From a1fbc143729b140f51c4a93dfc5a9a54a9a20486 Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Mon, 5 Dec 2022 19:06:32 +0000
Subject: [PATCH] Point at GAT where clause when unsatisfied

---
 .../src/traits/project.rs                     | 30 +++++++++++++++----
 .../own-bound-span.rs                         | 17 +++++++++++
 .../own-bound-span.stderr                     | 15 ++++++++++
 ...ject-safety-supertrait-mentions-GAT.stderr |  6 ++--
 4 files changed, 60 insertions(+), 8 deletions(-)
 create mode 100644 src/test/ui/generic-associated-types/own-bound-span.rs
 create mode 100644 src/test/ui/generic-associated-types/own-bound-span.stderr

diff --git a/compiler/rustc_trait_selection/src/traits/project.rs b/compiler/rustc_trait_selection/src/traits/project.rs
index 051660be9c474..71f6eae45aa54 100644
--- a/compiler/rustc_trait_selection/src/traits/project.rs
+++ b/compiler/rustc_trait_selection/src/traits/project.rs
@@ -2321,11 +2321,10 @@ fn assoc_ty_own_obligations<'cx, 'tcx>(
     nested: &mut Vec<PredicateObligation<'tcx>>,
 ) {
     let tcx = selcx.tcx();
-    for predicate in tcx
+    let own = tcx
         .predicates_of(obligation.predicate.item_def_id)
-        .instantiate_own(tcx, obligation.predicate.substs)
-        .predicates
-    {
+        .instantiate_own(tcx, obligation.predicate.substs);
+    for (predicate, span) in std::iter::zip(own.predicates, own.spans) {
         let normalized = normalize_with_depth_to(
             selcx,
             obligation.param_env,
@@ -2334,9 +2333,30 @@ fn assoc_ty_own_obligations<'cx, 'tcx>(
             predicate,
             nested,
         );
+
+        let nested_cause = if matches!(
+            obligation.cause.code(),
+            super::CompareImplItemObligation { .. }
+                | super::CheckAssociatedTypeBounds { .. }
+                | super::AscribeUserTypeProvePredicate(..)
+        ) {
+            obligation.cause.clone()
+        } else if span.is_dummy() {
+            ObligationCause::new(
+                obligation.cause.span,
+                obligation.cause.body_id,
+                super::ItemObligation(obligation.predicate.item_def_id),
+            )
+        } else {
+            ObligationCause::new(
+                obligation.cause.span,
+                obligation.cause.body_id,
+                super::BindingObligation(obligation.predicate.item_def_id, span),
+            )
+        };
         nested.push(Obligation::with_depth(
             tcx,
-            obligation.cause.clone(),
+            nested_cause,
             obligation.recursion_depth + 1,
             obligation.param_env,
             normalized,
diff --git a/src/test/ui/generic-associated-types/own-bound-span.rs b/src/test/ui/generic-associated-types/own-bound-span.rs
new file mode 100644
index 0000000000000..3699f7296f5d0
--- /dev/null
+++ b/src/test/ui/generic-associated-types/own-bound-span.rs
@@ -0,0 +1,17 @@
+struct S;
+
+trait D {
+    type P<T: Copy>;
+    //~^ NOTE required by this bound in `D::P`
+    //~| NOTE required by a bound in `D::P`
+}
+
+impl D for S {
+    type P<T: Copy> = ();
+}
+
+fn main() {
+    let _: <S as D>::P<String>;
+    //~^ ERROR the trait bound `String: Copy` is not satisfied
+    //~| NOTE the trait `Copy` is not implemented for `String`
+}
diff --git a/src/test/ui/generic-associated-types/own-bound-span.stderr b/src/test/ui/generic-associated-types/own-bound-span.stderr
new file mode 100644
index 0000000000000..8ab8ea623b2c9
--- /dev/null
+++ b/src/test/ui/generic-associated-types/own-bound-span.stderr
@@ -0,0 +1,15 @@
+error[E0277]: the trait bound `String: Copy` is not satisfied
+  --> $DIR/own-bound-span.rs:14:12
+   |
+LL |     let _: <S as D>::P<String>;
+   |            ^^^^^^^^^^^^^^^^^^^ the trait `Copy` is not implemented for `String`
+   |
+note: required by a bound in `D::P`
+  --> $DIR/own-bound-span.rs:4:15
+   |
+LL |     type P<T: Copy>;
+   |               ^^^^ required by this bound in `D::P`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/src/test/ui/object-safety/object-safety-supertrait-mentions-GAT.stderr b/src/test/ui/object-safety/object-safety-supertrait-mentions-GAT.stderr
index c1aaad31e8125..f05b0cd65381b 100644
--- a/src/test/ui/object-safety/object-safety-supertrait-mentions-GAT.stderr
+++ b/src/test/ui/object-safety/object-safety-supertrait-mentions-GAT.stderr
@@ -3,10 +3,10 @@ error[E0311]: the parameter type `Self` may not live long enough
    = help: consider adding an explicit lifetime bound `Self: 'a`...
    = note: ...so that the type `Self` will meet its required lifetime bounds...
 note: ...that is required by this bound
-  --> $DIR/object-safety-supertrait-mentions-GAT.rs:9:39
+  --> $DIR/object-safety-supertrait-mentions-GAT.rs:6:15
    |
-LL | trait SuperTrait<T>: for<'a> GatTrait<Gat<'a> = T> {
-   |                                       ^^^^^^^^^^^
+LL |         Self: 'a;
+   |               ^^
 
 error: associated item referring to unboxed trait object for its own trait
   --> $DIR/object-safety-supertrait-mentions-GAT.rs:10:20