Freeze SmallMap without relying on SmallMap internals

Summary: This makes freeze slightly more expensive (recreate index), but allows hiding `VecMap` from `starlark_map` API D40923063.

Reviewed By: bobyangyf

Differential Revision: D40923062

fbshipit-source-id: 3258f49816992832621d42e58ea70c1388783a10

Author: stepancheg

starlark/src/values/freeze.rs

@@ -21,7 +21,6 @@ use std::marker::PhantomData;
 
 use gazebo::prelude::*;
 use starlark_map::small_map::SmallMap;
-use starlark_map::vec_map::VecMap;
 use starlark_map::Hashed;
 
 use crate::values::Freezer;
@@ -180,36 +179,24 @@ impl<K: Freeze> Freeze for Hashed<K> {
     }
 }
 
-impl<K, V> Freeze for VecMap<K, V>
+impl<K, V> Freeze for SmallMap<K, V>
 where
     K: Freeze,
     V: Freeze,
 {
-    type Frozen = VecMap<K::Frozen, V::Frozen>;
+    type Frozen = SmallMap<K::Frozen, V::Frozen>;
 
-    fn freeze(self, freezer: &Freezer) -> anyhow::Result<Self::Frozen> {
-        let mut result = VecMap::with_capacity(self.len());
+    fn freeze(self, freezer: &Freezer) -> anyhow::Result<SmallMap<K::Frozen, V::Frozen>> {
+        let mut new = SmallMap::with_capacity(self.len());
         for (key, value) in self.into_iter_hashed() {
-            let key = key.freeze(freezer)?;
+            let hash = key.hash();
+            let key = key.into_key().freeze(freezer)?;
+            // TODO(nga): verify hash unchanged after freeze.
+            let key = Hashed::new_unchecked(hash, key);
             let value = value.freeze(freezer)?;
-            result.insert_hashed_unique_unchecked(key, value);
+            new.insert_hashed_unique_unchecked(key, value);
         }
-        Ok(result)
-    }
-}
-
-impl<K, V> Freeze for SmallMap<K, V>
-where
-    K: Freeze,
-    V: Freeze,
-{
-    type Frozen = SmallMap<K::Frozen, V::Frozen>;
-
-    fn freeze(mut self, freezer: &Freezer) -> anyhow::Result<SmallMap<K::Frozen, V::Frozen>> {
-        self.maybe_drop_index();
-        let (entries, index) = self.into_raw_parts();
-        let entries = entries.freeze(freezer)?;
-        unsafe { Ok(SmallMap::from_raw_parts(entries, index)) }
+        Ok(new)
     }
 }