Auto merge of rust-lang#116125 - RalfJung:const-param-ty-eq, r=compiler-errors

ConstParamTy: require Eq as supertrait

As discussed with `@BoxyUwu` [on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/260443-project-const-generics/topic/.60ConstParamTy.60.20and.20.60Eq.60).

We want to say that valtree equality on const generic params agrees with `==`, but that only makes sense if `==` actually exists, hence we should have an appropriate bound. Valtree equality is an equivalence relation, so such a type can always be `Eq` and not just `PartialEq`.

Author: bors

core/src/marker.rs

@@ -986,11 +986,16 @@ pub trait Tuple {}
 pub trait PointerLike {}
 
 /// A marker for types which can be used as types of `const` generic parameters.
+///
+/// These types must have a proper equivalence relation (`Eq`) and it must be automatically
+/// derived (`StructuralPartialEq`). There's a hard-coded check in the compiler ensuring
+/// that all fields are also `ConstParamTy`, which implies that recursively, all fields
+/// are `StructuralPartialEq`.
 #[lang = "const_param_ty"]
 #[unstable(feature = "adt_const_params", issue = "95174")]
 #[rustc_on_unimplemented(message = "`{Self}` can't be used as a const parameter type")]
 #[allow(multiple_supertrait_upcastable)]
-pub trait ConstParamTy: StructuralEq + StructuralPartialEq {}
+pub trait ConstParamTy: StructuralEq + StructuralPartialEq + Eq {}
 
 /// Derive macro generating an impl of the trait `ConstParamTy`.
 #[rustc_builtin_macro]