Add move constructor to Status.

This will result in smaller code generation when Status instances are
passed around.

Benchmarks don't indicate a significant change either way.
CPU:        48 * Intel(R) Xeon(R) CPU E5-2690 v3 @ 2.60GHz
CPUCache:   30720 KB
Keys:       16 bytes each
Values:     100 bytes each (50 bytes after compression)
Entries:    1000000
RawSize:    110.6 MB (estimated)
FileSize:   62.9 MB (estimated)

Baseline:
fillseq      :       3.589 micros/op;   30.8 MB/s
fillsync     :    4165.299 micros/op;    0.0 MB/s (1000 ops)
fillrandom   :       5.864 micros/op;   18.9 MB/s
overwrite    :       7.830 micros/op;   14.1 MB/s
readrandom   :       5.534 micros/op; (1000000 of 1000000 found)
readrandom   :       4.292 micros/op; (1000000 of 1000000 found)
readseq      :       0.312 micros/op;  354.1 MB/s
readreverse  :       0.501 micros/op;  220.8 MB/s
compact      :  886211.000 micros/op;
readrandom   :       3.518 micros/op; (1000000 of 1000000 found)
readseq      :       0.251 micros/op;  441.2 MB/s
readreverse  :       0.456 micros/op;  242.4 MB/s
fill100K     :    1329.723 micros/op;   71.7 MB/s (1000 ops)
crc32c       :       1.976 micros/op; 1976.7 MB/s (4K per op)
snappycomp   :       4.705 micros/op;  830.2 MB/s (output: 55.1%)
snappyuncomp :       0.958 micros/op; 4079.1 MB/s
acquireload  :       0.727 micros/op; (each op is 1000 loads)

New:
fillseq      :       3.129 micros/op;   35.4 MB/s
fillsync     :    2748.099 micros/op;    0.0 MB/s (1000 ops)
fillrandom   :       5.394 micros/op;   20.5 MB/s
overwrite    :       7.253 micros/op;   15.3 MB/s
readrandom   :       5.655 micros/op; (1000000 of 1000000 found)
readrandom   :       4.425 micros/op; (1000000 of 1000000 found)
readseq      :       0.298 micros/op;  371.3 MB/s
readreverse  :       0.508 micros/op;  217.9 MB/s
compact      :  885842.000 micros/op;
readrandom   :       3.545 micros/op; (1000000 of 1000000 found)
readseq      :       0.252 micros/op;  438.2 MB/s
readreverse  :       0.425 micros/op;  260.2 MB/s
fill100K     :    1418.347 micros/op;   67.2 MB/s (1000 ops)
crc32c       :       1.987 micros/op; 1966.0 MB/s (4K per op)
snappycomp   :       4.767 micros/op;  819.4 MB/s (output: 55.1%)
snappyuncomp :       0.916 micros/op; 4264.9 MB/s
acquireload  :       0.665 micros/op; (each op is 1000 loads)

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=194002392

3 files changed, 62 insertions, 11 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 4bf3df3..d49c31e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -277,6 +277,7 @@ if(LEVELDB_BUILD_TESTS)
   leveldb_test("${PROJECT_SOURCE_DIR}/issues/issue200_test.cc")
 
   leveldb_test("${PROJECT_SOURCE_DIR}/util/env_test.cc")
+  leveldb_test("${PROJECT_SOURCE_DIR}/util/status_test.cc")
 
   if(NOT BUILD_SHARED_LIBS)
     leveldb_test("${PROJECT_SOURCE_DIR}/db/autocompact_test.cc")
diff --git a/include/leveldb/status.h b/include/leveldb/status.h
index 39d692d..ee9fac2 100644
--- a/include/leveldb/status.h
+++ b/include/leveldb/status.h
@@ -13,6 +13,7 @@
 #ifndef STORAGE_LEVELDB_INCLUDE_STATUS_H_
 #define STORAGE_LEVELDB_INCLUDE_STATUS_H_
 
+#include <algorithm>
 #include <string>
 #include "leveldb/export.h"
 #include "leveldb/slice.h"
@@ -22,12 +23,14 @@ namespace leveldb {
 class LEVELDB_EXPORT Status {
  public:
   // Create a success status.
-  Status() : state_(nullptr) { }
+  Status() noexcept : state_(nullptr) { }
   ~Status() { delete[] state_; }
 
-  // Copy the specified status.
-  Status(const Status& s);
-  void operator=(const Status& s);
+  Status(const Status& rhs);
+  Status& operator=(const Status& rhs);
+
+  Status(Status&& rhs) noexcept : state_(rhs.state_) { rhs.state_ = nullptr; }
+  Status& operator=(Status&& rhs) noexcept;
 
   // Return a success status.
   static Status OK() { return Status(); }
@@ -96,16 +99,21 @@ class LEVELDB_EXPORT Status {
   static const char* CopyState(const char* s);
 };
 
-inline Status::Status(const Status& s) {
-  state_ = (s.state_ == nullptr) ? nullptr : CopyState(s.state_);
+inline Status::Status(const Status& rhs) {
+  state_ = (rhs.state_ == nullptr) ? nullptr : CopyState(rhs.state_);
 }
-inline void Status::operator=(const Status& s) {
-  // The following condition catches both aliasing (when this == &s),
-  // and the common case where both s and *this are ok.
-  if (state_ != s.state_) {
+inline Status& Status::operator=(const Status& rhs) {
+  // The following condition catches both aliasing (when this == &rhs),
+  // and the common case where both rhs and *this are ok.
+  if (state_ != rhs.state_) {
     delete[] state_;
-    state_ = (s.state_ == nullptr) ? nullptr : CopyState(s.state_);
+    state_ = (rhs.state_ == nullptr) ? nullptr : CopyState(rhs.state_);
   }
+  return *this;
+}
+inline Status& Status::operator=(Status&& rhs) noexcept {
+  std::swap(state_, rhs.state_);
+  return *this;
 }
 
 }  // namespace leveldb
diff --git a/util/status_test.cc b/util/status_test.cc
new file mode 100644
index 0000000..7ed3b9e
--- /dev/null
+++ b/util/status_test.cc
@@ -0,0 +1,42 @@
+// Copyright (c) 2018 The LevelDB Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file. See the AUTHORS file for names of contributors.
+
+#include <utility>
+
+#include "leveldb/slice.h"
+#include "leveldb/status.h"
+#include "util/testharness.h"
+
+namespace leveldb {
+
+TEST(Status, MoveConstructor) {
+  {
+    Status ok = Status::OK();
+    Status ok2 = std::move(ok);
+
+    ASSERT_TRUE(ok2.ok());
+  }
+
+  {
+    Status status = Status::NotFound("custom NotFound status message");
+    Status status2 = std::move(status);
+
+    ASSERT_TRUE(status2.IsNotFound());
+    ASSERT_EQ("NotFound: custom NotFound status message", status2.ToString());
+  }
+
+  {
+    Status self_moved = Status::IOError("custom IOError status message");
+
+    // Needed to bypass compiler warning about explicit move-assignment.
+    Status& self_moved_reference = self_moved;
+    self_moved_reference = std::move(self_moved);
+  }
+}
+
+}  // namespace leveldb
+
+int main(int argc, char** argv) {
+  return leveldb::test::RunAllTests();
+}