From 7f97dfe700be69d359eaebf97275285f8a85faf4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Esteban=20K=C3=BCber?= <esteban@kuber.com.ar>
Date: Thu, 29 Feb 2024 16:30:01 +0000
Subject: [PATCH] Account for unmet `T: !Copy` in E0277 message

---
 .../src/traits/error_reporting/suggestions.rs   | 17 ++++++++++++-----
 tests/ui/traits/negative-bounds/simple.stderr   |  8 ++++----
 2 files changed, 16 insertions(+), 9 deletions(-)

diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
index e82e94993d249..06f908f01a97e 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
@@ -4773,11 +4773,18 @@ pub(super) fn get_explanation_based_on_obligation<'tcx>(
             Some(desc) => format!(" {desc}"),
             None => String::new(),
         };
-        format!(
-            "{pre_message}the trait `{}` is not implemented for{desc} `{}`{post}",
-            trait_predicate.print_modifiers_and_trait_path(),
-            tcx.short_ty_string(trait_ref.skip_binder().self_ty(), &mut None),
-        )
+        if let ty::ImplPolarity::Positive = trait_predicate.polarity() {
+            format!(
+                "{pre_message}the trait `{}` is not implemented for{desc} `{}`{post}",
+                trait_predicate.print_modifiers_and_trait_path(),
+                tcx.short_ty_string(trait_ref.skip_binder().self_ty(), &mut None),
+            )
+        } else {
+            // "the trait bound `T: !Send` is not satisfied" reads better than "`!Send` is
+            // not implemented for `T`".
+            // FIXME: add note explaining explicit negative trait bounds.
+            format!("{pre_message}the trait bound `{trait_predicate}` is not satisfied{post}")
+        }
     }
 }
 
diff --git a/tests/ui/traits/negative-bounds/simple.stderr b/tests/ui/traits/negative-bounds/simple.stderr
index 6d750739e197c..b8d12138794bf 100644
--- a/tests/ui/traits/negative-bounds/simple.stderr
+++ b/tests/ui/traits/negative-bounds/simple.stderr
@@ -2,7 +2,7 @@ error[E0277]: the trait bound `T: !Copy` is not satisfied
   --> $DIR/simple.rs:10:16
    |
 LL |     not_copy::<T>();
-   |                ^ the trait `!Copy` is not implemented for `T`
+   |                ^ the trait bound `T: !Copy` is not satisfied
    |
 note: required by a bound in `not_copy`
   --> $DIR/simple.rs:3:16
@@ -14,7 +14,7 @@ error[E0277]: the trait bound `T: !Copy` is not satisfied
   --> $DIR/simple.rs:15:16
    |
 LL |     not_copy::<T>();
-   |                ^ the trait `!Copy` is not implemented for `T`
+   |                ^ the trait bound `T: !Copy` is not satisfied
    |
 note: required by a bound in `not_copy`
   --> $DIR/simple.rs:3:16
@@ -26,7 +26,7 @@ error[E0277]: the trait bound `Copyable: !Copy` is not satisfied
   --> $DIR/simple.rs:30:16
    |
 LL |     not_copy::<Copyable>();
-   |                ^^^^^^^^ the trait `!Copy` is not implemented for `Copyable`
+   |                ^^^^^^^^ the trait bound `Copyable: !Copy` is not satisfied
    |
    = help: the trait `Copy` is implemented for `Copyable`
 note: required by a bound in `not_copy`
@@ -44,7 +44,7 @@ error[E0277]: the trait bound `NotNecessarilyCopyable: !Copy` is not satisfied
   --> $DIR/simple.rs:37:16
    |
 LL |     not_copy::<NotNecessarilyCopyable>();
-   |                ^^^^^^^^^^^^^^^^^^^^^^ the trait `!Copy` is not implemented for `NotNecessarilyCopyable`
+   |                ^^^^^^^^^^^^^^^^^^^^^^ the trait bound `NotNecessarilyCopyable: !Copy` is not satisfied
    |
 note: required by a bound in `not_copy`
   --> $DIR/simple.rs:3:16