Correctly lift empty lists of clauses and goals

diff --git a/src/librustc/ty/context.rs b/src/librustc/ty/context.rs
index 7bb547dbceda22572c76cc2f6837f3e61a3103f9..289ef4da9922766cceeb3e92bd3ec7b392ea0468 100644 (file)
--- a/src/librustc/ty/context.rs
+++ b/src/librustc/ty/context.rs
@@ -1714,6 +1714,10 @@ fn lift_to_tcx<'b, 'gcx>(
         &self,
         tcx: TyCtxt<'b, 'gcx, 'tcx>,
     ) -> Option<&'tcx List<Goal<'tcx>>> {
+        if self.is_empty() {
+            return Some(List::empty());
+        }
+
         if tcx.interners.arena.in_arena(*self as *const _) {
             return Some(unsafe { mem::transmute(*self) });
         }
@@ -1732,6 +1736,10 @@ fn lift_to_tcx<'b, 'gcx>(
         &self,
         tcx: TyCtxt<'b, 'gcx, 'tcx>,
     ) -> Option<&'tcx List<Clause<'tcx>>> {
+        if self.is_empty() {
+            return Some(List::empty());
+        }
+
         if tcx.interners.arena.in_arena(*self as *const _) {
             return Some(unsafe { mem::transmute(*self) });
         }