(Wangle) Clean up move constructors

Summary:
These were doing more assignments than necessary.
1) We don't need to set `core_` to `nullptr` and then immediately
overwrite it with `other.core_`.
2) Don't need to swap the booleans (this was actually an unitialized
memory access).

Test Plan: Ran all the tests.

Reviewed By: hans@fb.com

Subscribers: trunkagent, folly-diffs@, jsedgwick, yfeldblum

FB internal diff: D1852386

Signature: t1:1852386:1424375154:25997209e76ccd16169125597e868932a5143ffb

folly/futures/Future-inl.h

@@ -32,13 +32,14 @@ namespace detail {
 }
 
 template <class T>
-Future<T>::Future(Future<T>&& other) noexcept : core_(nullptr) {
-  *this = std::move(other);
+Future<T>::Future(Future<T>&& other) noexcept : core_(other.core_) {
+  other.core_ = nullptr;
 }
 
 template <class T>
-Future<T>& Future<T>::operator=(Future<T>&& other) {
-  std::swap(core_, other.core_);
+Future<T>& Future<T>::operator=(Future<T>&& other) noexcept {
+  core_ = other.core_;
+  other.core_ = nullptr;
   return *this;
 }
 

folly/futures/Future.h

@@ -192,7 +192,7 @@ class Future {
 
   // movable
   Future(Future&&) noexcept;
-  Future& operator=(Future&&);
+  Future& operator=(Future&&) noexcept;
 
   // makeFuture
   template <class F = T>

folly/futures/Promise-inl.h

@@ -29,14 +29,16 @@ Promise<T>::Promise() : retrieved_(false), core_(new detail::Core<T>())
 {}
 
 template <class T>
-Promise<T>::Promise(Promise<T>&& other) : core_(nullptr) {
-  *this = std::move(other);
+Promise<T>::Promise(Promise<T>&& other) noexcept
+    : retrieved_(other.retrieved_), core_(other.core_) {
+  other.core_ = nullptr;
 }
 
 template <class T>
-Promise<T>& Promise<T>::operator=(Promise<T>&& other) {
-  std::swap(core_, other.core_);
-  std::swap(retrieved_, other.retrieved_);
+Promise<T>& Promise<T>::operator=(Promise<T>&& other) noexcept {
+  retrieved_ = other.retrieved_;
+  core_ = other.core_;
+  other.core_ = nullptr;
   return *this;
 }
 

folly/futures/Promise.h

@@ -36,8 +36,8 @@ public:
   Promise& operator=(Promise const&) = delete;
 
   // movable
-  Promise(Promise<T>&&);
-  Promise& operator=(Promise<T>&&);
+  Promise(Promise<T>&&) noexcept;
+  Promise& operator=(Promise<T>&&) noexcept;
 
   /** Return a Future tied to the shared core state. This can be called only
     once, thereafter Future already retrieved exception will be raised. */