adt_const_params does not explicitly check for a union ConstParamTy impl

[BoxyUwU]

I am not sure there is actually a piece of code that is incorrectly compiled since you cannot manually impl StructuralEq and StructuralPartialEq and you also cant derive(Eq, PartialEq) on a union. regardless there is currently no check that a manual impl ConstParamTy for MyType { is not applied to a union. There should be one to ensure that we do not accidentally allow usage of unions in const param tys.

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rustc --version --verbose:

Nightly channel

Build using the Nightly version: 1.72.0-nightly

(2023-05-29 498553fc04f6a3fdc534)

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