Implement Reflect for PhantomData

[BenjaminBrienen]

Objective

• Fixed PhantomData does not implement Reflect #5144

Solution

• impl_reflect! following the example

Testing

N/A
I don't see tests for the other core impl_reflects.

[BenjaminBrienen]

#5145 (comment)

I'm curious why we want this now and not before.

[MrGVSV]

#5145 (comment)

I'm curious why we want this now and not before.

It's still the same issues as before. I think the biggest problem is serialization. It really only adds noise to the serialized output. And if we make it Reflect, then it becomes easy for library authors to forget to ignore it themselves, which means downstream users will see it if they serialize it.

If do this, we may want to consider skipping it during serialization (and other operations such as reflect_partial_eq).

[MrGVSV]

Also, can we add a serialization test for PhantomData in the serde module? Whether or not we're allowing this type to be serialized, we should ensure any type containing it can be deserialized and FromReflect-ed.

[soqb]

do we want really need this? i feel like PhantomData is supposed to act as if its not there at all (its a 👻 after all). is there anything not accomplished by annotating #[reflect(ignore)] or is it simply an ergonomic improvement? i'd really rather not have all the "noise" that @MrGVSV mentioned (not just in serialization mind, also in runtime and binary size) just for the sake of a few less tokens to type out.

tldr; the implementation is correct but i'm not sure the feature is desirable.