bevy_reflect: Re-reflect hashbrown types (#18944)

# Objective

Fixes #18943

## Solution

Reintroduces support for `hashbrown`'s `HashMap` and `HashSet` types.
These were inadvertently removed when `bevy_platform` newtyped the
`hashbrown` types.

Since we removed our `hashbrown` dependency, I gated these impls behind
a `hashbrown` feature. Not entirely sure if this is necessary since we
enabled it for `bevy_reflect` through `bevy_platform` anyways. (Complex
features still confuse me a bit so let me know if I can just remove it!)

I also went ahead and preemptively implemented `TypePath` for `PassHash`
while I was here.

## Testing

You can test that it works by adding the following to a Bevy example
based on this PR (you'll also need to include `hashbrown` of course):

```rust
#[derive(Reflect)]
struct Foo(hashbrown::HashMap<String, String>);
```

Then check it compiles with:

```
cargo check --example hello_world --no-default-features --features=bevy_reflect/hashbrown
```

crates/bevy_reflect/Cargo.toml

@@ -33,6 +33,9 @@ debug_stack = ["std"]
 ## Adds reflection support to `glam` types.
 glam = ["dep:glam"]
 
+## Adds reflection support to `hashbrown` types.
+hashbrown = ["dep:hashbrown"]
+
 ## Adds reflection support to `petgraph` types.
 petgraph = ["dep:petgraph", "std"]
 
@@ -87,6 +90,7 @@ bevy_platform = { path = "../bevy_platform", version = "0.16.0", default-feature
 
 # used by bevy-utils, but it also needs reflect impls
 foldhash = { version = "0.1.3", default-features = false }
+hashbrown = { version = "0.15.1", optional = true, default-features = false }
 
 # other
 erased-serde = { version = "0.4", default-features = false, features = [

crates/bevy_reflect/src/impls/std.rs

@@ -1001,6 +1001,19 @@ crate::func::macros::impl_function_traits!(::bevy_platform::collections::HashMap
     >
 );
 
+#[cfg(feature = "hashbrown")]
+impl_reflect_for_hashmap!(hashbrown::hash_map::HashMap<K, V, S>);
+#[cfg(feature = "hashbrown")]
+impl_type_path!(::hashbrown::hash_map::HashMap<K, V, S>);
+#[cfg(all(feature = "functions", feature = "hashbrown"))]
+crate::func::macros::impl_function_traits!(::hashbrown::hash_map::HashMap<K, V, S>;
+    <
+        K: FromReflect + MaybeTyped + TypePath + GetTypeRegistration + Eq + Hash,
+        V: FromReflect + MaybeTyped + TypePath + GetTypeRegistration,
+        S: TypePath + BuildHasher + Default + Send + Sync
+    >
+);
+
 macro_rules! impl_reflect_for_hashset {
     ($ty:path) => {
         impl<V, S> Set for $ty
@@ -1208,6 +1221,7 @@ macro_rules! impl_reflect_for_hashset {
 
 impl_type_path!(::bevy_platform::hash::NoOpHash);
 impl_type_path!(::bevy_platform::hash::FixedHasher);
+impl_type_path!(::bevy_platform::hash::PassHash);
 impl_reflect_opaque!(::core::net::SocketAddr(
     Clone,
     Debug,
@@ -1239,6 +1253,18 @@ crate::func::macros::impl_function_traits!(::bevy_platform::collections::HashSet
     >
 );
 
+#[cfg(feature = "hashbrown")]
+impl_reflect_for_hashset!(::hashbrown::hash_set::HashSet<V,S>);
+#[cfg(feature = "hashbrown")]
+impl_type_path!(::hashbrown::hash_set::HashSet<V, S>);
+#[cfg(all(feature = "functions", feature = "hashbrown"))]
+crate::func::macros::impl_function_traits!(::hashbrown::hash_set::HashSet<V, S>;
+    <
+        V: Hash + Eq + FromReflect + TypePath + GetTypeRegistration,
+        S: TypePath + BuildHasher + Default + Send + Sync
+    >
+);
+
 impl<K, V> Map for ::alloc::collections::BTreeMap<K, V>
 where
     K: FromReflect + MaybeTyped + TypePath + GetTypeRegistration + Eq + Ord,
@@ -2848,4 +2874,15 @@ mod tests {
         let output = <&'static str as FromReflect>::from_reflect(&expected).unwrap();
         assert_eq!(expected, output);
     }
+
+    #[test]
+    fn should_reflect_hashmaps() {
+        assert_impl_all!(std::collections::HashMap<u32, f32>: Reflect);
+        assert_impl_all!(bevy_platform::collections::HashMap<u32, f32>: Reflect);
+
+        // We specify `foldhash::fast::RandomState` directly here since without the `default-hasher`
+        // feature, hashbrown uses an empty enum to force users to specify their own
+        #[cfg(feature = "hashbrown")]
+        assert_impl_all!(hashbrown::HashMap<u32, f32, foldhash::fast::RandomState>: Reflect);
+    }
 }