Point at specific field in struct literal when trait fulfillment fails

compiler/rustc_middle/src/traits/mod.rs

@@ -467,6 +467,8 @@ pub enum WellFormedLoc {
 pub struct ImplDerivedObligationCause<'tcx> {
     pub derived: DerivedObligationCause<'tcx>,
     pub impl_def_id: DefId,
+    /// The index of the derived predicate in the parent impl's predicates.
+    pub impl_def_predicate_index: Option<usize>,
     pub span: Span,
 }