Fix some lifetime issues that the compiler complains about.

Presumably these are related to the 2018 edition.

Author: nnethercote

src/snapshot_vec.rs

@@ -49,7 +49,7 @@ impl<D> fmt::Debug for SnapshotVec<D>
           D::Undo: fmt::Debug,
           D::Value: fmt::Debug
 {
-    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
         fmt.debug_struct("SnapshotVec")
             .field("values", &self.values)
             .field("undo_log", &self.undo_log)

src/unify/mod.rs

@@ -422,7 +422,7 @@ impl<S: UnificationStore> UnificationTable<S> {
 /// ////////////////////////////////////////////////////////////////////////
 /// Public API
 
-impl<'tcx, S, K, V> UnificationTable<S>
+impl<S, K, V> UnificationTable<S>
 where
     S: UnificationStore<Key = K, Value = V>,
     K: UnifyKey<Value = V>,