Point to the empty trait alias.

compiler/rustc_error_messages/locales/en-US/typeck.ftl

@@ -45,6 +45,7 @@ typeck-copy-impl-on-non-adt =
 
 typeck-trait-object-declared-with-no-traits =
     at least one trait is required for an object type
+    .alias-span = this alias does not contain a trait
 
 typeck-ambiguous-lifetime-bound =
     ambiguous lifetime bound, explicit lifetime bound required

compiler/rustc_typeck/src/astconv/mod.rs

@@ -1370,7 +1370,13 @@ trait here instead: `trait NewTrait: {} {{}}`",
         }
 
         if regular_traits.is_empty() && auto_traits.is_empty() {
-            tcx.sess.emit_err(TraitObjectDeclaredWithNoTraits { span });
+            let trait_alias_span = bounds
+                .trait_bounds
+                .iter()
+                .map(|&(trait_ref, _, _)| trait_ref.def_id())
+                .find(|&trait_ref| tcx.is_trait_alias(trait_ref))
+                .map(|trait_ref| tcx.def_span(trait_ref));
+            tcx.sess.emit_err(TraitObjectDeclaredWithNoTraits { span, trait_alias_span });
             return tcx.ty_error();
         }
 

compiler/rustc_typeck/src/errors.rs

@@ -103,6 +103,8 @@ pub struct CopyImplOnNonAdt {
 pub struct TraitObjectDeclaredWithNoTraits {
     #[primary_span]
     pub span: Span,
+    #[label = "alias-span"]
+    pub trait_alias_span: Option<Span>,
 }
 
 #[derive(SessionDiagnostic)]

src/test/ui/traits/alias/only-maybe-bound.stderr

@@ -1,12 +1,18 @@
 error[E0224]: at least one trait is required for an object type
   --> $DIR/only-maybe-bound.rs:13:12
    |
+LL | trait _1 = _0;
+   | -------------- this alias does not contain a trait
+...
 LL | type _T0 = dyn _1;
    |            ^^^^^^
 
 error[E0224]: at least one trait is required for an object type
   --> $DIR/only-maybe-bound.rs:19:12
    |
+LL | trait _2 = _1 + _1;
+   | ------------------- this alias does not contain a trait
+LL | 
 LL | type _T1 = dyn _2;
    |            ^^^^^^
 

src/test/ui/traits/issue-65673.stderr

@@ -1,6 +1,9 @@
 error[E0224]: at least one trait is required for an object type
   --> $DIR/issue-65673.rs:9:16
    |
+LL | trait Alias<T> = where T: Trait;
+   | -------------------------------- this alias does not contain a trait
+...
 LL |     type Ctx = dyn Alias<T>;
    |                ^^^^^^^^^^^^