Use a diagnostic item instead of filtering for Index::Output

compiler/rustc_hir_typeck/src/expr.rs

@@ -2871,6 +2871,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         base_ty: Ty<'tcx>,
     ) -> Option<(ErrorGuaranteed, Ty<'tcx>, Ty<'tcx>)> {
         let index_trait_def_id = self.tcx.lang_items().index_trait()?;
+        let index_trait_output_def_id = self.tcx.get_diagnostic_item(sym::IndexOutput)?;
 
         let mut relevant_impls = vec![];
         self.tcx.for_each_relevant_impl(index_trait_def_id, base_ty, |impl_def_id| {
@@ -2917,15 +2918,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             let element_ty = ocx.normalize(
                 &cause,
                 self.param_env,
-                self.tcx.mk_projection(
-                    self.tcx
-                        .associated_items(index_trait_def_id)
-                        .filter_by_name_unhygienic(sym::Output)
-                        .next()
-                        .unwrap()
-                        .def_id,
-                    impl_trait_ref.substs,
-                ),
+                self.tcx.mk_projection(index_trait_output_def_id, impl_trait_ref.substs),
             );
 
             let errors = ocx.select_where_possible();

compiler/rustc_span/src/symbol.rs

@@ -204,6 +204,7 @@ symbols! {
         HashSet,
         Hasher,
         Implied,
+        IndexOutput,
         Input,
         Into,
         IntoDiagnostic,

library/core/src/ops/index.rs

@@ -59,6 +59,7 @@
 pub trait Index<Idx: ?Sized> {
     /// The returned type after indexing.
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_diagnostic_item = "IndexOutput"]
     type Output: ?Sized;
 
     /// Performs the indexing (`container[index]`) operation.