Make the structural_match error diagnostic for const generics clearer

src/librustc_typeck/collect/type_of.rs

@@ -346,7 +346,9 @@ pub(super) fn type_of(tcx: TyCtxt<'_>, def_id: DefId) -> Ty<'_> {
                         tcx.sess,
                         hir_ty.span,
                         E0741,
-                        "the types of const generic parameters must derive `PartialEq` and `Eq`",
+                        "`{}` must be annotated with `#[derive(PartialEq, Eq)]` to be used as the \
+                         type of a const parameter",
+                        ty,
                     )
                     .span_label(
                         hir_ty.span,

src/test/ui/const-generics/const-param-type-depends-on-type-param-ungated.rs

@@ -1,6 +1,6 @@
 use std::marker::PhantomData;
 
 struct B<T, const N: T>(PhantomData<[T; N]>); //~ ERROR const generics are unstable
-//~^ ERROR the types of const generic parameters must derive `PartialEq` and `Eq`
+//~^ ERROR `T` must be annotated with `#[derive(PartialEq, Eq)]`
 
 fn main() {}

src/test/ui/const-generics/const-param-type-depends-on-type-param-ungated.stderr

@@ -7,7 +7,7 @@ LL | struct B<T, const N: T>(PhantomData<[T; N]>);
    = note: see issue #44580 <https://github.com/rust-lang/rust/issues/44580> for more information
    = help: add `#![feature(const_generics)]` to the crate attributes to enable
 
-error[E0741]: the types of const generic parameters must derive `PartialEq` and `Eq`
+error[E0741]: `T` must be annotated with `#[derive(PartialEq, Eq)]` to be used as the type of a const parameter
   --> $DIR/const-param-type-depends-on-type-param-ungated.rs:3:22
    |
 LL | struct B<T, const N: T>(PhantomData<[T; N]>);

src/test/ui/const-generics/const-param-type-depends-on-type-param.rs

@@ -7,6 +7,6 @@
 // details.
 
 pub struct Dependent<T, const X: T>([(); X]);
-//~^ ERROR the types of const generic parameters must derive `PartialEq` and `Eq`
+//~^ ERROR `T` must be annotated with `#[derive(PartialEq, Eq)]`
 
 fn main() {}

src/test/ui/const-generics/const-param-type-depends-on-type-param.stderr

@@ -6,7 +6,7 @@ LL | #![feature(const_generics)]
    |
    = note: `#[warn(incomplete_features)]` on by default
 
-error[E0741]: the types of const generic parameters must derive `PartialEq` and `Eq`
+error[E0741]: `T` must be annotated with `#[derive(PartialEq, Eq)]` to be used as the type of a const parameter
   --> $DIR/const-param-type-depends-on-type-param.rs:9:34
    |
 LL | pub struct Dependent<T, const X: T>([(); X]);

src/test/ui/const-generics/forbid-non-structural_match-types.rs

@@ -8,6 +8,6 @@ struct B<const X: A>; // ok
 
 struct C;
 
-struct D<const X: C>; //~ ERROR the types of const generic parameters must derive
+struct D<const X: C>; //~ ERROR `C` must be annotated with `#[derive(PartialEq, Eq)]`
 
 fn main() {}

src/test/ui/const-generics/forbid-non-structural_match-types.stderr

@@ -6,7 +6,7 @@ LL | #![feature(const_generics)]
    |
    = note: `#[warn(incomplete_features)]` on by default
 
-error[E0741]: the types of const generic parameters must derive `PartialEq` and `Eq`
+error[E0741]: `C` must be annotated with `#[derive(PartialEq, Eq)]` to be used as the type of a const parameter
   --> $DIR/forbid-non-structural_match-types.rs:11:19
    |
 LL | struct D<const X: C>;