Rollup merge of rust-lang#72807 - xiaotianrandom:fix-assoc-type-diagnostics, r=estebank

Avoid setting wrong obligation cause span of associated type mismatch

Removes code that sets wrong obligation cause span of associated type mismatch. See the linked issue for details.

Closes rust-lang#72806.

Author: Dylan-DPC

src/librustc_trait_selection/traits/wf.rs

@@ -172,25 +172,18 @@ fn extend_cause_with_original_assoc_item_obligation<'tcx>(
         };
     match pred.kind() {
         ty::PredicateKind::Projection(proj) => {
-            // The obligation comes not from the current `impl` nor the `trait` being
-            // implemented, but rather from a "second order" obligation, like in
-            // `src/test/ui/associated-types/point-at-type-on-obligation-failure.rs`.
-            let trait_assoc_item = tcx.associated_item(proj.projection_def_id());
-            if let Some(impl_item_span) =
-                items.iter().find(|item| item.ident == trait_assoc_item.ident).map(fix_span)
-            {
-                cause.span = impl_item_span;
-            } else {
-                let kind = &proj.ty().skip_binder().kind;
-                if let ty::Projection(projection_ty) = kind {
-                    // This happens when an associated type has a projection coming from another
-                    // associated type. See `traits-assoc-type-in-supertrait-bad.rs`.
-                    let trait_assoc_item = tcx.associated_item(projection_ty.item_def_id);
-                    if let Some(impl_item_span) =
-                        items.iter().find(|item| item.ident == trait_assoc_item.ident).map(fix_span)
-                    {
-                        cause.span = impl_item_span;
-                    }
+            // The obligation comes not from the current `impl` nor the `trait` being implemented,
+            // but rather from a "second order" obligation, where an associated type has a
+            // projection coming from another associated type. See
+            // `src/test/ui/associated-types/point-at-type-on-obligation-failure.rs` and
+            // `traits-assoc-type-in-supertrait-bad.rs`.
+            let kind = &proj.ty().skip_binder().kind;
+            if let ty::Projection(projection_ty) = kind {
+                let trait_assoc_item = tcx.associated_item(projection_ty.item_def_id);
+                if let Some(impl_item_span) =
+                    items.iter().find(|item| item.ident == trait_assoc_item.ident).map(fix_span)
+                {
+                    cause.span = impl_item_span;
                 }
             }
         }

src/test/ui/associated-types/issue-72806.rs

@@ -0,0 +1,20 @@
+trait Bar {
+    type Ok;
+    type Sibling: Bar2<Ok=char>;
+}
+trait Bar2 {
+    type Ok;
+}
+
+struct Foo;
+struct Foo2;
+
+impl Bar for Foo {  //~ ERROR type mismatch resolving `<Foo2 as Bar2>::Ok == char`
+    type Ok = ();
+    type Sibling = Foo2;
+}
+impl Bar2 for Foo2 {
+    type Ok = u32;
+}
+
+fn main() {}

src/test/ui/associated-types/issue-72806.stderr

@@ -0,0 +1,9 @@
+error[E0271]: type mismatch resolving `<Foo2 as Bar2>::Ok == char`
+  --> $DIR/issue-72806.rs:12:6
+   |
+LL | impl Bar for Foo {
+   |      ^^^ expected `u32`, found `char`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0271`.