rvalueification of Future::filter(...): 1/n

Summary:
This is part of "the great r-valuification of folly::Future":
* This is something we should do for safety in general.
* Several of folly::Future's methods are lvalue-qualified even though they act as though they are rvalue-qualified, that is, they provide a postcondition that says, in effect, callers should act as though the method invalidated its `this` object (regardless of whether that invalidation was actual or logical).
* This violates the C++ principle to "Express ideas directly in code" (see Core Guidelines), and generally makes it more confusing for callers as well as hiding the actual semantics from tools (linters, compilers, etc.).
* This dichotomy and confusion has manifested itself by some failures around D7840699 since lvalue-qualification hides that operation's move-out semantics - leads to some use of future operations that are really not correct, but are not obviously incorrect.
* The goal of rvalueification is to make sure methods that are logically rvalue-qualified are actually rvalue-qualified, which forces callsites to acknowledge that rvalueification, e.g., `std::move(f).filter(...)` instead of `f.filter(...)`. This syntactic change in the callsites forces callers to acknowledge the method's rvalue semantics.

Reviewed By: yfeldblum

Differential Revision: D9308741

fbshipit-source-id: 671b1b7a39f16bcc20ad9f584ed0ca8def53d21c

folly/futures/Future-inl.h

@@ -2266,7 +2266,7 @@ Future<bool> Future<T>::willEqual(Future<T>& f) {
 
 template <class T>
 template <class F>
-Future<T> Future<T>::filter(F&& predicate) {
+Future<T> Future<T>::filter(F&& predicate) && {
   return std::move(*this).then([p = std::forward<F>(predicate)](T val) {
     T const& valConstRef = val;
     if (!p(valConstRef)) {

folly/futures/Future.h

@@ -1765,7 +1765,12 @@ class Future : private futures::detail::FutureBase<T> {
   ///   i.e., as if `*this` was moved into RESULT.
   /// - `RESULT.valid() == true`
   template <class F>
-  Future<T> filter(F&& predicate);
+  Future<T> filter(F&& predicate) &&;
+
+  template <class F>
+  Future<T> filter(F&& predicate) & {
+    return std::move(*this).filter(std::forward<F>(predicate));
+  }
 
   /// Like reduce, but works on a Future<std::vector<T / Try<T>>>, for example
   /// the result of collect or collectAll