Make the `structural_match` error diagnostic for const generics clearer

7 files changed, 9 insertions, 7 deletions

diff --git a/src/librustc_typeck/collect/type_of.rs b/src/librustc_typeck/collect/type_of.rs
index e17b736058f..ddf0b247ae3 100644
--- a/src/librustc_typeck/collect/type_of.rs
+++ b/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,
diff --git a/src/test/ui/const-generics/const-param-type-depends-on-type-param-ungated.rs b/src/test/ui/const-generics/const-param-type-depends-on-type-param-ungated.rs
index 78bd549ba79..59bb06690a1 100644
--- a/src/test/ui/const-generics/const-param-type-depends-on-type-param-ungated.rs
+++ b/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() {}
diff --git a/src/test/ui/const-generics/const-param-type-depends-on-type-param-ungated.stderr b/src/test/ui/const-generics/const-param-type-depends-on-type-param-ungated.stderr
index 14ade3f33fd..70f9fc4184c 100644
--- a/src/test/ui/const-generics/const-param-type-depends-on-type-param-ungated.stderr
+++ b/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]>);
diff --git a/src/test/ui/const-generics/const-param-type-depends-on-type-param.rs b/src/test/ui/const-generics/const-param-type-depends-on-type-param.rs
index 684ccf71082..4dcda3b87a8 100644
--- a/src/test/ui/const-generics/const-param-type-depends-on-type-param.rs
+++ b/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() {}
diff --git a/src/test/ui/const-generics/const-param-type-depends-on-type-param.stderr b/src/test/ui/const-generics/const-param-type-depends-on-type-param.stderr
index 724e3909e45..290d53c4e19 100644
--- a/src/test/ui/const-generics/const-param-type-depends-on-type-param.stderr
+++ b/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]);
diff --git a/src/test/ui/const-generics/forbid-non-structural_match-types.rs b/src/test/ui/const-generics/forbid-non-structural_match-types.rs
index 7bc4f3986eb..a30cdc3efdf 100644
--- a/src/test/ui/const-generics/forbid-non-structural_match-types.rs
+++ b/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() {}
diff --git a/src/test/ui/const-generics/forbid-non-structural_match-types.stderr b/src/test/ui/const-generics/forbid-non-structural_match-types.stderr
index d2469ca766e..4f343146263 100644
--- a/src/test/ui/const-generics/forbid-non-structural_match-types.stderr
+++ b/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>;