1 files changed, 20 insertions, 4 deletions

diff --git a/compiler/rustc_infer/src/traits/engine.rs b/compiler/rustc_infer/src/traits/engine.rs
index d3519f4b3..f75344f20 100644
--- a/compiler/rustc_infer/src/traits/engine.rs
+++ b/compiler/rustc_infer/src/traits/engine.rs
@@ -1,6 +1,5 @@
 use crate::infer::InferCtxt;
 use crate::traits::Obligation;
-use rustc_data_structures::fx::FxHashMap;
 use rustc_hir::def_id::DefId;
 use rustc_middle::ty::{self, ToPredicate, Ty};
 
@@ -37,13 +36,19 @@ pub trait TraitEngine<'tcx>: 'tcx {
         obligation: PredicateObligation<'tcx>,
     );
 
-    fn select_all_or_error(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>>;
-
     fn select_where_possible(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>>;
 
+    fn collect_remaining_errors(&mut self) -> Vec<FulfillmentError<'tcx>>;
+
     fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>>;
 
-    fn relationships(&mut self) -> &mut FxHashMap<ty::TyVid, ty::FoundRelationships>;
+    /// Among all pending obligations, collect those are stalled on a inference variable which has
+    /// changed since the last call to `select_where_possible`. Those obligations are marked as
+    /// successful and returned.
+    fn drain_unstalled_obligations(
+        &mut self,
+        infcx: &InferCtxt<'tcx>,
+    ) -> Vec<PredicateObligation<'tcx>>;
 }
 
 pub trait TraitEngineExt<'tcx> {
@@ -52,6 +57,8 @@ pub trait TraitEngineExt<'tcx> {
         infcx: &InferCtxt<'tcx>,
         obligations: impl IntoIterator<Item = PredicateObligation<'tcx>>,
     );
+
+    fn select_all_or_error(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>>;
 }
 
 impl<'tcx, T: ?Sized + TraitEngine<'tcx>> TraitEngineExt<'tcx> for T {
@@ -64,4 +71,13 @@ impl<'tcx, T: ?Sized + TraitEngine<'tcx>> TraitEngineExt<'tcx> for T {
             self.register_predicate_obligation(infcx, obligation);
         }
     }
+
+    fn select_all_or_error(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
+        let errors = self.select_where_possible(infcx);
+        if !errors.is_empty() {
+            return errors;
+        }
+
+        self.collect_remaining_errors()
+    }
 }