Explain that in paths generics belong on the enum, not the variant

```
error[E0109]: type arguments are not allowed on tuple variant `TSVariant`
  --> $DIR/enum-variant-generic-args.rs:54:29
   |
LL |     TSVariant::<()>(());
   |     ---------   ^^ type argument not allowed
   |     |
   |     not allowed on tuple variant `TSVariant`
   |
   = note: type arguments are not allowed for enum variants; specify them on the enum itself
```

Fix #93993.

Author: estebank

compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs

@@ -6,7 +6,7 @@ use rustc_errors::{
     Applicability, Diag, ErrorGuaranteed, MultiSpan, pluralize, struct_span_code_err,
 };
 use rustc_hir as hir;
-use rustc_hir::def::{DefKind, Res};
+use rustc_hir::def::{CtorOf, DefKind, Res};
 use rustc_hir::def_id::DefId;
 use rustc_middle::bug;
 use rustc_middle::ty::print::{PrintPolyTraitRefExt as _, PrintTraitRefExt as _};
@@ -1048,15 +1048,15 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 } else {
                     Some((
                         match segment.res {
-                            hir::def::Res::PrimTy(ty) => {
+                            Res::PrimTy(ty) => {
                                 format!("{} `{}`", segment.res.descr(), ty.name())
                             }
-                            hir::def::Res::Def(_, def_id)
+                            Res::Def(_, def_id)
                                 if let Some(name) = self.tcx().opt_item_name(def_id) =>
                             {
                                 format!("{} `{name}`", segment.res.descr())
                             }
-                            hir::def::Res::Err => "this type".to_string(),
+                            Res::Err => "this type".to_string(),
                             _ => segment.res.descr().to_string(),
                         },
                         segment.ident.span,
@@ -1118,6 +1118,14 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         for (what, span) in types_and_spans {
             err.span_label(span, format!("not allowed on {what}"));
         }
+        if segments.clone().any(|segment| {
+            !segment.args().args.is_empty()
+                && matches!(segment.res, Res::Def(DefKind::Ctor(CtorOf::Variant, _), _))
+        }) {
+            let msg = "generic arguments are not allowed for enum variants; specify them on the \
+                       enum itself";
+            err.note(msg);
+        }
         generics_args_err_extend(self.tcx(), segments, &mut err, err_extend);
         err.emit()
     }

tests/ui/type-alias-enum-variants/enum-variant-generic-args.stderr

@@ -279,6 +279,8 @@ LL |     Enum::<()>::TSVariant::<()>(());
    |                 ---------   ^^ type argument not allowed
    |                 |
    |                 not allowed on tuple variant `TSVariant`
+   |
+   = note: generic arguments are not allowed for enum variants; specify them on the enum itself
 
 error[E0109]: type arguments are not allowed on this type
   --> $DIR/enum-variant-generic-args.rs:57:24
@@ -445,6 +447,8 @@ LL |     Enum::<()>::UVariant::<()>;
    |                 --------   ^^ type argument not allowed
    |                 |
    |                 not allowed on unit variant `UVariant`
+   |
+   = note: generic arguments are not allowed for enum variants; specify them on the enum itself
 
 error[E0109]: type arguments are not allowed on this type
   --> $DIR/enum-variant-generic-args.rs:93:23