Make co_current_executor look like nullptr, std::nullopt, std::in_place

Summary:
[Folly] Make `co_current_executor` look like `nullptr`, `std::nullopt`, `std::in_place`.

* Use a `co_` prefix to indicate that it is offers a useful result when awaited.
* Offer a well-named value with a well-named type or type alias.
  * There is the `nullptr` value and `std::nullptr_t` type or type alias.
  * There is the `std::nullopt` value and the `std::nullopt_t` type or type alias.
  * There is the `std::in_place` value and the `std::in_place_t` type or type alias.

Reviewed By: andriigrynenko, lewissbaker

Differential Revision: D13561713

fbshipit-source-id: 835da086e7165d37a952a1f169318cb566401d12

folly/experimental/coro/Task.h

@@ -32,7 +32,16 @@
 namespace folly {
 namespace coro {
 
-struct getCurrentExecutor {};
+namespace detail {
+struct co_current_executor_ {
+  enum class secret_ { token_ };
+  explicit constexpr co_current_executor_(secret_) {}
+};
+} // namespace detail
+
+using co_current_executor_t = detail::co_current_executor_;
+constexpr co_current_executor_t co_current_executor{
+    co_current_executor_t::secret_::token_};
 
 template <typename T = void>
 class Task;
@@ -82,7 +91,7 @@ class TaskPromiseBase {
     return co_viaIfAsync(executor_, static_cast<Awaitable&&>(awaitable));
   }
 
-  auto await_transform(folly::coro::getCurrentExecutor) noexcept {
+  auto await_transform(co_current_executor_t) noexcept {
     return AwaitableReady<folly::Executor*>{executor_};
   }
 

folly/experimental/coro/test/CoroTest.cpp

@@ -203,9 +203,9 @@ TEST(Coro, NestedThreads) {
 }
 
 coro::Task<int> taskGetCurrentExecutor(Executor* executor) {
-  auto currentExecutor = co_await coro::getCurrentExecutor();
-  EXPECT_EQ(executor, currentExecutor);
-  co_return co_await task42().scheduleOn(currentExecutor);
+  auto current = co_await coro::co_current_executor;
+  EXPECT_EQ(executor, current);
+  co_return co_await task42().scheduleOn(current);
 }
 
 TEST(Coro, CurrentExecutor) {