From b20e23dd415b379ca2d91f1a9832c11087d42a15 Mon Sep 17 00:00:00 2001
From: mgi388 <135186256+mgi388@users.noreply.github.com>
Date: Thu, 9 Jan 2025 18:30:54 +1100
Subject: [PATCH] Add compile-time dyn compatible checks for `DynEq`, `DynHash`
 (#17254)

# Objective

- Shrink `bevy_utils` more.
- Refs #11478

## Solution

- Removes `assert_object_safe` from `bevy_utils` by using a compile time
check instead.

## Testing

- CI.

---

## Migration Guide

`assert_object_safe` is no longer exported by `bevy_utils`. Instead, you
can write a compile time check that your trait is "dyn compatible":

```rust
/// Assert MyTrait is dyn compatible
const _: Option<Box<dyn MyTrait>> = None;
```

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
---
 crates/bevy_ecs/src/label.rs         | 22 ++++++--------------
 crates/bevy_utils/src/lib.rs         |  2 --
 crates/bevy_utils/src/object_safe.rs | 30 ----------------------------
 3 files changed, 6 insertions(+), 48 deletions(-)
 delete mode 100644 crates/bevy_utils/src/object_safe.rs

diff --git a/crates/bevy_ecs/src/label.rs b/crates/bevy_ecs/src/label.rs
index e3f5078b22f71..10a23cfb898c3 100644
--- a/crates/bevy_ecs/src/label.rs
+++ b/crates/bevy_ecs/src/label.rs
@@ -22,6 +22,9 @@ pub trait DynEq: Any {
     fn dyn_eq(&self, other: &dyn DynEq) -> bool;
 }
 
+// Tests that this trait is dyn-compatible
+const _: Option<Box<dyn DynEq>> = None;
+
 impl<T> DynEq for T
 where
     T: Any + Eq,
@@ -48,6 +51,9 @@ pub trait DynHash: DynEq {
     fn dyn_hash(&self, state: &mut dyn Hasher);
 }
 
+// Tests that this trait is dyn-compatible
+const _: Option<Box<dyn DynHash>> = None;
+
 impl<T> DynHash for T
 where
     T: DynEq + Hash,
@@ -201,19 +207,3 @@ macro_rules! define_label {
             $crate::intern::Interner::new();
     };
 }
-
-#[cfg(test)]
-mod tests {
-    use super::{DynEq, DynHash};
-    use bevy_utils::assert_object_safe;
-
-    #[test]
-    fn dyn_eq_object_safe() {
-        assert_object_safe::<dyn DynEq>();
-    }
-
-    #[test]
-    fn dyn_hash_object_safe() {
-        assert_object_safe::<dyn DynHash>();
-    }
-}
diff --git a/crates/bevy_utils/src/lib.rs b/crates/bevy_utils/src/lib.rs
index 73d12f319754b..0fd3fa6a7e0f3 100644
--- a/crates/bevy_utils/src/lib.rs
+++ b/crates/bevy_utils/src/lib.rs
@@ -26,8 +26,6 @@ pub mod synccell;
 pub mod syncunsafecell;
 
 mod default;
-mod object_safe;
-pub use object_safe::assert_object_safe;
 mod once;
 #[cfg(feature = "std")]
 mod parallel_queue;
diff --git a/crates/bevy_utils/src/object_safe.rs b/crates/bevy_utils/src/object_safe.rs
deleted file mode 100644
index 6e74d5ef82908..0000000000000
--- a/crates/bevy_utils/src/object_safe.rs
+++ /dev/null
@@ -1,30 +0,0 @@
-/// Assert that a given `T` is [object safe](https://doc.rust-lang.org/reference/items/traits.html#object-safety).
-/// Will fail to compile if that is not the case.
-///
-/// # Examples
-///
-/// ```rust
-/// # use bevy_utils::assert_object_safe;
-/// // Concrete types are always object safe
-/// struct MyStruct;
-/// assert_object_safe::<MyStruct>();
-///
-/// // Trait objects are where that safety comes into question.
-/// // This trait is object safe...
-/// trait ObjectSafe { }
-/// assert_object_safe::<dyn ObjectSafe>();
-/// ```
-///
-/// ```compile_fail
-/// # use bevy_utils::assert_object_safe;
-/// // ...but this trait is not.
-/// trait NotObjectSafe {
-///     const VALUE: usize;
-/// }
-/// assert_object_safe::<dyn NotObjectSafe>();
-/// // Error: the trait `NotObjectSafe` cannot be made into an object
-/// ```
-pub fn assert_object_safe<T: ?Sized>() {
-    // This space is left intentionally blank. The type parameter T is sufficient to induce a compiler
-    // error without a function body.
-}