenes, rollback snapshots etc. This function returns
+/// such a hasher.
+///
+/// [`Reflect::reflect_hash`]: crate::Reflect
+#[inline]
+pub fn reflect_hasher() -> bevy_utils::AHasher {
+    FixedState.build_hasher()
+}