From e947fad68cc7dce9285364984931dc1f286b9fa3 Mon Sep 17 00:00:00 2001
From: Camille GILLOT <gillot.camille@gmail.com>
Date: Mon, 9 May 2022 19:03:37 +0200
Subject: [PATCH] Point to the empty trait alias.

---
 compiler/rustc_error_messages/locales/en-US/typeck.ftl | 1 +
 compiler/rustc_typeck/src/astconv/mod.rs               | 8 +++++++-
 compiler/rustc_typeck/src/errors.rs                    | 2 ++
 src/test/ui/traits/alias/only-maybe-bound.stderr       | 6 ++++++
 src/test/ui/traits/issue-65673.stderr                  | 3 +++
 5 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/compiler/rustc_error_messages/locales/en-US/typeck.ftl b/compiler/rustc_error_messages/locales/en-US/typeck.ftl
index 6a3235fc772..e279b39a690 100644
--- a/compiler/rustc_error_messages/locales/en-US/typeck.ftl
+++ b/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
diff --git a/compiler/rustc_typeck/src/astconv/mod.rs b/compiler/rustc_typeck/src/astconv/mod.rs
index a21e9f4a0a5..b0244a44ffe 100644
--- a/compiler/rustc_typeck/src/astconv/mod.rs
+++ b/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();
         }
 
diff --git a/compiler/rustc_typeck/src/errors.rs b/compiler/rustc_typeck/src/errors.rs
index 3d2f93537e4..d99ae35bb9a 100644
--- a/compiler/rustc_typeck/src/errors.rs
+++ b/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)]
diff --git a/src/test/ui/traits/alias/only-maybe-bound.stderr b/src/test/ui/traits/alias/only-maybe-bound.stderr
index 99589edb535..e9e846c2ff3 100644
--- a/src/test/ui/traits/alias/only-maybe-bound.stderr
+++ b/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;
    |            ^^^^^^
 
diff --git a/src/test/ui/traits/issue-65673.stderr b/src/test/ui/traits/issue-65673.stderr
index 79f071ba302..71f3a0e7c7c 100644
--- a/src/test/ui/traits/issue-65673.stderr
+++ b/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>;
    |                ^^^^^^^^^^^^