reverse obligations for better diagnostics on multiple conflicting fn bounds

Author: compiler-errors

compiler/rustc_hir_typeck/src/closure.rs

@@ -231,7 +231,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         for obligation in traits::elaborate_obligations(
             self.tcx,
-            self.obligations_for_self_ty(expected_vid).collect(),
+            // Reverse the obligations here, since `elaborate_*` uses a stack,
+            // and we want to keep inference generally in the same order of
+            // the registered obligations.
+            self.obligations_for_self_ty(expected_vid).rev().collect(),
         ) {
             debug!(?obligation.predicate);
             let bound_predicate = obligation.predicate.kind();

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -670,7 +670,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     pub(in super::super) fn obligations_for_self_ty<'b>(
         &'b self,
         self_ty: ty::TyVid,
-    ) -> impl Iterator<Item = traits::PredicateObligation<'tcx>> + Captures<'tcx> + 'b {
+    ) -> impl DoubleEndedIterator<Item = traits::PredicateObligation<'tcx>> + Captures<'tcx> + 'b
+    {
         // FIXME: consider using `sub_root_var` here so we
         // can see through subtyping.
         let ty_var_root = self.root_var(self_ty);