Replace assertion

Author: pierwill

chalk-solve/src/coherence.rs

@@ -61,7 +61,8 @@ impl<I: Interner> SpecializationPriorities<I> {
     /// Store the priority of an impl (used during construction).
     /// Panics if we have already stored the priority for this impl.
     fn insert(&mut self, impl_id: ImplId<I>, p: SpecializationPriority) {
-        self.map.insert(impl_id, p);
+        let old_value = self.map.insert(impl_id, p);
+        assert!(old_value.is_none());
     }
 }