`MapEntities` is not called for Components

[Zeenobit]

Bevy version

0.16

What you did

From my testing, it doesn't seem like MapEntities is being called for components as it used to.

I maintain the moonshine-save library, and this specific test is failing for me:

https://github.com/Zeenobit/moonshine_save/blob/bevy-0.16/tests/basic.rs

What went wrong

After some debugging, I realized MapEntities is not being called at all. Looking at this CL:
3c8fae2#diff-717fbf581c75ed1fc65c9755af2bf8a2ddc97c02e89f4044f86efa70a2ea4260L110
(see dynamic_scene.rs, write_to_world_with)

It looks like this functionality was removed in favor of relations:

bevy/crates/bevy_scene/src/dynamic_scene.rs

Line 107 in 3c8fae2

{

To me, it seems like the check for map entities is just missing. I don't see why we would support it for resources (further below in the function) and not for components.

But I wanted to check before opening a PR!

[rendaoer]

#[derive(Component, MapEntities, Reflect)]
#[reflect(Component, MapEntities)]
struct FooBar(#[entities] Entity);

Basic test can be run correctly if it is changed to this

You can check this PR #18432

[Zeenobit]

Yup, you're absolutely correct.

Although this feels like a bit of a footgun. Most other derives don't require an explicit marked member if they only have one member.

[Henauxg]

(relevant migration guide https://bevyengine.org/learn/migration-guides/0-15-to-0-16/#replace-visitentities-with-mapentities)

I think we can close this ? (Even if it means opening up a PR later to improve the situation on single fields structs)