Fix incorrect TypeId retrieval in reflect component clone

[Mysvac]

Objective

Fix incorrect TypeId retrieval in ComponentCloneCtx::write_target_component_reflect().

When cloning reflected components, the code incorrectly called .type_id() on a Box<dyn Reflect>, which returns the container's TypeId rather than the underlying concrete type's TypeId.

For example:

let boxed: Box<dyn Reflect> = Box::new(String::from("test"));

// Correct: dereferencing gets the concrete type's TypeId
assert_eq!((*boxed).type_id(), TypeId::of::<String>());

// Wrong: calling on Box returns something else
assert_ne!(boxed.type_id(), TypeId::of::<String>());

See Rust documentation: https://doc.rust-lang.org/std/any/index.html#smart-pointers-and-dyn-any.

Solution

Dereference the Box<dyn Reflect> before calling .type_id() to obtain the concrete type's identifier.

Changed from:

let component_type_id = component.type_id();  // Wrong: returns Box's TypeId

To:

let component_type_id = (*component).type_id();  // Correct: returns concrete type's TypeId

Testing

• cargo fmt --check
• cargo test -p bevy_reflect
• cargo test -p bevy_ecs

Additional Notes

This pattern is error-prone. A safer long-term solution would be for Reflect trait to provide an unambiguous method like typeid() or reflect_type_id() that clearly indicates it returns the underlying type's identifier, not the trait object's.

---

Code Change

View the specific change

File: crates/bevy_ecs/src/entity/clone_entities.rs

 pub fn write_target_component_reflect(&mut self, component: Box<dyn bevy_reflect::Reflect>) {
     /* ... */
     let source_type_id = self
         .component_info
         .type_id()
         .expect("Source component must have TypeId");
     
-    let component_type_id = component.type_id();
+    let component_type_id = (*component).type_id();
     
     if source_type_id != component_type_id {
         panic!("Passed component TypeId does not match source component TypeId")
     }
     /* ... */
 }

[eugineerd]

I'm a bit confused about this.

1. The provided example doesn't run (assert_ne! fails)
2. Tests already cover this path, and if type ids were incorrect they would've panicked

[Mysvac]

Okay, that's true, but I've discovered a more troublesome issue.

If the scope does not import Any, then boxed.type_id() will call type_id() of dyn Reflect. However, if core::any::Any is imported, then boxed.type_id() will call type_id() of Box<dyn Reflect>.

#[test]
fn unused_any() {
    use bevy_reflect::Reflect;

    let boxed: Box<dyn Reflect> = Box::new(String::from("test"));
    assert_eq!((*boxed).type_id(), TypeId::of::<String>());
    assert_eq!(boxed.type_id(), TypeId::of::<String>());
}

#[test]
fn used_any() {
    use bevy_reflect::Reflect;
    use core::any::Any;

    let boxed: Box<dyn Reflect> = Box::new(String::from("test"));
    assert_eq!((*boxed).type_id(), TypeId::of::<String>());
    assert_ne!(boxed.type_id(), TypeId::of::<String>()); // !!!
}

The source code does not import Any, so it can run normally, but this is quite "subtle".

[eugineerd]

If the scope does not import Any, then boxed.type_id() will call type_id() of dyn Reflect. However, if core::any::Any is imported, then boxed.type_id() will call type_id() of Box<dyn Reflect>.

Oh, that's true. Looks like clippy catches this by default at least:

warning: calling `.type_id()` on `Box<dyn Reflect>`
   --> crates/bevy_ecs/src/entity/clone_entities.rs:247:33
    |
247 |         let component_type_id = component.type_id();
    |                                 ---------^^^^^^^^^^
    |                                 |
    |                                 help: consider dereferencing first: `(*component)`
    |
    = note: this returns the type id of the literal type `Box<_>` instead of the type id of the boxed value, which is most likely not what you want
    = note: if this is intentional, use `TypeId::of::<Box<dyn Reflect>>()` instead, which makes it more clear
    = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.92.0/index.html#type_id_on_box
    = note: `#[warn(clippy::type_id_on_box)]` on by default