RUST-1062 More efficiently serialize array indexes (#384)

Author: patrickfreed

src/ser/raw/document_serializer.rs

@@ -30,17 +30,29 @@ impl<'a> DocumentSerializer<'a> {
         })
     }
 
+    /// Serialize a document key using the provided closure.
+    fn serialize_doc_key_custom<F: FnOnce(&mut Serializer) -> Result<()>>(
+        &mut self,
+        f: F,
+    ) -> Result<()> {
+        // push a dummy element type for now, will update this once we serialize the value
+        self.root_serializer.reserve_element_type();
+        f(self.root_serializer)?;
+        self.num_keys_serialized += 1;
+        Ok(())
+    }
+
+    /// Serialize a document key to string using `KeySerializer`.
     fn serialize_doc_key<T>(&mut self, key: &T) -> Result<()>
     where
         T: serde::Serialize + ?Sized,
     {
-        // push a dummy element type for now, will update this once we serialize the value
-        self.root_serializer.reserve_element_type();
-        key.serialize(KeySerializer {
-            root_serializer: &mut *self.root_serializer,
+        self.serialize_doc_key_custom(|rs| {
+            key.serialize(KeySerializer {
+                root_serializer: rs,
+            })?;
+            Ok(())
         })?;
-
-        self.num_keys_serialized += 1;
         Ok(())
     }
 
@@ -63,7 +75,13 @@ impl<'a> serde::ser::SerializeSeq for DocumentSerializer<'a> {
     where
         T: serde::Serialize,
     {
-        self.serialize_doc_key(&self.num_keys_serialized.to_string())?;
+        let index = self.num_keys_serialized;
+        self.serialize_doc_key_custom(|rs| {
+            use std::io::Write;
+            write!(&mut rs.bytes, "{}", index)?;
+            rs.bytes.push(0);
+            Ok(())
+        })?;
         value.serialize(&mut *self.root_serializer)
     }
 
@@ -160,6 +178,7 @@ impl<'a> serde::ser::SerializeTupleStruct for DocumentSerializer<'a> {
 }
 
 /// Serializer used specifically for serializing document keys.
+/// Only keys that serialize to strings will be accepted.
 struct KeySerializer<'a> {
     root_serializer: &'a mut Serializer,
 }