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

src/librustc_typeck/collect/type_of.rs

@@ -342,12 +342,37 @@ pub(super) fn type_of(tcx: TyCtxt<'_>, def_id: DefId) -> Ty<'_> {
                 if traits::search_for_structural_match_violation(param.hir_id, param.span, tcx, ty)
                     .is_some()
                 {
+                    // We use the same error code in both branches, because this is really the same
+                    // issue: we just special-case the message for type parameters to make it
+                    // clearer.
+                    if let ty::Param(_) = ty.peel_refs().kind {
+                        // Const parameters may not have type parameters as their types,
+                        // because we cannot be sure that the type parameter derives `PartialEq`
+                        // and `Eq` (just implementing them is not enough for `structural_match`).
                         struct_span_err!(
                             tcx.sess,
                             hir_ty.span,
                             E0741,
-                        "`{}` must be annotated with `#[derive(PartialEq, Eq)]` to be used as the \
-                         type of a const parameter",
+                            "`{}` is not guaranteed to `#[derive(PartialEq, Eq)]`, so may not be \
+                             used as the type of a const parameter",
+                            ty,
+                        )
+                        .span_label(
+                            hir_ty.span,
+                            format!("`{}` may not derive both `PartialEq` and `Eq`", ty),
+                        )
+                        .note(
+                            "it is not currently possible to use a type parameter as the type of a \
+                             const parameter",
+                        )
+                        .emit();
+                    } else {
+                        struct_span_err!(
+                            tcx.sess,
+                            hir_ty.span,
+                            E0741,
+                            "`{}` must be annotated with `#[derive(PartialEq, Eq)]` to be used as \
+                             the type of a const parameter",
                             ty,
                         )
                         .span_label(
@@ -356,6 +381,7 @@ pub(super) fn type_of(tcx: TyCtxt<'_>, def_id: DefId) -> Ty<'_> {
                         )
                         .emit();
                     }
+                }
                 ty
             }
             x => bug!("unexpected non-type Node::GenericParam: {:?}", x),

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

 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