Introduce in_snapshot and assert_open_snapshot methods.

This makes the two snapshot implementations more consistent with each
other and with crate `ena`.

Author: nnethercote

src/librustc/infer/region_constraints/mod.rs

@@ -429,10 +429,14 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
         }
     }
 
-    pub fn commit(&mut self, snapshot: RegionSnapshot) {
-        debug!("RegionConstraintCollector: commit({})", snapshot.length);
+    fn assert_open_snapshot(&self, snapshot: &RegionSnapshot) {
         assert!(self.undo_log.len() > snapshot.length);
         assert!(self.undo_log[snapshot.length] == OpenSnapshot);
+    }
+
+    pub fn commit(&mut self, snapshot: RegionSnapshot) {
+        debug!("RegionConstraintCollector: commit({})", snapshot.length);
+        self.assert_open_snapshot(&snapshot);
 
         if snapshot.length == 0 {
             self.undo_log.clear();
@@ -444,8 +448,7 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
 
     pub fn rollback_to(&mut self, snapshot: RegionSnapshot) {
         debug!("RegionConstraintCollector: rollback_to({:?})", snapshot);
-        assert!(self.undo_log.len() > snapshot.length);
-        assert!(self.undo_log[snapshot.length] == OpenSnapshot);
+        self.assert_open_snapshot(&snapshot);
         while self.undo_log.len() > snapshot.length + 1 {
             let undo_entry = self.undo_log.pop().unwrap();
             self.rollback_undo_entry(undo_entry);

src/librustc_data_structures/snapshot_map/mod.rs

@@ -55,16 +55,20 @@ impl<K, V> SnapshotMap<K, V>
         self.undo_log.clear();
     }
 
+    fn in_snapshot(&self) -> bool {
+        !self.undo_log.is_empty()
+    }
+
     pub fn insert(&mut self, key: K, value: V) -> bool {
         match self.map.insert(key.clone(), value) {
             None => {
-                if !self.undo_log.is_empty() {
+                if self.in_snapshot() {
                     self.undo_log.push(UndoLog::Inserted(key));
                 }
                 true
             }
             Some(old_value) => {
-                if !self.undo_log.is_empty() {
+                if self.in_snapshot() {
                     self.undo_log.push(UndoLog::Overwrite(key, old_value));
                 }
                 false
@@ -75,7 +79,7 @@ impl<K, V> SnapshotMap<K, V>
     pub fn remove(&mut self, key: K) -> bool {
         match self.map.remove(&key) {
             Some(old_value) => {
-                if !self.undo_log.is_empty() {
+                if self.in_snapshot() {
                     self.undo_log.push(UndoLog::Overwrite(key, old_value));
                 }
                 true