Don't drop Upcast candidate in intercrate mode

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

@@ -921,6 +921,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         param_env: ty::ParamEnv<'tcx>,
         cause: &ObligationCause<'tcx>,
     ) -> Option<ty::PolyExistentialTraitRef<'tcx>> {
+        // Don't drop any candidates in intercrate mode, as it's incomplete.
+        // (Not that it matters, since `Unsize` is not a stable trait.)
+        if self.infcx.intercrate {
+            return None;
+        }
+
         let tcx = self.tcx();
         if tcx.features().trait_upcasting {
             return None;