Point at specific field in struct literal when trait fulfillment fails

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -1224,6 +1224,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 ImplDerivedObligation(Box::new(ImplDerivedObligationCause {
                     derived,
                     impl_def_id,
+                    impl_def_predicate_index: None,
                     span: obligation.cause.span,
                 }))
             });

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -2562,11 +2562,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         assert_eq!(predicates.parent, None);
         let predicates = predicates.instantiate_own(tcx, substs);
         let mut obligations = Vec::with_capacity(predicates.len());
-        for (predicate, span) in predicates {
+        for (index, (predicate, span)) in predicates.into_iter().enumerate() {
             let cause = cause.clone().derived_cause(parent_trait_pred, |derived| {
                 ImplDerivedObligation(Box::new(ImplDerivedObligationCause {
                     derived,
                     impl_def_id: def_id,
+                    impl_def_predicate_index: Some(index),
                     span,
                 }))
             });