Add comments.

src/Compilers/Core/Portable/Serialization/ObjectBinder.cs

@@ -27,11 +27,13 @@ public static ObjectBinderState AllocateStateCopy()
         {
             lock (s_gate)
             {
+                // If we have any pooled copies, then just return one of those.
                 if (s_pool.Count > 0)
                 {
                     return s_pool.Pop();
                 }
 
+                // Otherwise, create copy from our current state and return that.
                 var state = ObjectBinderState.Create(s_version);
                 state.CopyFrom(s_state);
 
@@ -43,6 +45,8 @@ public static void FreeStateCopy(ObjectBinderState state)
         {
             lock (s_gate)
             {
+                // If our version changed between now and when we returned the state object,
+                // then we don't want to keep around this verion in the pool.  
                 if (state.Version == s_version)
                 {
                     if (s_pool.Count < 128)

src/Compilers/Core/Portable/Serialization/ObjectBinderState.cs

@@ -29,13 +29,6 @@ internal struct ObjectBinderState
         public static ObjectBinderState Create(int version)
             => new ObjectBinderState(version, new Dictionary<Type, int>(), new List<Type>(), new List<Func<ObjectReader, object>>());
 
-        public void Clear()
-        {
-            _typeToIndex.Clear();
-            _types.Clear();
-            _typeReaders.Clear();
-        }
-
         public void CopyFrom(ObjectBinderState other)
         {
             if (_types.Count == 0)
@@ -82,6 +75,7 @@ public bool RegisterTypeReader(Type type, Func<ObjectReader, object> typeReader)
         {
             if (_typeToIndex.ContainsKey(type))
             {
+                // We already knew about this type, nothing to register.
                 return false;
             }