Add error message for using type parameter as the type of a const parameter

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 `T` must be annotated with `#[derive(PartialEq, Eq)]`
+//~^ ERROR `T` is not guaranteed to `#[derive(PartialEq, Eq)]`
 
 fn main() {}

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

@@ -7,11 +7,13 @@ 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]: `T` must be annotated with `#[derive(PartialEq, Eq)]` to be used as the type of a const parameter
+error[E0741]: `T` is not guaranteed to `#[derive(PartialEq, Eq)]`, so may not 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]>);
-   |                      ^ `T` doesn't derive both `PartialEq` and `Eq`
+   |                      ^ `T` may not derive both `PartialEq` and `Eq`
+   |
+   = note: it is not currently possible to use a type parameter as the type of a const parameter
 
 error: aborting due to 2 previous errors
 

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 `T` must be annotated with `#[derive(PartialEq, Eq)]`
+//~^ ERROR `T` is not guaranteed to `#[derive(PartialEq, Eq)]`
 
 fn main() {}

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

@@ -6,11 +6,13 @@ LL | #![feature(const_generics)]
    |
    = note: `#[warn(incomplete_features)]` on by default
 
-error[E0741]: `T` must be annotated with `#[derive(PartialEq, Eq)]` to be used as the type of a const parameter
+error[E0741]: `T` is not guaranteed to `#[derive(PartialEq, Eq)]`, so may not 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]);
-   |                                  ^ `T` doesn't derive both `PartialEq` and `Eq`
+   |                                  ^ `T` may not derive both `PartialEq` and `Eq`
+   |
+   = note: it is not currently possible to use a type parameter as the type of a const parameter
 
 error: aborting due to previous error; 1 warning emitted