Fix ICE from projection cycle

Cycles in normalization can cause evaluations to change from Unknown to
Err. This means that some selection that were applicable no longer are.

To avoid this:
* Selection candidates that are known to be applicable are prefered
  over candidates that are not.
* We don't ICE if a candidate is no longer applicable.

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

@@ -69,7 +69,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             }
 
             ProjectionCandidate(idx) => {
-                let obligations = self.confirm_projection_candidate(obligation, idx);
+                let obligations = self.confirm_projection_candidate(obligation, idx)?;
                 Ok(ImplSource::Param(obligations))
             }
 
@@ -120,7 +120,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         &mut self,
         obligation: &TraitObligation<'tcx>,
         idx: usize,
-    ) -> Vec<PredicateObligation<'tcx>> {
+    ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
         self.infcx.commit_unconditionally(|_| {
             let tcx = self.tcx();
 
@@ -148,19 +148,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 &mut obligations,
             );
 
-            obligations.extend(
+            obligations.extend(self.infcx.commit_if_ok(|_| {
                 self.infcx
                     .at(&obligation.cause, obligation.param_env)
                     .sup(placeholder_trait_predicate.trait_ref.to_poly_trait_ref(), candidate)
                     .map(|InferOk { obligations, .. }| obligations)
-                    .unwrap_or_else(|_| {
-                        bug!(
-                            "Projection bound `{:?}` was applicable to `{:?}` but now is not",
-                            candidate,
-                            obligation
-                        );
-                    }),
-            );
+                    .map_err(|_| Unimplemented)
+            })?);
 
             if let ty::Projection(..) = placeholder_self_ty.kind() {
                 for predicate in tcx.predicates_of(def_id).instantiate_own(tcx, substs).predicates {
@@ -181,7 +175,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 }
             }
 
-            obligations
+            Ok(obligations)
         })
     }