add a reserve API to allow pre-allocation

Author: nikomatsakis

src/snapshot_vec.rs

@@ -119,6 +119,12 @@ impl<D: SnapshotVecDelegate> SnapshotVec<D> {
         &self.values[index]
     }
 
+    /// Reserve space for new values, just like an ordinary vec.
+    pub fn reserve(&mut self, additional: usize) {
+        // This is not affected by snapshots or anything.
+        self.values.reserve(additional);
+    }
+
     /// Returns a mutable pointer into the vec; whatever changes you make here cannot be undone
     /// automatically, so you should be sure call `record()` with some sort of suitable undo
     /// action.

src/unify/backing_vec.rs

@@ -29,6 +29,8 @@ pub trait UnificationStore: ops::Index<usize, Output = VarValue<Key<Self>>> + Cl
 
     fn push(&mut self, value: VarValue<Self::Key>);
 
+    fn reserve(&mut self, num_new_values: usize);
+
     fn update<F>(&mut self, index: usize, op: F)
         where F: FnOnce(&mut VarValue<Self::Key>);
 
@@ -79,6 +81,11 @@ impl<K: UnifyKey> UnificationStore for InPlace<K> {
         self.values.push(value);
     }
 
+    #[inline]
+    fn reserve(&mut self, num_new_values: usize) {
+        self.values.reserve(num_new_values);
+    }
+
     #[inline]
     fn update<F>(&mut self, index: usize, op: F)
         where F: FnOnce(&mut VarValue<Self::Key>)
@@ -147,6 +154,11 @@ impl<K: UnifyKey> UnificationStore for Persistent<K> {
         self.values.push(value);
     }
 
+    #[inline]
+    fn reserve(&mut self, _num_new_values: usize) {
+        // not obviously relevant to DVec.
+    }
+
     #[inline]
     fn update<F>(&mut self, index: usize, op: F)
         where F: FnOnce(&mut VarValue<Self::Key>)

src/unify/mod.rs

@@ -274,6 +274,12 @@ impl<S: UnificationStore> UnificationTable<S> {
         key
     }
 
+    /// Reserve memory for `num_new_keys` to be created. Does not
+    /// actually create the new keys; you must then invoke `new_key`.
+    pub fn reserve(&mut self, num_new_keys: usize) {
+        self.values.reserve(num_new_keys);
+    }
+
     /// Returns the number of keys created so far.
     pub fn len(&self) -> usize {
         self.values.len()