Doing `#[reflect(ignore)]` on fields in a struct still requires `Default` on that field

[torsteingrindvik]

Bevy version

Latest: 1395e36

What you did

What went wrong

I couldn't ignore a field on my struct which I wanted to derive reflect on, since that field did not impl Default and I had no control over that field.

My expectation was that the field is ignored entirely with no limitations.

[MrGVSV]

This is actually expected behavior since FromReflect (which is automatically derived alongside Reflect) requires Default for ignored fields.

There are two possible solutions besides implementing Default for Foo:

1. Opt out of FromReflect by adding #[reflect(from_reflect = false)] to your Bar type
2. Add a custom default function for Foo like #[reflect(ignore, default = "create_foo")], #[reflect(ignore, default = "Foo::new")], etc.

Hopefully that helps!

[MrGVSV]

As for how we can avoid this in the future, I think we could update the macro code to make FromReflect opt-in. If any field is ignored, we can skip implementing FromReflect unless a default attribute is set for that field. I did something like this in #13432.

One possible issue is that this would mean that users won't be 100% sure their type implements FromReflect unless they try to use it.

[torsteingrindvik]

As for how we can avoid this in the future, I think we could update the macro code to make FromReflect opt-in. If any field is ignored, we can skip implementing FromReflect unless a default attribute is set for that field. I did something like this in #13432.

One possible issue is that this would mean that users won't be 100% sure their type implements FromReflect unless they try to use it.

If this is a longer term solution, is the near term is it possible in the derive macro to detect the lacking default impl and report on it?

Opt out of FromReflect by adding #[reflect(from_reflect = false)] to your Bar type

If I was guided towards this in the error msg that would have helped me right away, which would be very nice

[MrGVSV]

is it possible in the derive macro to detect the lacking default impl and report on it?

Unfortunately no. Macros don't have any type information available to them, including what traits a type implements.

Opt out of FromReflect by adding #[reflect(from_reflect = false)] to your Bar type

If I was guided towards this in the error msg that would have helped me right away, which would be very nice

Yeah ideally we would do this, but it's not possible to know whether or not a type implements Default via the macro. We'd have to generate this error based on the lack of default attributes when not opting out, which may be controversial compared to just returning None.